# std::rand

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< cpp‎ | numeric‎ | random

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.

### Parameters

(none)

### Return value

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

### Notes

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)

### Example

Run this code

    #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

uniform_int_distribution (C++11) | produces integer values evenly distributed across a range (class template)
srand | seeds pseudo-random number generator (function)
RAND_MAX | maximum possible value generated by std::rand (macro constant)
randint | generates a random integer in the specified range (function template)
C documentation for rand

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