This little software project illustrates techniques for working with random numbers in C++. The project consists of 3 individual files:

1. main.cpp, the entry point for the application,
2. RandomNumber.cpp, the demonstration code,
3. and RandomNumber.h, the prototype for the RandomNumber method.
   

The target compiler is Visual Studio C++, but the code should be functional on any mainstream C++ compiler as well. GCC would be another possibility if you don't have Visual Studio.

There is no Random class, since C++ is a hybrid OOP language and all that legacy stuff must be supported. This code can be easily wrapped into a class if necessary.

To create the tutorial in Visual Studio, create each of the three files, below, then create a Visual Studio C++ project and add the three files to the project. The "Empty Project" type works well for this example. An Empty Project has no default modules added to it.

/******************************************************************************
 * Fun with Random Numbers                                  RandomNumbers.cpp *
 *                                                                            *
 * Author: nicomp                                                             *
 * Abstract: Random Number stuff.                                             *
 *                                                                            *
 *                                                                            *
 *  Call randomize() to initialize the random number generator.               *
 *  Call srand(seed) to initialize the generator with a specific seed value.  *
 *  "Random" number sequences can be repeated by using the same seed.         *
 *                                                                            *
 * Revision History:                                                          *
 ******************************************************************************/

#include <iostream>
#include <stdlib.h>            // random(), srand() function prototypes
#include <time.h>              // time() function prototype
#include "RandomNumbers.h"

using namespace std;           // Not the default in some compilers

void RandomNumberDemo()
{
//  Using srand() allows repeatable random number sequences...
    srand(42);     // randomize using a specific seed. Any seed will work for this demo.

    cout << "\n integer random numbers...";
//  random returns a number between 0 and RAND_MAX in stdlib.h
    for (int i = 0; i < 5; i++)    cout << "\n => " << rand();

//  ====================================================================
    cout << "\n 3 decimal place floating point random numbers between 0 and 1...";
    srand(unsigned int(time(0)));                // Get a little more random
    for (int k = 0; k < 5; k++)    {
        int lottoNo;                // variable is local to this block
        float result;
        lottoNo= rand();
//      Generate a random # between 1001 and 1999 where the ones digit is not zero
        while (1)
        {
//            cout << "\n<<" << lottoNo << ">>";
            if ((lottoNo>= 1001) && (lottoNo<= 1999))    // between 1001 and 1999
                if ((lottoNo / 10) * 10 != lottoNo)        // does it have a non-zero last digit?
                    break;
            lottoNo = rand();
        }
        result = ((float)lottoNo / 1000) - 1;
        cout << "\n =>" ;

//      The numbers only have 3 decimal places anyway. This is not needed
//      cout.precision(4);        // This specifier resets after each output

        cout.width(6);            // Field width - blank padded on the left
        cout << result;
        cout << "<<";
    }
    cout << "\n"; // Just to clean up the display
}

// Include this header only once.
// It's not an issues in this particular project, but it's a good habit.
#pragma once
/******************************************************************************
 * Fun with Random Numbers                                    RandomNumbers.h *
 *                                                                            *
 * Author: nicomp                                                             *
 * Abstract: prototypes for random numbers functions in this demo.            *
 *                                                                            *
 * Revision History:                                                          *
 *                                                                            *
 *                                                                            *
 ******************************************************************************/

// Function Prototype
void RandomNumberDemo();

/******************************************************************************
 * main() for Fun with Random Numbers                                main.cpp *
 *                                                                            *
 * Author: nicomp                                                             *
 * Abstract: Random Number stuff.                                             *
 *                                                                            *
 * Revision History:                                                          *
 *                                                                            *
 *                                                                            *
 ******************************************************************************/

//#include <iostream>
#include "RandomNumbers.h"

using namespace std;

void main()
{
    RandomNumberDemo(); // invoke the demo function
}

Random numbers are useful in simulations and testing. The concept of the random number seed is important; in most cases we want a repeatable sequence of random numbers. If we can't repeat a set of test parameters, we can't replicate our results. Selecting the same seed guarantees that scenario.

In other words, we want numbers that are randomly distributed over a known range, but we need to retrieve the same sequence of numbers on-demand.

Random numbers can also be used to play the lottery, which, as we all know, is simply a tax on people who are bad at math.