Generate random numbers uniformly over an entire range

c++, random

Solution

Why `rand` is a bad idea

Most of the answers you got here make use of the `rand` function and the modulus operator. That method may not generate numbers uniformly (it depends on the range and the value of `RAND_MAX`), and is therefore discouraged.

C++11 and generation over a range

With C++11 multiple other options have risen. One of which fits your requirements, for generating a random number in a range, pretty nicely: `std::uniform_int_distribution`. Here's an example:

#include <iostream>
#include <random>
int main()
{    
    const int range_from  = 0;
    const int range_to    = 1000;
    std::random_device                  rand_dev;
    std::mt19937                        generator(rand_dev());
    std::uniform_int_distribution<int>  distr(range_from, range_to);

    std::cout << distr(generator) << '\n';
}

Try it online on Godbolt

And here's the running example.

Template function may help some:

template<typename T>
T random(T range_from, T range_to) {
    std::random_device                  rand_dev;
    std::mt19937                        generator(rand_dev());
    std::uniform_int_distribution<T>    distr(range_from, range_to);
    return distr(generator);
}

Other random generators

The `<random>` header offers innumerable other random number generators with different kind of distributions including Bernoulli, Poisson and normal.

How can I shuffle a container?

The standard provides `std::shuffle`, which can be used as follows:

#include <iostream>
#include <random>
#include <vector>
int main()
{    
    std::vector<int> vec = {4, 8, 15, 16, 23, 42};
 
    std::random_device random_dev;
    std::mt19937       generator(random_dev());
 
    std::shuffle(vec.begin(), vec.end(), generator);
    std::for_each(vec.begin(), vec.end(), [](auto i){std::cout << i << '\n';});
}

Try it online on Godbolt

The algorithm will reorder the elements randomly, with a linear complexity.

Boost.Random

Another alternative, in case you don't have access to a C++11+ compiler, is to use Boost.Random. Its interface is very similar to the C++11 one.

Problem

I need to generate random numbers within a specified interval, [max;min]. Also, the random numbers should be uniformly distributed over the interval, not located to a particular point. Currenly I am generating as: ``` for(int i=0; i<6; i++) { DWORD random = rand()%(max-min+1) + min; } ``` From my tests, random numbers are generated around one point only. ``` Example min = 3604607; max = 7654607; ``` Random numbers generated: ``` 3631594 3609293 3630000 3628441 3636376 3621404 ``` From answers below: OK, RAND_MAX is 32767. I am on C++ Windows platform. Is there any other method to generate random numbers with a uniform distribution?

Original source

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