std:: srand

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

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

#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';
}

Random value on [0, 2147483647]: 1373858591

See also