Defined in header <cstdlib>
int rand();

Returns a pseudo-random integral value between ​0​ and RAND_MAX (0 and RAND_MAX included).

std::srand() seeds the pseudo-random number generator used by rand(). If rand() is used before any calls to srand(), rand() behaves as if it was seeded with srand(1).

Each time rand() is seeded with srand(), it must produce the same sequence of values on successive calls. Other functions in the standard library may call rand, it is implementation-defined which functions do so.

It is implementation-defined whether rand() is thread-safe.



Return value

Pseudo-random integral value between ​0​ and RAND_MAX.


There are no guarantees as to the quality of the random sequence produced. In the past, some implementations of rand() have had serious shortcomings in the randomness, distribution and period of the sequence produced (in one well-known example, the low-order bit simply alternated between 1 and 0 between calls).

rand() is not recommended for serious random-number generation needs, like cryptography. It is recommended to use C++11's random number generation facilities to replace rand(). (since C++11).


#include <cstdlib>
#include <iostream>
#include <ctime>
int main() 
    std::srand(std::time(0)); // use current time as seed for random generator
    int random_variable = std::rand();
    std::cout << "Random value on [0 " << RAND_MAX << "]: " 
              << random_variable << '\n';

Possible output:

Random value on [0 2147483647]: 1373858591

See also

produces integer values evenly distributed across a range
(class template)
seeds pseudo-random number generator
maximum possible value generated by std::rand
(macro constant)
generates a random integer in the specified range
(function template)
C documentation for rand

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