• The function rand generates an unsigned integer between 0 and RAND_MAX (a symbolic constant defined in the <cstdlib> header file).
• To demonstrate rand, let us develop a program to simulate 20 rolls of a six-sided die and print the value of each roll. The function prototype for the rand function is in <cstdlib>. To produce integers in the range 0 to 5, we use the modulus operator (%) with rand as follows:

rand() % 6

 1   
 2  // Shifted and scaled random integers.
 3  #include <iostream>
 4  using std::cout;
 5  using std::endl;
 6
 7  #include <iomanip>
 8  using std::setw;
 9
10  #include <cstdlib> // contains function prototype for rand
11  using std::rand;                                          
12
13  int main()
14  {
15     // loop 20 times
16     for ( int counter = 1; counter <= 20; counter++ )
17     {
18        // pick random number from 1 to 6 and output it
19        cout << setw( 10 ) << ( 1 + rand() % 6 );      
20
21        // if counter is divisible by 5, start a new line of output
22        if ( counter % 5 == 0 )
23           cout << endl;
24     } // end for
25
26     return 0; // indicates successful termination
27  } // end main

• This is called scaling. The number 6 is called the scaling factor. We then shift the range of numbers produced by adding 1 to our previous result. That confirms that the results are in the range 1 to 6.

Function rand actually generates pseudorandom numbers. Repeatedly calling rand produces a sequence of numbers that appears to be random. However, the sequence repeats itself each time the program executes. Once a program has been thoroughly debugged, it can be conditioned to produce a different sequence of random numbers for each execution. This is called randomizing and is accomplished with the C++ Standard Library function srand. Function srand takes an unsigned integer argument and seeds the rand function to produce a different sequence of random numbers for each execution of the program.

 
 1 
 2  // Randomizing die-rolling program.
 3  #include <iostream>
 4  using std::cout;
 5  using std::cin;
 6  using std::endl;
 7
 8  #include <iomanip>
 9  using std::setw;
10
11  #include <cstdlib> // contains prototypes for functions srand and rand
12  using std::rand;                                                      
13  using std::srand;                                                     
14
15  int main()
16  {
17     unsigned seed; // stores the seed entered by the user
18
19     cout << "Enter seed: ";
20     cin >> seed;
21     srand( seed ); // seed random number generator
22
23     // loop 10 times
24     for ( int counter = 1; counter <= 10; counter++ )
25     {
26        // pick random number from 1 to 6 and output it
27        cout << setw( 10 ) << ( 1 + rand() % 6 );
28
29        // if counter is divisible by 5, start a new line of output
30        if ( counter % 5 == 0 )
31           cout << endl;
32     } // end for
33
34     return 0; // indicates successful termination
35  } // end main

• To randomize without having to enter a seed each time, we may use a statement like

srand( time( 0 ) );

• This causes the computer to read its clock to obtain the value for the seed. Function time (with the argument 0 as written in the preceding statement) returns the current time as the number of seconds since January 1, 1970 at midnight Greenwich Mean Time (GMT). This value is converted to an unsigned integer and used as the seed to the random number generator. The function prototype for time is in <ctime>.
• Calling function srand more than once in a program restarts the pseudorandom number sequence and can affect the randomness of the numbers produced by rand.