5.2. Essentials of Counter-Controlled Repetition

This section uses the while repetition statement introduced in Chapter 4 to formalize the elements required to perform counter-controlled repetition. Counter-controlled repetition requires

Consider the simple program in Fig. 5.1, which prints the numbers from 1 to 10. The declaration in line 9 names the control variable (counter), declares it to be an integer, reserves space for it in memory and sets it to an initial value of 1. Declarations that require initialization are, in effect, executable statements. In C++, it is more precise to call a declaration that also reserves memory—as the preceding declaration does—a definition. Because definitions are declarations, too, we'll use the term "declaration" except when the distinction is important.

 1   // Fig. 5.1: fig05_01.cpp
 2   // Counter-controlled repetition.
 3   #include <iostream>
 4   using std::cout;
 5   using std::endl;
 6
 7   int main()
 8   {
 9      int counter = 1; // declare and initialize control variable
10
11      while ( counter <= 10 ) // loop-continuation condition
12      {
13         cout << counter << " ";
14         counter++; // increment control variable by 1
15      } // end while
16
17      cout << endl; // output a newline
18      return 0; // successful termination
19   } // end main

1 2 3 4 5 6 7 8 9 10

The declaration and initialization of counter (line 9) also could have been accomplished with the statements

int counter; // declare control variable
counter = 1; // initialize control variable to 1

We use both methods of initializing variables.

Line 14 increments the loop counter by 1 each time the loop's body is performed. The loop-continuation condition (line 11) in the while statement determines whether the value of the control variable is less than or equal to 10 (the final value for which the condition is true). Note that the body of this while executes even when the control variable is 10. The loop terminates when the control variable is greater than 10 (i.e., when counter becomes 11).

Figure 5.1 can be made more concise by initializing counter to 0 and by replacing the while statement with

while ( ++counter <= 10 ) // loop-continuation condition
   cout << counter << " ";

This code saves a statement, because the incrementing is done directly in the while condition before the condition is tested. Also, the code eliminates the braces around the body of the while, because the while now contains only one statement. Coding in such a condensed fashion takes some practice and can lead to programs that are more difficult to read, debug, modify and maintain.

Common Programming Error 5.1

Floating-point values are approximate, so controlling counting loops with floating-point variables can result in imprecise counter values and inaccurate tests for termination.

Error-Prevention Tip 5.1

Control counting loops with integer values.