Lecture 10: Loops II: while ... and do ... while

while ...

The loops in this lecture, while and do-while are used for repeating things that are not exactly countable. Normally we use this when it is not exactly clear when the loop will finish, for instance because the control variable changes within the loop (as is strictly disadvised in for loops). Also, when we want to loop over something with a floating point type variable we use the while and do-while loops.
As we will show, the difference between the while and the do-while loops is the moment the condition is tested.

The general format of the while loop is

 while (condition)
   Instruction;
 
while flow
This structure is repeating the instruction as long as the condition is true. The condition is any condition that results in a boolean value (TRUE or FALSE), as we have discussed in lecture 8. This can be a comparison, or anything else (still information in file?, user pressed a key?, etc).
Note that the structure while does not attribute a starting value to any variable like in the for-loop. So, we have to do this ourselves.
Example:
 
program code
main()
// while example
{
  float x;

  x = 0.0;
  while (x<=1.0)
    {
      printf("%0.1f\n", x);
      x = x + 0.1;
    }
}

  output
  0.0 
  0.1
  0.2
  0.3
  0.4
  0.5
  0.6
  0.7
  0.8
  0.9
  1.0

do ... while

The do-while structure is very similar to the while structure. Theonly difference is that now the condition is checked at the end of the instructions to be repeated. The general format is
 
 do
  Instruction;
 while (condition); 
 
do-while flow
A very important difference between while and do-while is that with while the condition is checked in the beginning of the loop, whereas in do-while it is checked at the end. Therefore, the instructions in do-while are at least executed once.
Look at the following programs (also an example with a for-loop is included). Only the code with the do-while structure has output.
program code
x = 100;
do
  {
    printf("Ajax\n");
    x = x + 1;
  }
while (x<=10);		 program code
x = 100;
while (x<=10)
  {
    printf("Ajax\n");
    x = x + 1;
  }		 program code
for (i=100; i<=10; i++)
  printf("Ajax\n");
output
Ajax		 output
output

for, while e do-while compared

Effectively, the for and while loops are exactly the same. Both have a condition that is checked in the beginning of the loop and a step instruction. The only difference is that the for-loop already includes a starting instruction. If we precede the while-loop with a starting instruction, the two loops are equal:
 
for
for (startI; testC; stepI) 
  instruction;
while
startI;
while (testC)
  {
    instruction;
    stepI;
  }
for (i=1; i<=10; i++)
  printf("%d", i);
 
 
 

i=1;
while (i<=10)
  {
    printf("%d", i);
    i++;
  }

Only for the sake of readability of the program will we use the for-loop for repeating things in a countable way. For all other types of loop will we use while and do-while. In the do-while loop the instructions are at least executed once. Always take these considerations into account when deciding for the type of loop to use.

Nesting II

Now that we know all the types of loops, let's take a look at the rules-of-good-behavior related to nesting: Examples of bad code:


for (i=1; i<=10; i++)
  {
    for (i=1; i<=20; i++) 
      printf("%d", i);
  }
for  (i=1; i<=10; i++)
  {
    x = i;
    do
      x = x+0.1;
  }
while (x<20);
Two loops with equal control variable i. Two loops not well nested.
Good indentation of your program will 
always avoid such errors.

Quick test:

To test your knowledge of what you have learned in this lesson, click here for an on-line test.
Peter Stallinga. Universidade do Algarve, 4  November 2002