while Loops in C++

By: Manoj Kumar in C++ Tutorials on 2007-09-09

A while loop causes your program to repeat a sequence of statements as long as the starting condition remains true. The following program shows advantages of a while loop instead of using goto statements.

while loops.

1:    // 
2:    // Looping with while
3:
4:    #include <iostream.h>
5:
6:    int main()
7:    {
8:      int counter = 0;               // initialize the condition
9:
10:      while(counter < 5)     // test condition still true
11:       {
12:          counter++;              // body of the loop
13:          cout << "counter: " << counter << "\n";
14:      }
15:
16:      cout << "Complete. Counter: " << counter << ".\n";
17:       return 0;
18: }
Output: counter: 1
counter: 2
counter: 3
counter: 4
counter: 5
Complete. Counter: 5.

Analysis: This simple program demonstrates the fundamentals of the while loop. A condition is tested, and if it is true, the body of the while loop is executed. In this case, the condition tested on line 10 is whether counter is less than 5. If the condition is true, the body of the loop is executed; on line 12 the counter is incremented, and on line 13 the value is printed. When the conditional statement on line 10 fails (when counter is no longer less than 5), the entire body of the while loop (lines 11-14) is skipped. Program execution falls through to line 15.

The while Statement

The syntax for the while statement is as follows:

while ( condition )
statement;

condition is any C++ expression, and statement is any valid C++ statement or block of statements. When condition evaluates to TRUE (1), statement is executed, and then condition is tested again. This continues until condition tests FALSE, at which time the while loop terminates and execution continues on the first line below statement.

Example

// count to 10
int x = 0;
while (x < 10)
cout << "X: " << x++;

More Complicated while Statements

The condition tested by a while loop can be as complex as any legal C++ expression. This can include expressions produced using the logical && (AND), || (OR), and ! (NOT) operators. Listing 7.3 is a somewhat more complicated while statement.

Complex while loops.

1:    // 
2:    // Complex while statements
3:
4:    #include <iostream.h>
5:
6:    int main()
7:    {
8:      unsigned short small;
9:      unsigned long  large;
10:      const unsigned short MAXSMALL=65535;
11:
12:      cout << "Enter a small number: ";
13:      cin >> small;
14:      cout << "Enter a large number: ";
15:      cin >> large;
16:
17:       cout << "small: " << small << "...";
18:
19:      // for each iteration, test three conditions
20:      while (small < large && large > 0 && small < MAXSMALL)
21:
22:      {
23:         if (small % 5000 == 0)  // write a dot every 5k lines
24:           cout << ".";
25:
26:         small++;
27:
28:         large-=2;
29:      }
30:
31:      cout << "\nSmall: " << small << " Large: " << large << endl;
32:     return 0;
33: }
Output: Enter a small number: 2
Enter a large number: 100000
small: 2.........
Small: 33335 Large: 33334

Analysis: This program is a game. Enter two numbers, one small and one large. The smaller number will count up by ones, and the larger number will count down by twos. The goal of the game is to guess when they'll meet.
On lines 12-15, the numbers are entered. Line 20 sets up a while loop, which will continue only as long as three conditions are met:

small is not bigger than large.

large isn't negative.

small doesn't overrun the size of a small integer (MAXSMALL).

On line 23, the value in small is calculated modulo 5,000. This does not change the value in small; however, it only returns the value 0 when small is an exact multiple of 5,000. Each time it is, a dot (.) is printed to the screen to show progress. On line 26, small is incremented, and on line 28, large is decremented by 2.
When any of the three conditions in the while loop fails, the loop ends and execution of the program continues after the while loop's closing brace on line 29.