do...while Loops in C++

By: Abinaya Viewed: 153270 times  Printer Friendly Format    


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' );

 


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.


Most Viewed Articles (in C++ )

Latest Articles (in C++)

Comment on this tutorial

Subscribe to Tutorials

Related Tutorials

Archived Comments

1. HELP ME HOW TO ANALZE THE PROGRAM??????????
View Tutorial          By: MICHELLE FLORES at 2009-01-21 23:24:52

2. .. nice one ..
.. aNg gLing nka2'amazing .

View Tutorial          By: ma'kHuLet at 2010-01-19 19:24:51

3. .. nice one ..
.. aNg gLing nka2'amazing .

View Tutorial          By: ma'kHuLet at 2010-01-19 19:25:08

4. .. nice one ..
.. aNg gLing nka2'amazing .

View Tutorial          By: ma'kHuLet at 2010-01-19 19:25:19

5. thanks for all thanks for your help good bye
View Tutorial          By: ARAS at 2010-05-14 06:50:47

6. pls help me .....its hard to understand for me th
View Tutorial          By: heidi at 2010-06-27 02:31:29

7. Thank you......... It's very helpful.
View Tutorial          By: Ramesh Kumar Mahto at 2010-09-01 12:12:30

8. help me pls example ng c++ program code using do
View Tutorial          By: diana bernardo at 2010-10-15 23:34:19

9. amf..ang hrap hnd ko magets.....
View Tutorial          By: joshua javier at 2010-10-17 05:07:10

10. Hy plz help me for also draw to flowchart.........
View Tutorial          By: khadija at 2010-11-13 11:38:31

11. HI guys i am also studying c++ And now almost mast
View Tutorial          By: Inayat Ullah at 2011-02-08 09:11:43

12. HI guys i am also studying c++ And now almost mast
View Tutorial          By: Inayat Ullah at 2011-02-08 09:29:44

13. help me nmn qng pnu gawin s c++ ang pg ad ng 2loy2
View Tutorial          By: nheb at 2011-07-22 02:58:42

14. There something at Example #2! because when i try
View Tutorial          By: Paul at 2011-08-01 14:38:14

15. can you help me do this? - write a program that wi
View Tutorial          By: rence at 2011-08-16 13:33:14

16. 0
View Tutorial          By: zareen at 2011-08-20 08:27:28

17. i really hate do while loop statement, can u pls h
View Tutorial          By: mARjO at 2011-09-01 03:50:18

18. Can you give some examples of do while & pleas
View Tutorial          By: mhilan at 2011-09-01 04:54:13

19. 3 - To build features that realize the control ele
View Tutorial          By: kosovari at 2012-01-30 13:52:54

20. /
//BSIT
//
#include<iost

View Tutorial          By: Joshua Torres at 2012-10-05 16:59:17

21. Please, I need help, how can i write a program &qu
View Tutorial          By: LULU at 2012-11-23 01:34:07

22. nice tutorial, for c++ lecture notes, and example
View Tutorial          By: Jawad Khan at 2013-04-06 07:54:31