# 29 Numerics library [numerics]

## 29.6 Random number generation [rand]

This subclause defines a facility for generating (pseudo-)random numbers.

In addition to a few utilities, four categories of entities are described: uniform random bit generators, random number engines, random number engine adaptors, and random number distributions. These categorizations are applicable to types that satisfy the corresponding requirements, to objects instantiated from such types, and to templates producing such types when instantiated. [Note: These entities are specified in such a way as to permit the binding of any uniform random bit generator object e as the argument to any random number distribution object d, thus producing a zero-argument function object such as given by bind(d,e). end note]

Each of the entities specified via this subclause has an associated arithmetic type identified as result_­type. With T as the result_­type thus associated with such an entity, that entity is characterized:

1. a)as boolean or equivalently as boolean-valued, if T is bool;

2. b)otherwise as integral or equivalently as integer-valued, if numeric_­limits<T>​::​is_­integer is true;

3. c)otherwise as floating or equivalently as real-valued.

If integer-valued, an entity may optionally be further characterized as signed or unsigned, according to numeric_­limits<T>​::​is_­signed.

Unless otherwise specified, all descriptions of calculations in this subclause use mathematical real numbers.

Throughout this subclause, the operators bitand , bitor , and xor denote the respective conventional bitwise operations. Further:

1. a)the operator rshift denotes a bitwise right shift with zero-valued bits appearing in the high bits of the result, and

2. b)the operator lshiftw denotes a bitwise left shift with zero-valued bits appearing in the low bits of the result, and whose result is always taken modulo 2w.

### 29.6.1 Requirements [rand.req]

#### 29.6.1.1 General requirements [rand.req.genl]

Throughout this subclause [rand], the effect of instantiating a template:

1. a)that has a template type parameter named Sseq is undefined unless the corresponding template argument is cv-unqualified and satisfies the requirements of seed sequence.

2. b)that has a template type parameter named URBG is undefined unless the corresponding template argument is cv-unqualified and satisfies the requirements of uniform random bit generator.

3. c)that has a template type parameter named Engine is undefined unless the corresponding template argument is cv-unqualified and satisfies the requirements of random number engine.

4. d)that has a template type parameter named RealType is undefined unless the corresponding template argument is cv-unqualified and is one of float, double, or long double.

5. e)that has a template type parameter named IntType is undefined unless the corresponding template argument is cv-unqualified and is one of short, int, long, long long, unsigned short, unsigned int, unsigned long, or unsigned long long.

6. f)that has a template type parameter named UIntType is undefined unless the corresponding template argument is cv-unqualified and is one of unsigned short, unsigned int, unsigned long, or unsigned long long.

Throughout this subclause [rand], phrases of the form “x is an iterator of a specific kind” shall be interpreted as equivalent to the more formal requirement that “x is a value of a type satisfying the requirements of the specified iterator type”.

Throughout this subclause [rand], any constructor that can be called with a single argument and that satisfies a requirement specified in this subclause shall be declared explicit.

#### 29.6.1.2 Seed sequence requirements [rand.req.seedseq]

A seed sequence is an object that consumes a sequence of integer-valued data and produces a requested number of unsigned integer values i, 0i<232, based on the consumed data. [Note: Such an object provides a mechanism to avoid replication of streams of random variates. This can be useful, for example, in applications requiring large numbers of random number engines. end note]

A class S satisfies the requirements of a seed sequence if the expressions shown in Table 102 are valid and have the indicated semantics, and if S also satisfies all other requirements of this section [rand.req.seedseq]. In that Table and throughout this section:

1. a)T is the type named by S's associated result_­type;

2. b)q is a value of S and r is a possibly const value of S;

3. c)ib and ie are input iterators with an unsigned integer value_­type of at least 32 bits;

4. d)rb and re are mutable random access iterators with an unsigned integer value_­type of at least 32 bits;

5. e)ob is an output iterator; and

6. f)il is a value of initializer_­list<T>.

Table 102 — Seed sequence requirements
 Expression Return type Pre/post-condition Complexity S​::​result_­type T T is an unsigned integer type of at least 32 bits. compile-time S() Creates a seed sequence with the same initial state as all other default-constructed seed sequences of type S. constant S(ib,ie) Creates a seed sequence having internal state that depends on some or all of the bits of the supplied sequence [ib,ie). O(ie−ib) S(il) Same as S(il.begin(), il.end()). same as S(il.begin(), il.end()) q.generate(rb,re) void Does nothing if rb == re. Otherwise, fills the supplied sequence [rb,re) with 32-bit quantities that depend on the sequence supplied to the constructor and possibly also depend on the history of generate's previous invocations. O(re−rb) r.size() size_­t The number of 32-bit units that would be copied by a call to r.param. constant r.param(ob) void Copies to the given destination a sequence of 32-bit units that can be provided to the constructor of a second object of type S, and that would reproduce in that second object a state indistinguishable from the state of the first object. O(r.size())

#### 29.6.1.3 Uniform random bit generator requirements [rand.req.urng]

A uniform random bit generator g of type G is a function object returning unsigned integer values such that each value in the range of possible results has (ideally) equal probability of being returned. [Note: The degree to which g's results approximate the ideal is often determined statistically. end note]

A class G satisfies the requirements of a uniform random bit generator if the expressions shown in Table 103 are valid and have the indicated semantics, and if G also satisfies all other requirements of this section [rand.req.urng]. In that Table and throughout this section:

1. a)T is the type named by G's associated result_­type, and

2. b)g is a value of G.

Table 103 — Uniform random bit generator requirements
 Expression Return type Pre/post-condition Complexity G​::​result_­type T T is an unsigned integer type. compile-time g() T Returns a value in the closed interval [G​::​min(), G​::​max()]. amortized constant G​::​min() T Denotes the least value potentially returned by operator(). compile-time G​::​max() T Denotes the greatest value potentially returned by operator(). compile-time

The following relation shall hold: G​::​min() < G​::​max().

#### 29.6.1.4 Random number engine requirements [rand.req.eng]

A random number engine (commonly shortened to engine) e of type E is a uniform random bit generator that additionally meets the requirements (e.g., for seeding and for input/output) specified in this section.

At any given time, e has a state ei for some integer i0. Upon construction, e has an initial state e0. An engine's state may be established via a constructor, a seed function, assignment, or a suitable operator>>.

E's specification shall define:

1. a)the size of E's state in multiples of the size of result_­type, given as an integral constant expression;

2. b)the transition algorithm TA by which e's state ei is advanced to its successor state ei+1; and

3. c)the generation algorithm GA by which an engine's state is mapped to a value of type result_­type.

A class E that satisfies the requirements of a uniform random bit generator also satisfies the requirements of a random number engine if the expressions shown in Table 104 are valid and have the indicated semantics, and if E also satisfies all other requirements of this section [rand.req.eng]. In that Table and throughout this section:

1. a)T is the type named by E's associated result_­type;

2. b)e is a value of E, v is an lvalue of E, x and y are (possibly const) values of E;

3. c)s is a value of T;

4. d)q is an lvalue satisfying the requirements of a seed sequence;

5. e)z is a value of type unsigned long long;

6. f)os is an lvalue of the type of some class template specialization basic_­ostream<charT, traits>; and

7. g)is is an lvalue of the type of some class template specialization basic_­istream<charT, traits>;

where charT and traits are constrained according to Clause [strings] and Clause [input.output].

Table 104 — Random number engine requirements
 Expression Return type Pre/post-condition Complexity E() Creates an engine with the same initial state as all other default-constructed engines of type E. O(size of state) E(x) Creates an engine that compares equal to x. O(size of state) E(s) Creates an engine with initial state determined by s. O(size of state) E(q)268 Creates an engine with an initial state that depends on a sequence produced by one call to q.generate. same as complexity of q.generate called on a sequence whose length is size of state e.seed() void Postconditions: e == E(). same as E() e.seed(s) void Postconditions: e == E(s). same as E(s) e.seed(q) void Postconditions: e == E(q). same as E(q) e() T Advances e's state ei to ei+1 =TA(ei) and returns GA(ei). per Table 103 e.discard(z) 269 void Advances e's state ei to ei+z by any means equivalent to z consecutive calls e(). no worse than the complexity of z consecutive calls e() x == y bool This operator is an equivalence relation. With Sx and Sy as the infinite sequences of values that would be generated by repeated future calls to x() and y(), respectively, returns true if Sx=Sy; else returns false. O(size of state) x != y bool !(x == y). O(size of state) os << x reference to the type of os With os.fmtflags set to ios_­base​::​dec|ios_­base​::​left and the fill character set to the space character, writes to os the textual representation of x's current state. In the output, adjacent numbers are separated by one or more space characters. Postconditions: The os.fmtflags and fill character are unchanged. O(size of state) is >> v reference to the type of is With is.fmtflags set to ios_­base​::​dec, sets v's state as determined by reading its textual representation from is. If bad input is encountered, ensures that v's state is unchanged by the operation and calls is.setstate(ios​::​failbit) (which may throw ios​::​failure ([iostate.flags])). If a textual representation written via os << x was subsequently read via is >> v, then x == v provided that there have been no intervening invocations of x or of v. Requires: is provides a textual representation that was previously written using an output stream whose imbued locale was the same as that of is, and whose type's template specialization arguments charT and traits were respectively the same as those of is. Postconditions: The is.fmtflags are unchanged. O(size of state)

E shall meet the requirements of CopyConstructible and CopyAssignable types. These operations shall each be of complexity no worse than O(size of state).

This constructor (as well as the subsequent corresponding seed() function) may be particularly useful to applications requiring a large number of independent random sequences.

This operation is common in user code, and can often be implemented in an engine-specific manner so as to provide significant performance improvements over an equivalent naive loop that makes z consecutive calls e().

#### 29.6.1.5 Random number engine adaptor requirements [rand.req.adapt]

A random number engine adaptor (commonly shortened to adaptor) a of type A is a random number engine that takes values produced by some other random number engine, and applies an algorithm to those values in order to deliver a sequence of values with different randomness properties. An engine b of type B adapted in this way is termed a base engine in this context. The expression a.base() shall be valid and shall return a const reference to a's base engine.

The requirements of a random number engine type shall be interpreted as follows with respect to a random number engine adaptor type.

```A::A(); ```

Effects: The base engine is initialized as if by its default constructor.

```bool operator==(const A& a1, const A& a2); ```

Returns: true if a1's base engine is equal to a2's base engine. Otherwise returns false.

```A::A(result_type s); ```

Effects: The base engine is initialized with s.

```template<class Sseq> A::A(Sseq& q); ```

Effects: The base engine is initialized with q.

```void seed(); ```

Effects: With b as the base engine, invokes b.seed().

```void seed(result_type s); ```

Effects: With b as the base engine, invokes b.seed(s).

```template<class Sseq> void seed(Sseq& q); ```

Effects: With b as the base engine, invokes b.seed(q).

A shall also satisfy the following additional requirements:

1. a)The complexity of each function shall not exceed the complexity of the corresponding function applied to the base engine.

2. b)The state of A shall include the state of its base engine. The size of A's state shall be no less than the size of the base engine.

3. c)Copying A's state (e.g., during copy construction or copy assignment) shall include copying the state of the base engine of A.

4. d)The textual representation of A shall include the textual representation of its base engine.

#### 29.6.1.6 Random number distribution requirements [rand.req.dist]

A random number distribution (commonly shortened to distribution) d of type D is a function object returning values that are distributed according to an associated mathematical probability density function p(z) or according to an associated discrete probability function P(zi). A distribution's specification identifies its associated probability function p(z) or P(zi).

An associated probability function is typically expressed using certain externally-supplied quantities known as the parameters of the distribution. Such distribution parameters are identified in this context by writing, for example, p(z|a,b) or P(zi|a,b), to name specific parameters, or by writing, for example, p(z|{p}) or P(zi|{p}), to denote a distribution's parameters p taken as a whole.

A class D satisfies the requirements of a random number distribution if the expressions shown in Table 105 are valid and have the indicated semantics, and if D and its associated types also satisfy all other requirements of this section [rand.req.dist]. In that Table and throughout this section,

1. a)T is the type named by D's associated result_­type;

2. b)P is the type named by D's associated param_­type;

3. c)d is a value of D, and x and y are (possibly const) values of D;

4. d)glb and lub are values of T respectively corresponding to the greatest lower bound and the least upper bound on the values potentially returned by d's operator(), as determined by the current values of d's parameters;

5. e)p is a (possibly const) value of P;

6. f)g, g1, and g2 are lvalues of a type satisfying the requirements of a uniform random bit generator;

7. g)os is an lvalue of the type of some class template specialization basic_­ostream<charT, traits>; and

8. h)is is an lvalue of the type of some class template specialization basic_­istream<charT, traits>;

where charT and traits are constrained according to Clauses [strings] and [input.output].

Table 105 — Random number distribution requirements
 Expression Return type Pre/post-condition Complexity D​::​result_­type T T is an arithmetic type. compile-time D​::​param_­type P compile-time D() Creates a distribution whose behavior is indistinguishable from that of any other newly default-constructed distribution of type D. constant D(p) Creates a distribution whose behavior is indistinguishable from that of a distribution newly constructed directly from the values used to construct p. same as p's construction d.reset() void Subsequent uses of d do not depend on values produced by any engine prior to invoking reset. constant x.param() P Returns a value p such that D(p).param() == p. no worse than the complexity of D(p) d.param(p) void Postconditions: d.param() == p. no worse than the complexity of D(p) d(g) T With p=d.param(), the sequence of numbers returned by successive invocations with the same object g is randomly distributed according to the associated p(z |{p}) or P(zi|{p}) function. amortized constant number of invocations of g d(g,p) T The sequence of numbers returned by successive invocations with the same objects g and p is randomly distributed according to the associated p(z |{p}) or P(zi|{p}) function. amortized constant number of invocations of g x.min() T Returns glb. constant x.max() T Returns lub. constant x == y bool This operator is an equivalence relation. Returns true if x.param() == y.param() and S1=S2, where S1 and S2 are the infinite sequences of values that would be generated, respectively, by repeated future calls to x(g1) and y(g2) whenever g1 == g2. Otherwise returns false. constant x != y bool !(x == y). same as x == y. os << x reference to the type of os Writes to os a textual representation for the parameters and the additional internal data of x. Postconditions: The os.fmtflags and fill character are unchanged. is >> d reference to the type of is Restores from is the parameters and additional internal data of the lvalue d. If bad input is encountered, ensures that d is unchanged by the operation and calls is.setstate(ios​::​failbit) (which may throw ios​::​failure ([iostate.flags])). Requires: is provides a textual representation that was previously written using an os whose imbued locale and whose type's template specialization arguments charT and traits were the same as those of is. Postconditions: The is.fmtflags are unchanged.

D shall satisfy the requirements of CopyConstructible and CopyAssignable types.

The sequence of numbers produced by repeated invocations of d(g) shall be independent of any invocation of os << d or of any const member function of D between any of the invocations d(g).

If a textual representation is written using os << x and that representation is restored into the same or a different object y of the same type using is >> y, repeated invocations of y(g) shall produce the same sequence of numbers as would repeated invocations of x(g).

It is unspecified whether D​::​param_­type is declared as a (nested) class or via a typedef. In this subclause [rand], declarations of D​::​param_­type are in the form of typedefs for convenience of exposition only.

P shall satisfy the requirements of CopyConstructible, CopyAssignable, and EqualityComparable types.

For each of the constructors of D taking arguments corresponding to parameters of the distribution, P shall have a corresponding constructor subject to the same requirements and taking arguments identical in number, type, and default values. Moreover, for each of the member functions of D that return values corresponding to parameters of the distribution, P shall have a corresponding member function with the identical name, type, and semantics.

P shall have a declaration of the form

`using distribution_type =  D;`

### 29.6.2 Header <random> synopsis [rand.synopsis]

```#include <initializer_list>

namespace std {
// [rand.eng.lcong], class template linear_­congruential_­engine
template<class UIntType, UIntType a, UIntType c, UIntType m>
class linear_congruential_engine;

// [rand.eng.mers], class template mersenne_­twister_­engine
template<class UIntType, size_t w, size_t n, size_t m, size_t r,
UIntType a, size_t u, UIntType d, size_t s,
UIntType b, size_t t,
UIntType c, size_t l, UIntType f>
class mersenne_twister_engine;

// [rand.eng.sub], class template subtract_­with_­carry_­engine
template<class UIntType, size_t w, size_t s, size_t r>
class subtract_with_carry_engine;

template<class Engine, size_t p, size_t r>

// [rand.adapt.ibits], class template independent_­bits_­engine
template<class Engine, size_t w, class UIntType>
class independent_bits_engine;

// [rand.adapt.shuf], class template shuffle_­order_­engine
template<class Engine, size_t k>
class shuffle_order_engine;

// [rand.predef], engines and engine adaptors with predefined parameters
using minstd_rand0  = see below;
using minstd_rand   = see below;
using mt19937       = see below;
using mt19937_64    = see below;
using ranlux24_base = see below;
using ranlux48_base = see below;
using ranlux24      = see below;
using ranlux48      = see below;
using knuth_b       = see below;

using default_random_engine = see below;

// [rand.device], class random_­device
class random_device;

// [rand.util.seedseq], class seed_­seq
class seed_seq;

// [rand.util.canonical], function template generate_­canonical
template<class RealType, size_t bits, class URBG>
RealType generate_canonical(URBG& g);

// [rand.dist.uni.int], class template uniform_­int_­distribution
template<class IntType = int>
class uniform_int_distribution;

// [rand.dist.uni.real], class template uniform_­real_­distribution
template<class RealType = double>
class uniform_real_distribution;

// [rand.dist.bern.bernoulli], class bernoulli_­distribution
class bernoulli_distribution;

// [rand.dist.bern.bin], class template binomial_­distribution
template<class IntType = int>
class binomial_distribution;

// [rand.dist.bern.geo], class template geometric_­distribution
template<class IntType = int>
class geometric_distribution;

// [rand.dist.bern.negbin], class template negative_­binomial_­distribution
template<class IntType = int>
class negative_binomial_distribution;

// [rand.dist.pois.poisson], class template poisson_­distribution
template<class IntType = int>
class poisson_distribution;

// [rand.dist.pois.exp], class template exponential_­distribution
template<class RealType = double>
class exponential_distribution;

// [rand.dist.pois.gamma], class template gamma_­distribution
template<class RealType = double>
class gamma_distribution;

// [rand.dist.pois.weibull], class template weibull_­distribution
template<class RealType = double>
class weibull_distribution;

// [rand.dist.pois.extreme], class template extreme_­value_­distribution
template<class RealType = double>
class extreme_value_distribution;

// [rand.dist.norm.normal], class template normal_­distribution
template<class RealType = double>
class normal_distribution;

// [rand.dist.norm.lognormal], class template lognormal_­distribution
template<class RealType = double>
class lognormal_distribution;

// [rand.dist.norm.chisq], class template chi_­squared_­distribution
template<class RealType = double>
class chi_squared_distribution;

// [rand.dist.norm.cauchy], class template cauchy_­distribution
template<class RealType = double>
class cauchy_distribution;

// [rand.dist.norm.f], class template fisher_­f_­distribution
template<class RealType = double>
class fisher_f_distribution;

// [rand.dist.norm.t], class template student_­t_­distribution
template<class RealType = double>
class student_t_distribution;

// [rand.dist.samp.discrete], class template discrete_­distribution
template<class IntType = int>
class discrete_distribution;

// [rand.dist.samp.pconst], class template piecewise_­constant_­distribution
template<class RealType = double>
class piecewise_constant_distribution;

// [rand.dist.samp.plinear], class template piecewise_­linear_­distribution
template<class RealType = double>
class piecewise_linear_distribution;
}```

### 29.6.3 Random number engine class templates [rand.eng]

Each type instantiated from a class template specified in this section [rand.eng] satisfies the requirements of a random number engine type.

Except where specified otherwise, the complexity of each function specified in this section [rand.eng] is constant.

Except where specified otherwise, no function described in this section [rand.eng] throws an exception.

Every function described in this section [rand.eng] that has a function parameter q of type Sseq& for a template type parameter named Sseq that is different from type seed_­seq throws what and when the invocation of q.generate throws.

Descriptions are provided in this section [rand.eng] only for engine operations that are not described in [rand.req.eng] or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses.

Each template specified in this section [rand.eng] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold.

For every random number engine and for every random number engine adaptor X defined in this subclause ([rand.eng]) and in subclause [rand.adapt]:

• if the constructor

`template <class Sseq> explicit X(Sseq& q);`

is called with a type Sseq that does not qualify as a seed sequence, then this constructor shall not participate in overload resolution;

• if the member function

`template <class Sseq> void seed(Sseq& q);`

is called with a type Sseq that does not qualify as a seed sequence, then this function shall not participate in overload resolution.

The extent to which an implementation determines that a type cannot be a seed sequence is unspecified, except that as a minimum a type shall not qualify as a seed sequence if it is implicitly convertible to X​::​result_­type.

#### 29.6.3.1 Class template linear_­congruential_­engine[rand.eng.lcong]

A linear_­congruential_­engine random number engine produces unsigned integer random numbers. The state xi of a linear_­congruential_­engine object x is of size 1 and consists of a single integer. The transition algorithm is a modular linear function of the form TA(xi)=(axi+c)modm; the generation algorithm is GA(xi)=xi+1.

```template<class UIntType, UIntType a, UIntType c, UIntType m>
class linear_congruential_engine {
public:
// types
using result_type = UIntType;

// engine characteristics
static constexpr result_type multiplier = a;
static constexpr result_type increment = c;
static constexpr result_type modulus = m;
static constexpr result_type min() { return c == 0u ? 1u: 0u; }
static constexpr result_type max() { return m - 1u; }
static constexpr result_type default_seed = 1u;

// constructors and seeding functions
explicit linear_congruential_engine(result_type s = default_seed);
template<class Sseq> explicit linear_congruential_engine(Sseq& q);
void seed(result_type s = default_seed);
template<class Sseq> void seed(Sseq& q);

// generating functions
result_type operator()();
void discard(unsigned long long z);
};```

If the template parameter m is 0, the modulus m used throughout this section [rand.eng.lcong] is numeric_­limits<result_­type>​::​max() plus 1. [Note: m need not be representable as a value of type result_­type. end note]

If the template parameter m is not 0, the following relations shall hold: a < m and c < m.

The textual representation consists of the value of xi.

```explicit linear_congruential_engine(result_type s = default_seed); ```

Effects: Constructs a linear_­congruential_­engine object. If cmodm is 0 and smodm is 0, sets the engine's state to 1, otherwise sets the engine's state to smodm.

```template<class Sseq> explicit linear_congruential_engine(Sseq& q); ```

Effects: Constructs a linear_­congruential_­engine object. With k=log2m32 and a an array (or equivalent) of length k+3, invokes q.generate(a+0, a+k+3) and then computes S=(k1j=0aj+3232j)modm. If cmodm is 0 and S is 0, sets the engine's state to 1, else sets the engine's state to S.

#### 29.6.3.2 Class template mersenne_­twister_­engine[rand.eng.mers]

A mersenne_­twister_­engine random number engine270 produces unsigned integer random numbers in the closed interval [0,2w1]. The state xi of a mersenne_­twister_­engine object x is of size n and consists of a sequence X of n values of the type delivered by x; all subscripts applied to X are to be taken modulo n.

The transition algorithm employs a twisted generalized feedback shift register defined by shift values n and m, a twist value r, and a conditional xor-mask a. To improve the uniformity of the result, the bits of the raw shift register are additionally tempered (i.e., scrambled) according to a bit-scrambling matrix defined by values u,d,s,b,t,c, and .

The state transition is performed as follows:

1. a)Concatenate the upper wr bits of Xin with the lower r bits of Xi+1n to obtain an unsigned integer value Y.

2. b)With α=a(Ybitand1), set Xi to Xi+mnxor(Yrshift1)xorα.

The sequence X is initialized with the help of an initialization multiplier f.

The generation algorithm determines the unsigned integer values z1,z2,z3,z4 as follows, then delivers z4 as its result:

1. a)Let z1=Xixor((Xirshiftu)bitandd).

2. b)Let z2=z1xor((z1lshiftws)bitandb).

3. c)Let z3=z2xor((z2lshiftwt)bitandc).

4. d)Let z4=z3xor(z3rshift).

```template<class UIntType, size_t w, size_t n, size_t m, size_t r,
UIntType a, size_t u, UIntType d, size_t s,
UIntType b, size_t t,
UIntType c, size_t l, UIntType f>
class mersenne_twister_engine {
public:
// types
using result_type = UIntType;

// engine characteristics
static constexpr size_t word_size = w;
static constexpr size_t state_size = n;
static constexpr size_t shift_size = m;
static constexpr size_t mask_bits = r;
static constexpr UIntType xor_mask = a;
static constexpr size_t tempering_u = u;
static constexpr UIntType tempering_d = d;
static constexpr size_t tempering_s = s;
static constexpr UIntType tempering_b = b;
static constexpr size_t tempering_t = t;
static constexpr UIntType tempering_c = c;
static constexpr size_t tempering_l = l;
static constexpr UIntType initialization_multiplier = f;
static constexpr result_type min() { return 0; }
static constexpr result_type max() { return  2w−1; }
static constexpr result_type default_seed = 5489u;

// constructors and seeding functions
explicit mersenne_twister_engine(result_type value = default_seed);
template<class Sseq> explicit mersenne_twister_engine(Sseq& q);
void seed(result_type value = default_seed);
template<class Sseq> void seed(Sseq& q);

// generating functions
result_type operator()();
void discard(unsigned long long z);
};```

The following relations shall hold: 0 < m, m <= n, 2u < w, r <= w, u <= w, s <= w, t <= w, l <= w, w <= numeric_­limits<UIntType>​::​digits, a <= (1u<<w) - 1u, b <= (1u<<w) - 1u, c <= (1u<<w) - 1u, d <= (1u<<w) - 1u, and f <= (1u<<w) - 1u.

The textual representation of xi consists of the values of Xin,,Xi1, in that order.

```explicit mersenne_twister_engine(result_type value = default_seed); ```

Effects: Constructs a mersenne_­twister_­engine object. Sets Xn to valuemod2w. Then, iteratively for i=1n,,1, sets Xi to

[f(Xi1xor(Xi1rshift(w2)))+imodn]mod2w.

Complexity: O(n).

```template<class Sseq> explicit mersenne_twister_engine(Sseq& q); ```

Effects: Constructs a mersenne_­twister_­engine object. With k=w/32 and a an array (or equivalent) of length nk, invokes q.generate(a+0, a+nk) and then, iteratively for i=n,,1, sets Xi to (k1j=0ak(i+n)+j232j)mod2w. Finally, if the most significant wr bits of Xn are zero, and if each of the other resulting Xi is 0, changes Xn to 2w1.

The name of this engine refers, in part, to a property of its period: For properly-selected values of the parameters, the period is closely related to a large Mersenne prime number.

#### 29.6.3.3 Class template subtract_­with_­carry_­engine[rand.eng.sub]

A subtract_­with_­carry_­engine random number engine produces unsigned integer random numbers.

The state xi of a subtract_­with_­carry_­engine object x is of size O(r), and consists of a sequence X of r integer values 0Xi<m=2w; all subscripts applied to X are to be taken modulo r. The state xi additionally consists of an integer c (known as the carry) whose value is either 0 or 1.

The state transition is performed as follows:

1. a)Let Y=XisXirc.

2. b)Set Xi to y=Ymodm. Set c to 1 if Y<0, otherwise set c to 0.

[Note: This algorithm corresponds to a modular linear function of the form TA(xi)=(axi)modb, where b is of the form mrms+1 and a=b(b1)/m. end note]

The generation algorithm is given by GA(xi)=y, where y is the value produced as a result of advancing the engine's state as described above.

```template<class UIntType, size_t w, size_t s, size_t r>
class subtract_with_carry_engine {
public:
// types
using result_type = UIntType;

// engine characteristics
static constexpr size_t word_size = w;
static constexpr size_t short_lag = s;
static constexpr size_t long_lag = r;
static constexpr result_type min() { return 0; }
static constexpr result_type max() { return m−1; }
static constexpr result_type default_seed = 19780503u;

// constructors and seeding functions
explicit subtract_with_carry_engine(result_type value = default_seed);
template<class Sseq> explicit subtract_with_carry_engine(Sseq& q);
void seed(result_type value = default_seed);
template<class Sseq> void seed(Sseq& q);

// generating functions
result_type operator()();
void discard(unsigned long long z);
};```

The following relations shall hold: 0u < s, s < r, 0 < w, and w <= numeric_­limits<UIntType>​::​digits.

The textual representation consists of the values of Xir,,Xi1, in that order, followed by c.

```explicit subtract_with_carry_engine(result_type value = default_seed); ```

Effects: Constructs a subtract_­with_­carry_­engine object. Sets the values of Xr,,X1, in that order, as specified below. If X1 is then 0, sets c to 1; otherwise sets c to 0.

To set the values Xk, first construct e, a linear_­congruential_­engine object, as if by the following definition:

```linear_congruential_engine<result_type,
40014u,0u,2147483563u> e(value == 0u ? default_seed : value);```

Then, to set each Xk, obtain new values z0,,zn1 from n=w/32 successive invocations of e taken modulo 232. Set Xk to (n1j=0zj232j)modm.

Complexity: Exactly nr invocations of e.

```template<class Sseq> explicit subtract_with_carry_engine(Sseq& q); ```

Effects: Constructs a subtract_­with_­carry_­engine object. With k=w/32 and a an array (or equivalent) of length rk, invokes q.generate(a+0, a+rk) and then, iteratively for i=r,,1, sets Xi to (k1j=0ak(i+r)+j232j)modm. If X1 is then 0, sets c to 1; otherwise sets c to 0.

### 29.6.4 Random number engine adaptor class templates [rand.adapt]

#### 29.6.4.1 In general [rand.adapt.general]

Each type instantiated from a class template specified in this section [rand.adapt] satisfies the requirements of a random number engine adaptor type.

Except where specified otherwise, the complexity of each function specified in this section [rand.adapt] is constant.

Except where specified otherwise, no function described in this section [rand.adapt] throws an exception.

Every function described in this section [rand.adapt] that has a function parameter q of type Sseq& for a template type parameter named Sseq that is different from type seed_­seq throws what and when the invocation of q.generate throws.

Descriptions are provided in this section [rand.adapt] only for adaptor operations that are not described in section [rand.req.adapt] or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses.

Each template specified in this section [rand.adapt] requires one or more relationships, involving the value(s) of its non-type template parameter(s), to hold. A program instantiating any of these templates is ill-formed if any such required relationship fails to hold.

A discard_­block_­engine random number engine adaptor produces random numbers selected from those produced by some base engine e. The state xi of a discard_­block_­engine engine adaptor object x consists of the state ei of its base engine e and an additional integer n. The size of the state is the size of e's state plus 1.

The transition algorithm discards all but r>0 values from each block of pr values delivered by e. The state transition is performed as follows: If nr, advance the state of e from ei to ei+pr and set n to 0. In any case, then increment n and advance e's then-current state ej to ej+1.

The generation algorithm yields the value returned by the last invocation of e() while advancing e's state as described above.

```template<class Engine, size_t p, size_t r>
public:
// types
using result_type = typename Engine::result_type;

// engine characteristics
static constexpr size_t block_size = p;
static constexpr size_t used_block = r;
static constexpr result_type min() { return Engine::min(); }
static constexpr result_type max() { return Engine::max(); }

// constructors and seeding functions
explicit discard_block_engine(const Engine& e);
template<class Sseq> explicit discard_block_engine(Sseq& q);
void seed();
void seed(result_type s);
template<class Sseq> void seed(Sseq& q);

// generating functions
result_type operator()();
void discard(unsigned long long z);

// property functions
const Engine& base() const noexcept { return e; };

private:
Engine e;   // exposition only
int n;      // exposition only
};```

The following relations shall hold: 0 < r and r <= p.

The textual representation consists of the textual representation of e followed by the value of n.

In addition to its behavior pursuant to section [rand.req.adapt], each constructor that is not a copy constructor sets n to 0.

#### 29.6.4.3 Class template independent_­bits_­engine[rand.adapt.ibits]

An independent_­bits_­engine random number engine adaptor combines random numbers that are produced by some base engine e, so as to produce random numbers with a specified number of bits w. The state xi of an independent_­bits_­engine engine adaptor object x consists of the state ei of its base engine e; the size of the state is the size of e's state.

The transition and generation algorithms are described in terms of the following integral constants:

1. a)Let R=e.max() - e.min() + 1 and m=log2R.

2. b)With n as determined below, let w0=w/n, n0=nwmodn, y0=2w0R/2w0, and y1=2w0+1R/2w0+1.

3. c)Let n=w/m if and only if the relation Ry0y0/n holds as a result. Otherwise let n=1+w/m.

[Note: The relation w=n0w0+(nn0)(w0+1) always holds. end note]

The transition algorithm is carried out by invoking e() as often as needed to obtain n0 values less than y0+e.min() and nn0 values less than y1+e.min().

The generation algorithm uses the values produced while advancing the state as described above to yield a quantity S obtained as if by the following algorithm:

```S = 0;
for (k = 0; k≠n0; k += 1)  {
do u = e() - e.min(); while (u≥y0);
S = 2w0⋅S+umod2w0;
}
for (k = n0; k≠n; k += 1)  {
do u = e() - e.min(); while (u≥y1);
S = 2w0+1⋅S+umod2w0+1;
}```

```template<class Engine, size_t w, class UIntType>
class independent_bits_engine {
public:
// types
using result_type = UIntType;

// engine characteristics
static constexpr result_type min() { return 0; }
static constexpr result_type max() { return 2w−1; }

// constructors and seeding functions
independent_bits_engine();
explicit independent_bits_engine(const Engine& e);
explicit independent_bits_engine(Engine&& e);
explicit independent_bits_engine(result_type s);
template<class Sseq> explicit independent_bits_engine(Sseq& q);
void seed();
void seed(result_type s);
template<class Sseq> void seed(Sseq& q);

// generating functions
result_type operator()();
void discard(unsigned long long z);

// property functions
const Engine& base() const noexcept { return e; };

private:
Engine e;   // exposition only
};```

The following relations shall hold: 0 < w and w <= numeric_­limits<result_­type>​::​digits.

The textual representation consists of the textual representation of e.

#### 29.6.4.4 Class template shuffle_­order_­engine[rand.adapt.shuf]

A shuffle_­order_­engine random number engine adaptor produces the same random numbers that are produced by some base engine e, but delivers them in a different sequence. The state xi of a shuffle_­order_­engine engine adaptor object x consists of the state ei of its base engine e, an additional value Y of the type delivered by e, and an additional sequence V of k values also of the type delivered by e. The size of the state is the size of e's state plus k+1.

The transition algorithm permutes the values produced by e. The state transition is performed as follows:

1. a)Calculate an integer j=k(Yemin)emaxemin+1 .

2. b)Set Y to Vj and then set Vj to e().

The generation algorithm yields the last value of Y produced while advancing e's state as described above.

```template<class Engine, size_t k>
class shuffle_order_engine {
public:
// types
using result_type = typename Engine::result_type;

// engine characteristics
static constexpr size_t table_size = k;
static constexpr result_type min() { return Engine::min(); }
static constexpr result_type max() { return Engine::max(); }

// constructors and seeding functions
shuffle_order_engine();
explicit shuffle_order_engine(const Engine& e);
explicit shuffle_order_engine(Engine&& e);
explicit shuffle_order_engine(result_type s);
template<class Sseq> explicit shuffle_order_engine(Sseq& q);
void seed();
void seed(result_type s);
template<class Sseq> void seed(Sseq& q);

// generating functions
result_type operator()();
void discard(unsigned long long z);

// property functions
const Engine& base() const noexcept { return e; };

private:
Engine e;           // exposition only
result_type V[k];   // exposition only
result_type Y;      // exposition only
};```

The following relation shall hold: 0 < k.

The textual representation consists of the textual representation of e, followed by the k values of V, followed by the value of Y.

In addition to its behavior pursuant to section [rand.req.adapt], each constructor that is not a copy constructor initializes V[0],,V[k-1] and Y, in that order, with values returned by successive invocations of e().

### 29.6.5 Engines and engine adaptors with predefined parameters [rand.predef]

```using minstd_rand0 = linear_congruential_engine<uint_fast32_t, 16807, 0, 2147483647>; ```

Required behavior: The 10000th consecutive invocation of a default-constructed object of type minstd_­rand0 shall produce the value 1043618065.

```using minstd_rand = linear_congruential_engine<uint_fast32_t, 48271, 0, 2147483647>; ```

Required behavior: The 10000th consecutive invocation of a default-constructed object of type minstd_­rand shall produce the value 399268537.

```using mt19937 = mersenne_twister_engine<uint_fast32_t, 32,624,397,31,0x9908b0df,11,0xffffffff,7,0x9d2c5680,15,0xefc60000,18,1812433253>; ```

Required behavior: The 10000th consecutive invocation of a default-constructed object of type mt19937 shall produce the value 4123659995.

```using mt19937_64 = mersenne_twister_engine<uint_fast64_t, 64,312,156,31,0xb5026f5aa96619e9,29, 0x5555555555555555,17, 0x71d67fffeda60000,37, 0xfff7eee000000000,43, 6364136223846793005>; ```

Required behavior: The 10000th consecutive invocation of a default-constructed object of type mt19937_­64 shall produce the value 9981545732273789042.

```using ranlux24_base = subtract_with_carry_engine<uint_fast32_t, 24, 10, 24>; ```

Required behavior: The 10000th consecutive invocation of a default-constructed object of type ranlux24_­base shall produce the value 7937952.

```using ranlux48_base = subtract_with_carry_engine<uint_fast64_t, 48, 5, 12>; ```

Required behavior: The 10000th consecutive invocation of a default-constructed object of type ranlux48_­base shall produce the value 61839128582725.

```using ranlux24 = discard_block_engine<ranlux24_base, 223, 23>; ```

Required behavior: The 10000th consecutive invocation of a default-constructed object of type ranlux24 shall produce the value 9901578.

```using ranlux48 = discard_block_engine<ranlux48_base, 389, 11>; ```

Required behavior: The 10000th consecutive invocation of a default-constructed object of type ranlux48 shall produce the value 249142670248501.

```using knuth_b = shuffle_order_engine<minstd_rand0,256>; ```

Required behavior: The 10000th consecutive invocation of a default-constructed object of type knuth_­b shall produce the value 1112339016.

```using default_random_engine = implementation-defined; ```

Remarks: The choice of engine type named by this typedef is implementation-defined. [Note: The implementation may select this type on the basis of performance, size, quality, or any combination of such factors, so as to provide at least acceptable engine behavior for relatively casual, inexpert, and/or lightweight use. Because different implementations may select different underlying engine types, code that uses this typedef need not generate identical sequences across implementations. end note]

### 29.6.6 Class random_­device[rand.device]

A random_­device uniform random bit generator produces nondeterministic random numbers.

If implementation limitations prevent generating nondeterministic random numbers, the implementation may employ a random number engine.

```class random_device {
public:
// types
using result_type = unsigned int;

// generator characteristics
static constexpr result_type min() { return numeric_limits<result_type>::min(); }
static constexpr result_type max() { return numeric_limits<result_type>::max(); }

// constructors
explicit random_device(const string& token = implementation-defined);

// generating functions
result_type operator()();

// property functions
double entropy() const noexcept;

// no copy functions
random_device(const random_device& ) = delete;
void operator=(const random_device& ) = delete;
};```

```explicit random_device(const string& token = implementation-defined); ```

Effects: Constructs a random_­device nondeterministic uniform random bit generator object. The semantics and default value of the token parameter are implementation-defined.271

Throws: A value of an implementation-defined type derived from exception if the random_­device could not be initialized.

```double entropy() const noexcept; ```

Returns: If the implementation employs a random number engine, returns 0.0. Otherwise, returns an entropy estimate272 for the random numbers returned by operator(), in the range min() to log2(max()+1).

```result_type operator()(); ```

Returns: A nondeterministic random value, uniformly distributed between min() and max(), inclusive. It is implementation-defined how these values are generated.

Throws: A value of an implementation-defined type derived from exception if a random number could not be obtained.

The parameter is intended to allow an implementation to differentiate between different sources of randomness.

If a device has n states whose respective probabilities are P0,,Pn1, the device entropy S is defined as
S=n1i=0PilogPi.

### 29.6.7 Utilities [rand.util]

#### 29.6.7.1 Class seed_­seq[rand.util.seedseq]

```class seed_seq {
public:
// types
using result_type = uint_least32_t;

// constructors
seed_seq();
template<class T>
seed_seq(initializer_list<T> il);
template<class InputIterator>
seed_seq(InputIterator begin, InputIterator end);

// generating functions
template<class RandomAccessIterator>
void generate(RandomAccessIterator begin, RandomAccessIterator end);

// property functions
size_t size() const noexcept;
template<class OutputIterator>
void param(OutputIterator dest) const;

// no copy functions
seed_seq(const seed_seq& ) = delete;
void operator=(const seed_seq& ) = delete;

private:
vector<result_type> v;   // exposition only
};```

```seed_seq(); ```

Effects: Constructs a seed_­seq object as if by default-constructing its member v.

Throws: Nothing.

```template<class T> seed_seq(initializer_list<T> il); ```

Requires: T shall be an integer type.

Effects: Same as seed_­seq(il.begin(), il.end()).

```template<class InputIterator> seed_seq(InputIterator begin, InputIterator end); ```

Requires: InputIterator shall satisfy the requirements of an input iterator type. Moreover, iterator_­traits<InputIterator>​::​value_­type shall denote an integer type.

Effects: Constructs a seed_­seq object by the following algorithm:

```for( InputIterator s = begin; s != end; ++s)
v.push_back((*s)mod232);```

```template<class RandomAccessIterator> void generate(RandomAccessIterator begin, RandomAccessIterator end); ```

Requires: RandomAccessIterator shall meet the requirements of a mutable random access iterator. Moreover, iterator_­traits<RandomAccessIterator>​::​value_­type shall denote an unsigned integer type capable of accommodating 32-bit quantities.

Effects: Does nothing if begin == end. Otherwise, with s=v.size() and n=endbegin, fills the supplied range [begin,end) according to the following algorithm in which each operation is to be carried out modulo 232, each indexing operator applied to begin is to be taken modulo n, and T(x) is defined as xxor(xrshift27):

1. a)By way of initialization, set each element of the range to the value 0x8b8b8b8b. Additionally, for use in subsequent steps, let p=(nt)/2 and let q=p+t, where

t=(n623) ? 11 : (n68) ? 7 : (n39) ? 5 : (n7) ? 3 : (n1)/2;

2. b)With m as the larger of s+1 and n, transform the elements of the range: iteratively for k=0,,m1, calculate values

r1=1664525T(begin[k]xorbegin[k+p]xorbegin[k1])r2=r1+sk=0kmodn+v[k1]0<kskmodns<k

and, in order, increment begin[k+p] by r1, increment begin[k+q] by r2, and set begin[k] to r2.

3. c)Transform the elements of the range again, beginning where the previous step ended: iteratively for k=m,,m+n1, calculate values

r3=1566083941T(begin[k]+begin[k+p]+begin[k1])r4=r3(kmodn)

and, in order, update begin[k+p] by xoring it with r3, update begin[k+q] by xoring it with r4, and set begin[k] to r4.

Throws: What and when RandomAccessIterator operations of begin and end throw.

```size_t size() const noexcept; ```

Returns: The number of 32-bit units that would be returned by a call to param().

Complexity: Constant time.

```template<class OutputIterator> void param(OutputIterator dest) const; ```

Requires: OutputIterator shall satisfy the requirements of an output iterator. Moreover, the expression *dest = rt shall be valid for a value rt of type result_­type.

Effects: Copies the sequence of prepared 32-bit units to the given destination, as if by executing the following statement:

`copy(v.begin(), v.end(), dest);`

Throws: What and when OutputIterator operations of dest throw.

#### 29.6.7.2 Function template generate_­canonical[rand.util.canonical]

Each function instantiated from the template described in this section [rand.util.canonical] maps the result of one or more invocations of a supplied uniform random bit generator g to one member of the specified RealType such that, if the values gi produced by g are uniformly distributed, the instantiation's results tj, 0tj<1, are distributed as uniformly as possible as specified below.

[Note: Obtaining a value in this way can be a useful step in the process of transforming a value generated by a uniform random bit generator into a value that can be delivered by a random number distribution. end note]

```template<class RealType, size_t bits, class URBG> RealType generate_canonical(URBG& g); ```

Complexity: Exactly k=max(1,b/log2R) invocations of g, where b273 is the lesser of numeric_­limits<RealType>​::​digits and bits, and R is the value of g.max()g.min()+1.

Effects: Invokes g() k times to obtain values g0,,gk1, respectively. Calculates a quantity

S=k1i=0(gig.min())Ri

using arithmetic of type RealType.

Returns: S/Rk.

Throws: What and when g throws.

b is introduced to avoid any attempt to produce more bits of randomness than can be held in RealType.

### 29.6.8 Random number distribution class templates [rand.dist]

#### 29.6.8.1 In general [rand.dist.general]

Each type instantiated from a class template specified in this section [rand.dist] satisfies the requirements of a random number distribution type.

Descriptions are provided in this section [rand.dist] only for distribution operations that are not described in [rand.req.dist] or for operations where there is additional semantic information. In particular, declarations for copy constructors, for copy assignment operators, for streaming operators, and for equality and inequality operators are not shown in the synopses.

The algorithms for producing each of the specified distributions are implementation-defined.

The value of each probability density function p(z) and of each discrete probability function P(zi) specified in this section is 0 everywhere outside its stated domain.

#### 29.6.8.2.1 Class template uniform_­int_­distribution[rand.dist.uni.int]

A uniform_­int_­distribution random number distribution produces random integers i, aib, distributed according to the constant discrete probability function

P(i|a,b)=1/(ba+1).

```template<class IntType = int>
class uniform_int_distribution {
public:
// types
using result_type = IntType;
using param_type  = unspecified;

// constructors and reset functions
explicit uniform_int_distribution(IntType a = 0, IntType b = numeric_limits<IntType>::max());
explicit uniform_int_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
result_type a() const;
result_type b() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit uniform_int_distribution(IntType a = 0, IntType b = numeric_limits<IntType>::max()); ```

Requires: ab.

Effects: Constructs a uniform_­int_­distribution object; a and b correspond to the respective parameters of the distribution.

```result_type a() const; ```

Returns: The value of the a parameter with which the object was constructed.

```result_type b() const; ```

Returns: The value of the b parameter with which the object was constructed.

#### 29.6.8.2.2 Class template uniform_­real_­distribution[rand.dist.uni.real]

A uniform_­real_­distribution random number distribution produces random numbers x, ax<b, distributed according to the constant probability density function

p(x|a,b)=1/(ba).

[Note: This implies that p(x|a,b) is undefined when a == b. end note]

```template<class RealType = double>
class uniform_real_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructors and reset functions
explicit uniform_real_distribution(RealType a = 0.0, RealType b = 1.0);
explicit uniform_real_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
result_type a() const;
result_type b() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit uniform_real_distribution(RealType a = 0.0, RealType b = 1.0); ```

Requires: ab and banumeric_limits<RealType>::max().

Effects: Constructs a uniform_­real_­distribution object; a and b correspond to the respective parameters of the distribution.

```result_type a() const; ```

Returns: The value of the a parameter with which the object was constructed.

```result_type b() const; ```

Returns: The value of the b parameter with which the object was constructed.

#### 29.6.8.3.1 Class bernoulli_­distribution[rand.dist.bern.bernoulli]

A bernoulli_­distribution random number distribution produces bool values b distributed according to the discrete probability function

P(b|p)={pifb=true1pifb=false.

```class bernoulli_distribution {
public:
// types
using result_type = bool;
using param_type  = unspecified;

// constructors and reset functions
explicit bernoulli_distribution(double p = 0.5);
explicit bernoulli_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
double p() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit bernoulli_distribution(double p = 0.5); ```

Requires: 0p1.

Effects: Constructs a bernoulli_­distribution object; p corresponds to the parameter of the distribution.

```double p() const; ```

Returns: The value of the p parameter with which the object was constructed.

#### 29.6.8.3.2 Class template binomial_­distribution[rand.dist.bern.bin]

A binomial_­distribution random number distribution produces integer values i0 distributed according to the discrete probability function

P(i|t,p)=(ti)pi(1p)ti.

```template<class IntType = int>
class binomial_distribution {
public:
// types
using result_type = IntType;
using param_type  = unspecified;

// constructors and reset functions
explicit binomial_distribution(IntType t = 1, double p = 0.5);
explicit binomial_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
IntType t() const;
double p() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit binomial_distribution(IntType t = 1, double p = 0.5); ```

Requires: 0p1 and 0t.

Effects: Constructs a binomial_­distribution object; t and p correspond to the respective parameters of the distribution.

```IntType t() const; ```

Returns: The value of the t parameter with which the object was constructed.

```double p() const; ```

Returns: The value of the p parameter with which the object was constructed.

#### 29.6.8.3.3 Class template geometric_­distribution[rand.dist.bern.geo]

A geometric_­distribution random number distribution produces integer values i0 distributed according to the discrete probability function

P(i|p)=p(1p)i.

```template<class IntType = int>
class geometric_distribution {
public:
// types
using result_type = IntType;
using param_type  = unspecified;

// constructors and reset functions
explicit geometric_distribution(double p = 0.5);
explicit geometric_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
double p() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit geometric_distribution(double p = 0.5); ```

Requires: 0<p<1.

Effects: Constructs a geometric_­distribution object; p corresponds to the parameter of the distribution.

```double p() const; ```

Returns: The value of the p parameter with which the object was constructed.

#### 29.6.8.3.4 Class template negative_­binomial_­distribution[rand.dist.bern.negbin]

A negative_­binomial_­distribution random number distribution produces random integers i0 distributed according to the discrete probability function

P(i|k,p)=(k+i1i)pk(1p)i.

[Note: This implies that P(i|k,p) is undefined when p == 1. end note]

```template<class IntType = int>
class negative_binomial_distribution {
public:
// types
using result_type = IntType;
using param_type  = unspecified;

// constructor and reset functions
explicit negative_binomial_distribution(IntType k = 1, double p = 0.5);
explicit negative_binomial_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
IntType k() const;
double p() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit negative_binomial_distribution(IntType k = 1, double p = 0.5); ```

Requires: 0<p1 and 0<k.

Effects: Constructs a negative_­binomial_­distribution object; k and p correspond to the respective parameters of the distribution.

```IntType k() const; ```

Returns: The value of the k parameter with which the object was constructed.

```double p() const; ```

Returns: The value of the p parameter with which the object was constructed.

#### 29.6.8.4.1 Class template poisson_­distribution[rand.dist.pois.poisson]

A poisson_­distribution random number distribution produces integer values i0 distributed according to the discrete probability function

P(i|μ)=eμμii!.

The distribution parameter μ is also known as this distribution's mean.

```template<class IntType = int>
class poisson_distribution
{
public:
// types
using result_type = IntType;
using param_type  = unspecified;

// constructors and reset functions
explicit poisson_distribution(double mean = 1.0);
explicit poisson_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
double mean() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit poisson_distribution(double mean = 1.0); ```

Requires: 0<mean.

Effects: Constructs a poisson_­distribution object; mean corresponds to the parameter of the distribution.

```double mean() const; ```

Returns: The value of the mean parameter with which the object was constructed.

#### 29.6.8.4.2 Class template exponential_­distribution[rand.dist.pois.exp]

An exponential_­distribution random number distribution produces random numbers x>0 distributed according to the probability density function

p(x|λ)=λeλx.

```template<class RealType = double>
class exponential_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructors and reset functions
explicit exponential_distribution(RealType lambda = 1.0);
explicit exponential_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
RealType lambda() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit exponential_distribution(RealType lambda = 1.0); ```

Requires: 0<lambda.

Effects: Constructs an exponential_­distribution object; lambda corresponds to the parameter of the distribution.

```RealType lambda() const; ```

Returns: The value of the lambda parameter with which the object was constructed.

#### 29.6.8.4.3 Class template gamma_­distribution[rand.dist.pois.gamma]

A gamma_­distribution random number distribution produces random numbers x>0 distributed according to the probability density function

p(x|α,β)=ex/ββαΓ(α)xα1.

```template<class RealType = double>
class gamma_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructors and reset functions
explicit gamma_distribution(RealType alpha = 1.0, RealType beta = 1.0);
explicit gamma_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
RealType alpha() const;
RealType beta() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit gamma_distribution(RealType alpha = 1.0, RealType beta = 1.0); ```

Requires: 0<alpha and 0<beta.

Effects: Constructs a gamma_­distribution object; alpha and beta correspond to the parameters of the distribution.

```RealType alpha() const; ```

Returns: The value of the alpha parameter with which the object was constructed.

```RealType beta() const; ```

Returns: The value of the beta parameter with which the object was constructed.

#### 29.6.8.4.4 Class template weibull_­distribution[rand.dist.pois.weibull]

A weibull_­distribution random number distribution produces random numbers x0 distributed according to the probability density function

p(x|a,b)=ab(xb)a1exp((xb)a).

```template<class RealType = double>
class weibull_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructor and reset functions
explicit weibull_distribution(RealType a = 1.0, RealType b = 1.0);
explicit weibull_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
RealType a() const;
RealType b() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit weibull_distribution(RealType a = 1.0, RealType b = 1.0); ```

Requires: 0<a and 0<b.

Effects: Constructs a weibull_­distribution object; a and b correspond to the respective parameters of the distribution.

```RealType a() const; ```

Returns: The value of the a parameter with which the object was constructed.

```RealType b() const; ```

Returns: The value of the b parameter with which the object was constructed.

#### 29.6.8.4.5 Class template extreme_­value_­distribution[rand.dist.pois.extreme]

An extreme_­value_­distribution random number distribution produces random numbers x distributed according to the probability density function274

p(x|a,b)=1bexp(axbexp(axb)).

```template<class RealType = double>
class extreme_value_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructor and reset functions
explicit extreme_value_distribution(RealType a = 0.0, RealType b = 1.0);
explicit extreme_value_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
RealType a() const;
RealType b() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit extreme_value_distribution(RealType a = 0.0, RealType b = 1.0); ```

Requires: 0<b.

Effects: Constructs an extreme_­value_­distribution object; a and b correspond to the respective parameters of the distribution.

```RealType a() const; ```

Returns: The value of the a parameter with which the object was constructed.

```RealType b() const; ```

Returns: The value of the b parameter with which the object was constructed.

The distribution corresponding to this probability density function is also known (with a possible change of variable) as the Gumbel Type I, the log-Weibull, or the Fisher-Tippett Type I distribution.

#### 29.6.8.5.1 Class template normal_­distribution[rand.dist.norm.normal]

A normal_­distribution random number distribution produces random numbers x distributed according to the probability density function

p(x|μ,σ)=1σ2πexp((xμ)22σ2).

The distribution parameters μ and σ are also known as this distribution's mean and standard deviation.

```template<class RealType = double>
class normal_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructors and reset functions
explicit normal_distribution(RealType mean = 0.0, RealType stddev = 1.0);
explicit normal_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
RealType mean() const;
RealType stddev() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit normal_distribution(RealType mean = 0.0, RealType stddev = 1.0); ```

Requires: 0<stddev.

Effects: Constructs a normal_­distribution object; mean and stddev correspond to the respective parameters of the distribution.

```RealType mean() const; ```

Returns: The value of the mean parameter with which the object was constructed.

```RealType stddev() const; ```

Returns: The value of the stddev parameter with which the object was constructed.

#### 29.6.8.5.2 Class template lognormal_­distribution[rand.dist.norm.lognormal]

A lognormal_­distribution random number distribution produces random numbers x>0 distributed according to the probability density function

p(x|m,s)=1sx2πexp((lnxm)22s2).

```template<class RealType = double>
class lognormal_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructor and reset functions
explicit lognormal_distribution(RealType m = 0.0, RealType s = 1.0);
explicit lognormal_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
RealType m() const;
RealType s() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit lognormal_distribution(RealType m = 0.0, RealType s = 1.0); ```

Requires: 0<s.

Effects: Constructs a lognormal_­distribution object; m and s correspond to the respective parameters of the distribution.

```RealType m() const; ```

Returns: The value of the m parameter with which the object was constructed.

```RealType s() const; ```

Returns: The value of the s parameter with which the object was constructed.

#### 29.6.8.5.3 Class template chi_­squared_­distribution[rand.dist.norm.chisq]

A chi_­squared_­distribution random number distribution produces random numbers x>0 distributed according to the probability density function

p(x|n)=x(n/2)1ex/2Γ(n/2)2n/2.

```template<class RealType = double>
class chi_squared_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructor and reset functions
explicit chi_squared_distribution(RealType n = 1);
explicit chi_squared_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
RealType n() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit chi_squared_distribution(RealType n = 1); ```

Requires: 0<n.

Effects: Constructs a chi_­squared_­distribution object; n corresponds to the parameter of the distribution.

```RealType n() const; ```

Returns: The value of the n parameter with which the object was constructed.

#### 29.6.8.5.4 Class template cauchy_­distribution[rand.dist.norm.cauchy]

A cauchy_­distribution random number distribution produces random numbers x distributed according to the probability density function

p(x|a,b)=(πb(1+(xab)2))1.

```template<class RealType = double>
class cauchy_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructor and reset functions
explicit cauchy_distribution(RealType a = 0.0, RealType b = 1.0);
explicit cauchy_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
RealType a() const;
RealType b() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit cauchy_distribution(RealType a = 0.0, RealType b = 1.0); ```

Requires: 0<b.

Effects: Constructs a cauchy_­distribution object; a and b correspond to the respective parameters of the distribution.

```RealType a() const; ```

Returns: The value of the a parameter with which the object was constructed.

```RealType b() const; ```

Returns: The value of the b parameter with which the object was constructed.

#### 29.6.8.5.5 Class template fisher_­f_­distribution[rand.dist.norm.f]

A fisher_­f_­distribution random number distribution produces random numbers x≥0 distributed according to the probability density function

p(x|m,n)=Γ((m+n)/2)Γ(m/2)Γ(n/2)(mn)m/2x(m/2)1(1+mxn)(m+n)/2.

```template<class RealType = double>
class fisher_f_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructor and reset functions
explicit fisher_f_distribution(RealType m = 1, RealType n = 1);
explicit fisher_f_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
RealType m() const;
RealType n() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit fisher_f_distribution(RealType m = 1, RealType n = 1); ```

Requires: 0<m and 0<n.

Effects: Constructs a fisher_­f_­distribution object; m and n correspond to the respective parameters of the distribution.

```RealType m() const; ```

Returns: The value of the m parameter with which the object was constructed.

```RealType n() const; ```

Returns: The value of the n parameter with which the object was constructed.

#### 29.6.8.5.6 Class template student_­t_­distribution[rand.dist.norm.t]

A student_­t_­distribution random number distribution produces random numbers x distributed according to the probability density function

p(x|n)=1nπΓ((n+1)/2)Γ(n/2)(1+x2n)(n+1)/2.

```template<class RealType = double>
class student_t_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructor and reset functions
explicit student_t_distribution(RealType n = 1);
explicit student_t_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
RealType n() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```explicit student_t_distribution(RealType n = 1); ```

Requires: 0<n.

Effects: Constructs a student_­t_­distribution object; n corresponds to the parameter of the distribution.

```RealType n() const; ```

Returns: The value of the n parameter with which the object was constructed.

#### 29.6.8.6.1 Class template discrete_­distribution[rand.dist.samp.discrete]

A discrete_­distribution random number distribution produces random integers i, 0i<n, distributed according to the discrete probability function

P(i|p0,,pn1)=pi.

Unless specified otherwise, the distribution parameters are calculated as: pk=wk/S for k=0,,n1 , in which the values wk, commonly known as the weights, shall be non-negative, non-NaN, and non-infinity. Moreover, the following relation shall hold: 0<S=w0++wn1.

```template<class IntType = int>
class discrete_distribution {
public:
// types
using result_type = IntType;
using param_type  = unspecified;

// constructor and reset functions
discrete_distribution();
template<class InputIterator>
discrete_distribution(InputIterator firstW, InputIterator lastW);
discrete_distribution(initializer_list<double> wl);
template<class UnaryOperation>
discrete_distribution(size_t nw, double xmin, double xmax, UnaryOperation fw);
explicit discrete_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
vector<double> probabilities() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```discrete_distribution(); ```

Effects: Constructs a discrete_­distribution object with n=1 and p0=1. [Note: Such an object will always deliver the value 0. end note]

```template<class InputIterator> discrete_distribution(InputIterator firstW, InputIterator lastW); ```

Requires: InputIterator shall satisfy the requirements of an input iterator. Moreover, iterator_­traits<InputIterator>​::​value_­type shall denote a type that is convertible to double. If firstW == lastW, let n=1 and w0=1. Otherwise, [firstW,lastW) shall form a sequence w of length n>0.

Effects: Constructs a discrete_­distribution object with probabilities given by the formula above.

```discrete_distribution(initializer_list<double> wl); ```

Effects: Same as discrete_­distribution(wl.begin(), wl.end()).

```template<class UnaryOperation> discrete_distribution(size_t nw, double xmin, double xmax, UnaryOperation fw); ```

Requires: Each instance of type UnaryOperation shall be a function object whose return type shall be convertible to double. Moreover, double shall be convertible to the type of UnaryOperation's sole parameter. If nw=0, let n=1, otherwise let n=nw. The relation 0<δ=(xmaxxmin)/n shall hold.

Effects: Constructs a discrete_­distribution object with probabilities given by the formula above, using the following values: If nw=0, let w0=1. Otherwise, let wk=fw(xmin+kδ+δ/2) for k=0,,n1.

Complexity: The number of invocations of fw shall not exceed n.

```vector<double> probabilities() const; ```

Returns: A vector<double> whose size member returns n and whose operator[] member returns pk when invoked with argument k for k=0,,n1.

#### 29.6.8.6.2 Class template piecewise_­constant_­distribution[rand.dist.samp.pconst]

A piecewise_­constant_­distribution random number distribution produces random numbers x, b0x<bn, uniformly distributed over each subinterval [bi,bi+1) according to the probability density function

p(x|b0,,bn,ρ0,,ρn1)=ρi, for bix<bi+1.

The n+1 distribution parameters bi, also known as this distribution's interval boundaries, shall satisfy the relation bi<bi+1 for i=0,,n1. Unless specified otherwise, the remaining n distribution parameters are calculated as:

ρk=wkS(bk+1bk) for k=0,,n1,

in which the values wk, commonly known as the weights, shall be non-negative, non-NaN, and non-infinity. Moreover, the following relation shall hold: 0<S=w0++wn1.

```template<class RealType = double>
class piecewise_constant_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructor and reset functions
piecewise_constant_distribution();
template<class InputIteratorB, class InputIteratorW>
piecewise_constant_distribution(InputIteratorB firstB, InputIteratorB lastB,
InputIteratorW firstW);
template<class UnaryOperation>
piecewise_constant_distribution(initializer_list<RealType> bl, UnaryOperation fw);
template<class UnaryOperation>
piecewise_constant_distribution(size_t nw, RealType xmin, RealType xmax,
UnaryOperation fw);
explicit piecewise_constant_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
vector<result_type> intervals() const;
vector<result_type> densities() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```piecewise_constant_distribution(); ```

Effects: Constructs a piecewise_­constant_­distribution object with n=1, ρ0=1, b0=0, and b1=1.

```template<class InputIteratorB, class InputIteratorW> piecewise_constant_distribution(InputIteratorB firstB, InputIteratorB lastB, InputIteratorW firstW); ```

Requires: InputIteratorB and InputIteratorW shall each satisfy the requirements of an input iterator type. Moreover, iterator_­traits<InputIteratorB>​::​value_­type and iterator_­traits<InputIteratorW>​::​value_­type shall each denote a type that is convertible to double. If firstB == lastB or ++firstB == lastB, let n=1, w0=1, b0=0, and b1=1. Otherwise, [firstB,lastB) shall form a sequence b of length n+1, the length of the sequence w starting from firstW shall be at least n, and any wk for kn shall be ignored by the distribution.

Effects: Constructs a piecewise_­constant_­distribution object with parameters as specified above.

```template<class UnaryOperation> piecewise_constant_distribution(initializer_list<RealType> bl, UnaryOperation fw); ```

Requires: Each instance of type UnaryOperation shall be a function object whose return type shall be convertible to double. Moreover, double shall be convertible to the type of UnaryOperation's sole parameter.

Effects: Constructs a piecewise_­constant_­distribution object with parameters taken or calculated from the following values: If bl.size()<2, let n=1, w0=1, b0=0, and b1=1. Otherwise, let [bl.begin(),bl.end()) form a sequence b0,,bn, and let wk=fw((bk+1+bk)/2) for k=0,,n1.

Complexity: The number of invocations of fw shall not exceed n.

```template<class UnaryOperation> piecewise_constant_distribution(size_t nw, RealType xmin, RealType xmax, UnaryOperation fw); ```

Requires: Each instance of type UnaryOperation shall be a function object whose return type shall be convertible to double. Moreover, double shall be convertible to the type of UnaryOperation's sole parameter. If nw=0, let n=1, otherwise let n=nw. The relation 0<δ=(xmaxxmin)/n shall hold.

Effects: Constructs a piecewise_­constant_­distribution object with parameters taken or calculated from the following values: Let bk=xmin+kδ for k=0,,n, and wk=fw(bk+δ/2) for k=0,,n1.

Complexity: The number of invocations of fw shall not exceed n.

```vector<result_type> intervals() const; ```

Returns: A vector<result_­type> whose size member returns n+1 and whose operator[] member returns bk when invoked with argument k for k=0,,n.

```vector<result_type> densities() const; ```

Returns: A vector<result_­type> whose size member returns n and whose operator[] member returns ρk when invoked with argument k for k=0,,n1.

#### 29.6.8.6.3 Class template piecewise_­linear_­distribution[rand.dist.samp.plinear]

A piecewise_­linear_­distribution random number distribution produces random numbers x, b0x<bn, distributed over each subinterval [bi,bi+1) according to the probability density function

p(x|b0,,bn,ρ0,,ρn)=ρibi+1xbi+1bi+ρi+1xbibi+1bi, for bix<bi+1.

The n+1 distribution parameters bi, also known as this distribution's interval boundaries, shall satisfy the relation bi<bi+1 for i=0,,n1. Unless specified otherwise, the remaining n+1 distribution parameters are calculated as ρk=wk/S for k=0,,n, in which the values wk, commonly known as the weights at boundaries, shall be non-negative, non-NaN, and non-infinity. Moreover, the following relation shall hold:

0<S=12n1k=0(wk+wk+1)(bk+1bk).

```template<class RealType = double>
class piecewise_linear_distribution {
public:
// types
using result_type = RealType;
using param_type  = unspecified;

// constructor and reset functions
piecewise_linear_distribution();
template<class InputIteratorB, class InputIteratorW>
piecewise_linear_distribution(InputIteratorB firstB, InputIteratorB lastB,
InputIteratorW firstW);
template<class UnaryOperation>
piecewise_linear_distribution(initializer_list<RealType> bl, UnaryOperation fw);
template<class UnaryOperation>
piecewise_linear_distribution(size_t nw, RealType xmin, RealType xmax, UnaryOperation fw);
explicit piecewise_linear_distribution(const param_type& parm);
void reset();

// generating functions
template<class URBG>
result_type operator()(URBG& g);
template<class URBG>
result_type operator()(URBG& g, const param_type& parm);

// property functions
vector<result_type> intervals() const;
vector<result_type> densities() const;
param_type param() const;
void param(const param_type& parm);
result_type min() const;
result_type max() const;
};```

```piecewise_linear_distribution(); ```

Effects: Constructs a piecewise_­linear_­distribution object with n=1, ρ0=ρ1=1, b0=0, and b1=1.

```template<class InputIteratorB, class InputIteratorW> piecewise_linear_distribution(InputIteratorB firstB, InputIteratorB lastB, InputIteratorW firstW); ```

Requires: InputIteratorB and InputIteratorW shall each satisfy the requirements of an input iterator type. Moreover, iterator_­traits<InputIteratorB>​::​value_­type and iterator_­traits<InputIteratorW>​::​value_­type shall each denote a type that is convertible to double. If firstB == lastB or ++firstB == lastB, let n=1, ρ0=ρ1=1, b0=0, and b1=1. Otherwise, [firstB,lastB) shall form a sequence b of length n+1, the length of the sequence w starting from firstW shall be at least n+1, and any wk for kn+1 shall be ignored by the distribution.

Effects: Constructs a piecewise_­linear_­distribution object with parameters as specified above.

```template<class UnaryOperation> piecewise_linear_distribution(initializer_list<RealType> bl, UnaryOperation fw); ```

Requires: Each instance of type UnaryOperation shall be a function object whose return type shall be convertible to double. Moreover, double shall be convertible to the type of UnaryOperation's sole parameter.

Effects: Constructs a piecewise_­linear_­distribution object with parameters taken or calculated from the following values: If bl.size()<2, let n=1, ρ0=ρ1=1, b0=0, and b1=1. Otherwise, let [bl.begin(),bl.end()) form a sequence b0,,bn, and let wk=fw(bk) for k=0,,n.

Complexity: The number of invocations of fw shall not exceed n+1.

```template<class UnaryOperation> piecewise_linear_distribution(size_t nw, RealType xmin, RealType xmax, UnaryOperation fw); ```

Requires: Each instance of type UnaryOperation shall be a function object whose return type shall be convertible to double. Moreover, double shall be convertible to the type of UnaryOperation's sole parameter. If nw=0, let n=1, otherwise let n=nw. The relation 0<δ=(xmaxxmin)/n shall hold.

Effects: Constructs a piecewise_­linear_­distribution object with parameters taken or calculated from the following values: Let bk=xmin+kδ for k=0,,n, and wk=fw(bk) for k=0,,n.

Complexity: The number of invocations of fw shall not exceed n+1.

```vector<result_type> intervals() const; ```

Returns: A vector<result_­type> whose size member returns n+1 and whose operator[] member returns bk when invoked with argument k for k=0,,n.

```vector<result_type> densities() const; ```

Returns: A vector<result_­type> whose size member returns n and whose operator[] member returns ρk when invoked with argument k for k=0,,n.

### 29.6.9 Low-quality random number generation [c.math.rand]

[Note: The header <cstdlib> declares the functions described in this subclause. end note]

```int rand(); void srand(unsigned int seed); ```

Effects: The rand and srand functions have the semantics specified in the C standard library.

Remarks: The implementation may specify that particular library functions may call rand. It is implementation-defined whether the rand function may introduce data races ([res.on.data.races]). [Note: The other random number generation facilities in this International Standard ([rand]) are often preferable to rand, because rand's underlying algorithm is unspecified. Use of rand therefore continues to be non-portable, with unpredictable and oft-questionable quality and performance. end note]