Issues with c++ 11 mersenne_twister_engine class

Question

I have been trying to use the c++ 11 mersenne_twister_engine class( http://www.cplusplus.com/reference/random/mersenne_twister_engine/) to generate numbers in the interval [0,1] however I am continually getting 0 on every number.

Here is my code snippet

unsigned seed1 = std::chrono::system_clock::now().time_since_epoch().count();   

std::mt19937 generator(seed1); 
for (int i=0; i < WIDTH * HEIGHT; i++) //GENERATES THE MATRIX OF CONNECTIVITY
{

    double random = generator()/generator.max();
    if ( random  > VERT_CONNECTIVITY )
    vert_con[i] = 0;
    else vert_con[i] = 1;


}

generator() and generator.max() seem to be working... It is just random which gives me 0!

Thanks!

Answer 1

In C++, / on integer numbers results in an integer number. For example 11 / 2 results in 5, not 5.5. In the same way a / b is always zero if a < b.

Your problem is here:

generator()/generator.max()

generator() and generator.max() both return integers and of course, generator.max() >= generator(), so the result is zero (unless you are very lucky to get the max number in which case the result would be one).

To fix this, you can simply cast it:

(double)generator()/generator.max()