# std::rand

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< cpp‎ | numeric‎ | random

Defined in header `<cstdlib>` |  |
int rand(); |  |

Returns a pseudo-random integral value from the range `[`​0​`,` RAND_MAX`]`.

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

Each time `rand()` is seeded with std::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. 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 <ctime>
    #include <iostream>

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

        // roll 6-sided dice 20 times
        for (int n = 0; n != 20; ++n)
        {
            int x = 7;
            while (x > 6)
                x = 1 + std::rand() / ((RAND_MAX + 1u) / 6); // Note: 1 + rand() % 6 is biased
            std::cout << x << ' ';
        }
    }

Possible output:

    Random value on [0, 2147483647]: 726295113
    6 3 6 2 6 5 6 3 1 1 1 6 6 6 4 1 3 6 4 2

### 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)

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