c++ rand() insignificant change

February 8, 2024

I ran into an issue when trying to generate random numbers where it outputs an ascending sequence of numbers with increments of 3 every second. I am using mingw g++ 6.3.0 with the following compiler flags: -std=c++11 -Wall.

As an example, after running the following code and running the executable multiple times it provides the following output below:

main.cpp:

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

int main() {
  std::srand(std::time(0));

  int rnd = std::rand();
  std::cout << rnd << '\n';

  return 0;
}

Console output:

C:\...>make
g++ main.cpp -std=c++11 -Wall    -o a.exe

C:\...>a
21154

C:\...>a
21157

C:\...>a
21157

C:\...>a
21157

C:\...>a
21160

C:\...>a
21160

C:\...>a
21163

C:\...>a
21163

>Solution :

Well, rand() will return the same sequence of numbers for a given seed, which is why you see the change every second (you’re seeding it with the time in seconds).

As for incrementing by 3 each time, std::rand is not guaranteed to produce sequences of any particular quality, and yours appears particularly not-that-great (not uncommon for rand). There are better quality generators in <random> (see docs here).