std::rand

 

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Pseudo-random number generation

Uniform random bit generators   uniform_random_bit_generator (C++20) Random number engines linear_congruential_engine (C++11) mersenne_twister_engine (C++11) subtract_with_carry_engine (C++11) philox_engine (C++26) Random number engine adaptors discard_block_engine (C++11) independent_bits_engine (C++11) shuffle_order_engine (C++11) Predefined random number generators Non-deterministic random numbers random_device (C++11) Utilities generate_canonical (C++11) seed_seq (C++11) Random number algorithms ranges::generate_random (C++26) C random library rand    srand    RAND_MAX

Random number distributions Uniform distributions uniform_int_distribution (C++11) uniform_real_distribution (C++11) generate_canonical (C++11) Bernoulli distributions bernoulli_distribution (C++11) binomial_distribution (C++11) negative_binomial_distribution (C++11) geometric_distribution (C++11) Poisson distributions poisson_distribution (C++11) exponential_distribution (C++11) gamma_distribution (C++11) weibull_distribution (C++11) extreme_value_distribution (C++11) Normal distributions normal_distribution (C++11) lognormal_distribution (C++11) chi_squared_distribution (C++11) cauchy_distribution (C++11) fisher_f_distribution (C++11) student_t_distribution (C++11) Sampling distributions discrete_distribution (C++11) piecewise_constant_distribution (C++11) piecewise_linear_distribution (C++11)

 

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_value = std::rand();
    std::cout << "Random value on [0, " << RAND_MAX << "]: " << random_value << '\n';
 
    for (const int times = 8; const int sides : {2, 4, 6, 8})
    {
        std::cout << "Roll " << sides << "-sided dice " << times << " times: ";
        for (int n = 0; n != times; ++n)
        {
            int x = sides + 1;
            while (x > sides) 
                x = 1 + std::rand() / ((RAND_MAX + 1u) / sides); 
                    // Note: 1 + rand() % sides is biased
            std::cout << x << ' ';
        }
        std::cout << '\n';
    }
}

Possible output:

Random value on [0, 2147483647]: 948298199
Roll 2-sided dice 8 times: 2 2 1 2 1 1 2 2 
Roll 4-sided dice 8 times: 1 3 4 2 1 3 3 1 
Roll 6-sided dice 8 times: 3 2 1 6 6 4 4 2 
Roll 8-sided dice 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