CHAPTER 14: PROGRAMMING & LANGUAGES

The Difference Between Do While And Do Until




The difference between "do while" and "do until" is that a "do while" loops while the test case is true, whereas "do until" loops UNTIL the test case is true (which is equivalent to looping while the test case is false).

The difference between a "do ...while" loop and a "while {} " loop is that the while loop tests its condition before execution of the contents of the loop begins; the "do" loop tests its condition after it's been executed at least once. As noted above, if the test condition is false as the while loop is entered the block of code is never executed. Since the condition is tested at the bottom of a do loop, its block of code is always executed at least once.

To further clear your concept on this, understand the syntax and description of the two loop types:

while
The while loop is used to execute a block of code as long as some condition is true. If the condition is false from the start the block of code is not executed at al. The while loop tests the condition before it's executed so sometimes the loop may never be executed if initially the condition is not met. Its syntax is as follows.

while (tested condition is satisfied)
{
block of code
}

In all constructs, curly braces should only be used if the construct is to execute more than one line of code. The above program executes only one line of code so it not really necessary (same rules apply to if...else constructs) but you can use it to make the program seem more understandable or readable.

Here is a simple example of the use of the while loop. This program counts from 1 to 100.

#include <stdio.h>

int main(void)
{

int count = 1;

while (count <= 100)
{
printf("%d\n",count);
count += 1; // Notice this statement
}

return 0;

}

Note that no semi-colons ( ; ) are to be used after the while (condition) statement. These loops are very useful because the condition is tested before execution begins. However i never seem to like these loops as they are not as clear to read as the do ...while loops. The while loop is the favorite amongst most programmers but as for me, i definitely prefer the do ...while loop.

do ....while
The do loop also executes a block of code as long as a condition is satisfied.

Again, The difference between a "do ...while" loop and a "while {} " loop is that the while loop tests its condition before execution of the contents of the loop begins; the "do" loop tests its condition after it's been executed at least once. As noted above, if the test condition is false as the while loop is entered the block of code is never executed. Since the condition is tested at the bottom of a do loop, its block of code is always executed at least once.

Some people don't like these loops because it is always executed at least once. When i ask them "so what?", they normally reply that the loop executes even if the data is incorrect. Basically because the loop is always executed, it will execute no matter what value or type of data is supposed to be required. The "do ....while" loops syntax is as follows

do
{
block of code
} while (condition is satisfied);

Note that a semi-colon ( ; ) must be used at the end of the do ...while loop. This semi-colon is needed because it instructs whether the while (condition) statement is the beginning of a while loop or the end of a do ...while loop. Here is an example of the use of a do loop.

include <stdio.h>

int main(void)
{

int value, r_digit; printf("Enter a number to be reversed.\n");
scanf("%d", &value); do
{
r_digit = value % 10;
printf("%d", r_digit);
value = value / 10;
} while (value != 0); printf("\n"); return 0;

}