C++ Cookbook

11.6. Generating Random Numbers

Problem

You want to generate some random floating-point numbers in the interval of [0.0, 1.0) with a uniform distribution.

Solution

The C++ standard provides the C runtime function rand in the <cstdlib> header that returns a random number in the range of 0 to RAND_MAX inclusive. The RAND_MAX macro represents the highest value returnable by the rand function. A demonstration of using rand to generate random floating-point numbers is shown in Example 11-11.

Example 11-11. Generating random numbers using rand

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

using namespace std; 

double doubleRand() {
  return double(rand()) / (double(RAND_MAX) + 1.0);
}

int main() {
  srand(static_cast<unsigned int>(clock()));
  cout << "expect 5 numbers within the interval [0.0, 1.0)" << endl;
  for (int i=0; i < 5; i++) {
    cout << doubleRand() << "\n";
  }
  cout << endl;  
}

The program in Example 11-11 should produce output similar to:

expect 5 numbers within the interval [0.0, 1.0)
0.010437
0.740997
0.34906
0.369293
0.544373

Discussion

To be precise, random number generation functions, including rand, return pseudo-random numbers as opposed to truly random numbers, so whenever I say random, I actually mean pseudo-random.

Before using the rand function you need to seed (i.e., initialize) the random number generator with a call to srand. This assures that subsequent calls to rand won’t produce the same sequence of numbers each time the program is run. The simplest way to seed the ...