for, while and do while Loops

Prev: Variable Scope
Next: Quiz and Exercises on Program Flow

---

The Concept of Looping

The term LOOPING describes the way in which the program executes statements over and over again, before exiting the loop and continuing with program flow.

In C, there are three types of loops: for loops, while loops and do while loops.

It is possible to simulate each type by writing code using other loops.

---

for Loops

I use for loops a lot. Here's the basic form:

for (a ; b ; c) {
 /* statements */
}

Brief example:

for(i=10, j=0 ; i!=j ; i--, j++) {
  printf("%d - %d = %d\n", i, j, i-j);
}

a, b and c are expressions that are evaluated at different times.

a is evaluated once only - before entering the loop. It is a very good place to assign values to variables. If you wanted to assign values to multiple variables, you can separate each assignment with a comma like this: i=10, j=0

b determines whether or not the program loops. If b returns a non zero value, an iteration of the loop is performed. In other words, the program enters the loop and executes the statements inside. b should therefore be some sort of condition. You can use logical operators in your condition as usual.

c is evaluated after an iteration of the loop, i.e. after all the statements inside the loop body have been executed. If you have multiple expressions you want evaluating, you can separate each one with a comma like this: i--, j++

The round brackets and semi colons are required at all times. The three expressions aren't compulsory, but omitting b and failing to place a break statement somewhere in the loop can result in an infinite loop!

Let's look at a proper example:

#include <stdio.h>

int main() {
  int i,j; 

  for(i=10, j=0 ; i!=j ; i--, j++) {
    printf("%d - %d = %d\n", i, j, i-j);
  }

  return 0;
}

Output:

You should be familiar with the logic behind the printf statement, so I'll focus on the loop itself.

Before entering the loop, i is initialized with 10 and j with 0. Let's step through the loop one iteration at a time:

At the end of each iteration, i is decremented by 1, j is incremented by 1.

The sixth evaluation of i!=j returns 0 for false, because i and j are equal at that time, so the loop body is not executed.

---

while Loops

Some might say that while loops are simpler to use, since they only require one expression in the brackets:

while (expression) {
  /* statements */
}

The loop body is executed if expression returns a non zero value. Most of the time, you'd want to put some sort of condition to be evaluated.

I mentioned earlier on that you can simulate one type of loop with another type - there is no unique way to loop.

The previous example can be rewritten using a while loop.

#include <stdio.h>

int main() {
 int i=10; /* initialize variables */
 int j=0;  /* part a of a for loop */

 while (i!=j) { /* test for condition 
                   part b of for loop */
   printf("%d - %d = %d\n", i, j, i-j);
   i--;  /* do something to variables */
   j++;  /* part c of for loop        */
 }
 return 0;
}

---

do while Loops

The last type of loop I rarely use.

It's almost identical to a while loop, except the loop body is executed at least once - the

while (expression)

part is after the loop body, like this:

do {
  /* loop body */
} while (expression);

This example prints out the numbers 0-9:

#include <stdio.h>

int main() {
  int i = 0;
  do {
    printf("%d", i);
    i++;
  } while (i<10);
  return 0;
}

Don't forget the semi colon after the while(expression) part!

If you run the example again, but initialize i with 11 you'll notice that the loop body is still executed.

Simulating a do while loop with either a for or while loop is fiddley - most of the time I'd write the statements in the loop body once, before the loop:

#include <stdio.h>

int main() {
  int i = 0;

  printf("%d", i); /* "loop body" */
  i++;

  while (i<10) {
    printf("%d", i);
    i++;
  }
  return 0;
}

---

Watch Out! Infinite Loops Are About!

Having an expression that always returns a non zero value in the brackets of a while loop (or the second expression of a for loop) can result in an INFINTE LOOP. Missing the second expression in a for loop will also result in an infinite loop, but missing the expression in the brackets of a while loop causes a compilation error. This program should loop forever:

#include <stdio.h>

int main() {
  int i;

  for(i=0 ; /* no expression */ ; i++) {
    printf("%d\n", i);
  }

  return 0;
}

I say "should" loop forever, but sometimes the computer crashes and the program is shut down.

while (1) {} also causes an infinite loop.

To combat infinite loops, you can use the break statement (or in the last resort, Ctrl-Alt-Delete!).

#include <stdio.h>

int main() {
  int i;

  for(i=0 ; /* no expression */ ; i++) {
    printf("%d\n", i);
    if(i==1000) {
      break; /* exits the loop when */
    }       /* i equals 1000       */
  }

  return 0;
}

Prev: Variable Scope
Next: Quiz and Exercises on Program Flow

www.iota-six.co.uk Copyright © 2001-2003
Unauthorized copying not permitted

Designed and Developed Using Macromedia Studio MX