Rolling a Six-Sided Die

To demonstrate rand, Fig. 6.7 simulates 20 rolls of a six-sided die and displays 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

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 1   // Fig. 6.7: fig06_07.cpp
 2   // Shifted, scaled integers produced by 1 + rand() % 6.
 3   #include <iostream>
 4   #include <iomanip>
 5   #include <cstdlib> // contains function prototype for rand
 6   using namespace std;
 7
 8   int main()
 9   {
10      // loop 20 times
11      for ( unsigned int counter = 1; counter <= 20; ++counter )
12      {
13         // pick random number from 1 to 6 and output it
14         cout << setw( 10 ) << ( 1 + rand() % 6 );      
15
16         // if counter is divisible by 5, start a new line of output
17         if ( counter % 5 == 0 )
18            cout << endl;
19      } // end for
20   } // end main

         6         6         5         5         6
         5         1         1         5         3
         6         6         2         4         2
         6         2         3         4         1

Fig. 6.7. Shifted, scaled integers produced by 1 + rand() % 6.

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. Figure 6.7 confirms that the results are in the range 1 to 6. If you execute this program more than once, you’ll see that it produces the same “random” values each time. We’ll show how to fix this in Figure 6.9.

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