NAME¶

std::srand - std::srand

Synopsis¶

Defined in header <cstdlib>
void srand( unsigned seed );

Seeds the pseudo-random number generator used by std::rand() with the value seed.

If std::rand() is used before any calls to srand(), std::rand() behaves as if it was
seeded with srand(1).

Each time std::rand() is seeded with the same seed, it must produce the same
sequence of values.

srand() is not guaranteed to be thread-safe.

Parameters¶

seed - the seed value

Return value¶

(none)

Notes¶

Generally speaking, the pseudo-random number generator should only be seeded once,
before any calls to rand(), at the start of the program. It should not be repeatedly
seeded, or reseeded every time you wish to generate a new batch of pseudo-random
numbers.

Standard practice is to use the result of a call to std::time(0) as the seed.
However, std::time returns a std::time_t value, and std::time_t is not guaranteed to
be an integral type. In practice, though, every major implementation defines
std::time_t to be an integral type, and this is also what POSIX requires.

Example¶

// Run this code

#include <cstdlib>
#include <ctime>
#include <iostream>

int main()
{
std::srand(std::time(0)); // use current time as seed for random generator
std::cout << "Random value on [0, " << RAND_MAX << "]: " << std::rand() << '\n';
}

Possible output:¶

Random value on [0, 2147483647]: 1373858591

See also¶

rand generates a pseudo-random number
(function)
RAND_MAX maximum possible value generated by std::rand
(macro constant)
reseed reseeds the per-thread random engine
(function)
C documentation for
srand