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std::mersenne_twister_engine<UIntType,w,n,m,r,a,u,d,s,b,t,c,l,f>::mersenne_twister_engine

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Pseudo-random number generation
 
std::mersenne_twister_engine
Member functions
mersenne_twister_engine::mersenne_twister_engine
Generation
Characteristics
Non-member functions
(C++11)(C++11)(until C++20)
(C++11)(C++11)
 
mersenne_twister_engine() : mersenne_twister_engine(default_seed) {}
 (1) (since C++11)
explicit mersenne_twister_engine( result_type value );
 (2) (since C++11)
template< class SeedSeq >
explicit mersenne_twister_engine( SeedSeq& seq );
 (3) (since C++11)
mersenne_twister_engine( const mersenne_twister_engine& other );
 (4) (since C++11)
(implicitly declared)

Constructs the pseudo-random number engine.

1) The default constructor.
  • If the default-constructed engine is of type std::mt19937, the 10000th consecutive invocation of it produces the value 4123659995.
  • If the default-constructed engine is of type std::mt19937_64, the 10000th consecutive invocation of it produces the value 9981545732273789042.
2) Constructs the engine with a seed value value. Given 2w
 as p, the engine's initial state is determined as follows:
  1. Sets X
    -n     to value % p.
  2. For each integer i in [i - n, -1], sets X
    i     to [f·(X
    i-1     xor (X
    i-1     rshift (w-2)))+i mod n] mod p    , where xor and rshift stand for built-in bitwise XOR and bitwise right-shift respectively.
3) Constructs the engine with a seed sequence seq. Given std::size_t(w / 32) + 1 as k, the engine's initial state is determined as follows:
  1. Creates an invented array object a of length n * k.
  2. Calls seq.generate(a + 0, a + n * k).
  3. For each integer i in [-n, -1], sets X
    i     to (∑k-1
    j=0     a
    k(i+n)+j    ·232j
    ) mod 2w
    .
  4. If the most significant w − r bits of X
    -n     are zero, and if each of the other resulting X
    i     is ​0​, changes X
    -n     to 2w-1
    .
This overload participates in overload resolution only if SeedSeq meets the requirements of SeedSequence.
4) The copy constructor. Upon construction, *this == other is true.

Contents

[edit] Parameters

value - seed value to use in the initialization of the internal state
seq - seed sequence to use in the initialization of the internal state

[edit] Complexity

1,2) O(n).
3) Same as the complexity of the seq.generate call.
4) O(n).

[edit] Example

This section is incomplete
Reason: demos for overloads (2-4) required

Run this code
#include <cassert>
#include <random>
 
int main()
{
    std::mt19937 gen32; // overload (1)
    std::mt19937_64 gen64; // overload (1)
    gen32.discard(10000 - 1);
    gen64.discard(10000 - 1);
    assert(gen32() == 4123659995);
    assert(gen64() == 9981545732273789042ull);
}

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
P0935R0 C++11 the default constructor was explicit made implicit

[edit] See also

(C++11)
 sets the current state of the engine
(public member function) [edit]
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