Programming Tutorials

do...while Loops in C++

By: Abinaya in C++ Tutorials on 2007-09-09  

It is possible that the body of a while loop will never execute. The while statement checks its condition before executing any of its statements, and if the condition evaluates false, the entire body of the while loop is skipped. The following C++ program illustrates this.

Skipping the body of the while Loop.

1:     // 
2:      // Demonstrates skipping the body of
3:      // the while loop when the condition is false.
4:
5:      #include <iostream.h>
6:
7:      int main()
8:      {
9:         int counter;
10:        cout << "How many hellos?: ";
11:        cin >> counter;
12:        while (counter > 0)
13:        {
14:           cout << "Hello!\n";
15:           counter--;
16:        }
17:        cout << "Counter is OutPut: " << counter;
18:         return 0;
19: }
Output: How many hellos?: 2
Hello!
Hello!
Counter is OutPut: 0

How many hellos?: 0
Counter is OutPut: 0

Analysis: The user is prompted for a starting value on line 10. This starting value is stored in the integer variable counter. The value of counter is tested on line 12, and decremented in the body of the while loop. The first time through counter was set to 2, and so the body of the while loop ran twice. The second time through, however, the user typed in 0. The value of counter was tested on line 12 and the condition was false; counter was not greater than 0. The entire body of the while loop was skipped, and Hello was never printed.
What if you want to ensure that Hello is always printed at least once? The while loop can't accomplish this, because the if condition is tested before any printing is done. You can force the issue with an if statement just before entering the while:

if (counter < 1)  // force a minimum value
counter = 1;

but that is what programmers call a "kludge," an ugly and inelegant solution.

do...while

The do...while loop executes the body of the loop before its condition is tested and ensures that the body always executes at least one time. The program below rewrites the program shown above, this time using a do...while loop.

Demonstrates do...while loop.

1:      // 
2:      // Demonstrates do while
3:
4:      #include <iostream.h>
5:
6:      int main()
7:      {
8:         int counter;
9:         cout << "How many hellos? ";
10:        cin >> counter;
11:        do
12:        {
13:           cout << "Hello\n";
14:           counter--;
15:        }  while (counter >0 );
16:        cout << "Counter is: " << counter << endl;
17:         return 0;
18: }

Output: How many hellos? 2
Hello
Hello
Counter is: 0

Analysis: The user is prompted for a starting value on line 9, which is stored in the integer variable counter. In the do...while loop, the body of the loop is entered before the condition is tested, and therefore the body of the loop is guaranteed to run at least once. On line 13 the message is printed, on line 14 the counter is decremented, and on line 15 the condition is tested. If the condition evaluates TRUE, execution jumps to the top of the loop on line 13; otherwise, it falls through to line 16.
The continue and break statements work in the do...while loop exactly as they do in the while loop. The only difference between a while loop and a do...while loop is when the condition is tested.

The do...while Statement

The syntax for the do...while statement is as follows:

do
statement
while (condition);

statement is executed, and then condition is evaluated. If condition is TRUE, the loop is repeated; otherwise, the loop ends. The statements and conditions are otherwise identical to the while loop. Example 1

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

Example 2

// print lowercase alphabet.
char ch = `a';
do
{
cout << ch << ` `;
ch++;
} while ( ch <= `z' );

 

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DO use do...while when you want to ensure the loop is executed at least once. DO use while loops when you want to skip the loop if the condition is false. DO test all loops to make sure they do what you expect.

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