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C++ Cookbook
book

C++ Cookbook

by D. Ryan Stephens, Christopher Diggins, Jonathan Turkanis, Jeff Cogswell
November 2005
Beginner to intermediate content levelBeginner to intermediate
594 pages
16h 23m
English
O'Reilly Media, Inc.
Content preview from 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 ...

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Publisher Resources

ISBN: 0596007612Supplemental ContentErrata Page