Home » How to know the do while loop in C programming [closed]

How to know the do while loop in C programming [closed]


Well: Hope the following helps you.

  • Unlike for and while loops, which test the loop condition at the top
    of the loop, the do…while loop in C programming language checks its
    condition at the bottom of the loop
  • A do…while loop is similar to a while loop, except that a do…while loop is guaranteed to execute at least one time.

A sample syntax would be:



Notice that the conditional expression appears at the end of the loop, so the statement(x) in the loop execute once before the condition is tested.

If the condition is true, the flow of control jumps back up to do, and the statement(x) in the loop execute again. This process repeats until the given condition becomes false.

Try this code and you will be good:

#include <stdio.h>

int main ()
   // local variable definition 
   int x = 5;

   // do loop execution 
       printf("value of x: %dn", x);
       x = x + 1;
   }while( x < 10 );

   return 0;

Here is a great way to understand the difference between a while loop and a do / while loop.

(But first, a side note: do / while are actually pretty rare in practice. You don’t usually need them. The good, clean algorithms that you usually want to write, that naturally do the right thing for any input, and that properly do nothing when they’re given no input, usually end up being written in terms of while loops. “while (there's something to do) { do something; }“. But there are times when a do / while loop is exactly what you want.)

Suppose you’re trying to convert an integer to a string, that is, construct a string containing the base-10 representation of an integer, basically just like printf %d. This is a standard exercise. Here is the standard solution:

void itoa(int n, char buf[], int size)
    char *p = buf;
    while(n > 0)
        if(p >= &buf[size-1]) abort();
        *p++ = n % 10 + '0';
        n /= 10;
   *p = '';


We use the % operator to generate digits, filling them into a destination array. The standard additional difficulty with this technique is that it generates digits in the wrong order; the standard way to compensate for that is with a string-reverse function. Anyway, here’s the string-reverse function and a little test driver so you can try it out:

void strrev(char str[])
    char *p1 = str, *p2;
    for(p2 = str; *p2 != ''; p2++)
    while(p1 < p2)
        char tmp = *p1;
        *p1 = *p2; *p2 = tmp;
        p1++; p2--;

So go ahead and type that code in, and compile it, and play with it. You should discover that it works fine for just about any number you can type into it, except…

It does not work for zero! If you type in 0, the test while(n > 0) fails immediately, and we make zero trips through the loop, and we compute no digits, and we generate an empty string.

So: suppose you want to generate the string "0" for zero. In other words, suppose we always want to make at least one trip through the loop, even when n is initially 0. This sounds like a job for… do / while! Just rewrite the loop like this — same body, same condition (n > 0), but with the condition tested at the bottom of the loop instead of at the top:

do  {
    if(p >= &buf[size-1]) abort();
    *p++ = n % 10 + '0';
    n /= 10;
    } while(n > 0);

Try this, and you’ll see that it generates the same output for positive numbers, and now it works as expected for 0, too.

(But, in case you were wondering, the other thing this simple code doesn’t handle, and the do/while modification does nothing to address, is negative numbers.)

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