Difference between "while" loop and "do while" loop

Question

What is the difference between while loop and do while loop. I used to think both are completely same.Then I came across following piece of code:

do {
        printf("Word length... ");
        scanf("%d", &wdlen);
    } while(wdlen<2);

This code works perfectly. It prints word length and tascans the input. But when I changed it to

while(wdlen<2){
        printf("Word length... ");
        scanf("%d", &wdlen);
    } 

It gives a blank screen. It do not work. So there is some functional difference between both loops. Can anybody explain it?

Is there any other difference in these two?

Answer 1

The do while loop executes the content of the loop once before checking the condition of the while.

Whereas a while loop will check the condition first before executing the content.

In this case you are waiting for user input with scanf(), which will never execute in the while loop as wdlen is not initialized and may just contain a garbage value which may be greater than 2.

Answer 2

While : your condition is at the begin of the loop block, and makes possible to never enter the loop.

Do While : your condition is at the end of the loop block, and makes obligatory to enter the loop at least one time.

Answer 3

do {
    printf("Word length... ");
    scanf("%d", &wdlen);
} while(wdlen<2);

A do-while loop guarantees the execution of the loop at least once because it checks the loop condition AFTER the loop iteration. Therefore it'll print the string and call scanf, thus updating the wdlen variable.

while(wdlen<2){
    printf("Word length... ");
    scanf("%d", &wdlen);
} 

As for the while loop, it evaluates the loop condition BEFORE the loop body is executed. wdlen probably starts off as more than 2 in your code that's why you never reach the loop body.

Answer 4

do while in an exit control loop. while is an entry control loop.