# 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.

### 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

The function `bounded_rand()` below is an adapted version of Debiased Modulo (Once).

Run this code

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

    unsigned bounded_rand(unsigned range)
    {
        for (unsigned x, r;;)
            if (x = rand(), r = x % range, x - r <= -range)
                return r;
    }

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

        for (const unsigned sides : {2, 4, 6, 8})
        {
            std::cout << "Roll " << sides << "-sided die 8 times: ";
            for (int n = 8; n; --n)
                std::cout << 1 + bounded_rand(sides) << ' ';

            std::cout << '\n';
        }
    }

Possible output:

    Random value on [0, 2147483647]: 948298199
    Roll 2-sided die 8 times: 2 2 1 2 1 1 2 2
    Roll 4-sided die 8 times: 1 3 4 2 1 3 3 1
    Roll 6-sided die 8 times: 3 2 1 6 6 4 4 2
    Roll 8-sided die 8 times: 4 5 6 6 3 6 1 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)
C documentation for rand

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