True random integer between 20 and 30 in C++

Question

Forgive me if it's obvious, but I'm having some trouble with the idea of generating a truly random integer in range, I've just started using C++. My current code:

int start = rand() % 30 + 20;

always results in 31, as this is a "psuedo-random" integer. It is also above 30! Can true random numbers be achieved in this way, and also, how can I actually generate a number in range?

-Jai

http://stackoverflow.com/questions/1108780/why-do-i-always-get-the-same-sequence-of-random-numbers-with-rand — πάντα ῥεῖ, Nov 09 '14 at 14:50
You cannot get "a true random number" this way. To get a true random number you need a true random event. For example, elementary particle decay, white noise etc... — lapk, Nov 09 '14 at 14:52

deviantfan · Answer 1 · 2014-11-09T15:12:29.260

4

A normal computer can only generate pseudo-random numbers,
but that does not mean that the generated numbers will be the same every time.
Your problem is that you forgot srand, that rand isn´t that good either way,
and that the formula should be (rand() % 10) + 20.

For better generators than rand, see eg. Mersenne Twister (include <random>).
For "true" random numbers, you need special hardware.

edited Nov 09 '14 at 15:12

answered Nov 09 '14 at 14:53

deviantfan

11,268
3
32
49

(Side note: Capturing the input from a microphone, if done right, is a cheap "real" random source. But even a microphone isn´t built-in in every computer, and one could argue that the environment noise is predictable to a certain degree too). – deviantfan Nov 09 '14 at 15:02
There are much easier ways of getting randomness than that. Sites on the internet derive random numbers from natural phenomena that are random (like lightning strikes world wide). http://www.random.org/ – Martin York Nov 09 '14 at 15:33
It´s easier to get numbers from random.org in a C++ program at runtime than calling a method in ``? And in the case of security reasons, no internet (and no Mersenne twister) is good enough, even if the internet has perfect randomness. – deviantfan Nov 09 '14 at 15:39
Absolutely; Use the standard functions. I was just comparing your example of getting true randomness from a microphone. Which seems overly difficult (and easy to compromise) compared to other easier ways. – Martin York Nov 09 '14 at 15:47

score 1 · Answer 2 · answered Nov 09 '14 at 14:58

In C++, you might do something like this:

#include <iostream>
#include <random>
#include <chrono>

class Rand_int {
public:
  Rand_int(int low, int high)
    : re(std::chrono::system_clock::now().time_since_epoch().count()),
      dist{low,high} { }
  int operator()() { return dist(re); }
private:
  std::default_random_engine re;
  std::uniform_int_distribution<> dist;
};


int main() {
  Rand_int rnd {20, 30};
  for (int i=0; i<10; i++)
    std::cout << rnd() << '\n';
  return 0;
}

True random integer between 20 and 30 in C++

2 Answers2