Section 5.4 - Loop Iteration Schemes

There are two other common styles of loops that are directly supported in Ada: while loops and for loops. While and for loops are called `iteration schemes'; they are loops with information prepended to them on the kind of looping scheme desired.

The while loop is particularly easy. Write a normal loop block, as you saw in the previous section, and put in front of the block the keyword ``while'' and a condition. A while loop repeatedly executes the statements in the loop as long as the while condition is true. Here is an example of a loop that, while N is less than 20, it prints N and then adds one to it: 

 while N < 20
 loop
  Put(N);
  N := N + 1;
 end loop;

The for loop is similar, starting with the keyword ``for''. A for loop assigns a local loop parameter a lower value. It then repeatedly checks if the loop parameter is less than or equal to the higher value, and if so it executes a sequence of statements and then adds one to the loop parameter. Here's an example of a loop that prints "Hello" 20 times: 

 for Count in 1 .. 20
 loop
   Put_Line("Hello");
 end loop;

There are some key points about for loops that need mentioning:

  1. The variable named in the `for' loop is a local variable visible only in the statements inside it, and it cannot be modified by the statements inside (you can exit a for loop, using the exit statement we've already seen).
  2. Normally a loop always adds one. The reverse order can be requested by using the keyword `reverse' after the keyword `in'. In this case the loop value starts with the upper bound (given second) and decrements until it is less than the lower bound (given first). If you need to increment or decrement by more than one, use another kind of loop instead.

Both ``while'' and ``for'' loops check their conditions before executing each loop. That means that the loop can conceivably execute "zero" times if the loop condition starts as false. This does create a trap for beginning Ada programmers, though. The construct: 

  for J in 10 .. 1 loop

repeats zero times (i.e. it never executes the loop contents) because 10 is always greater than 1.

The construct 

  for J in reverse 10 .. 1 loop
repeats zero times as well; Ada considers 10 .. 1 an empty list, and doing nothing in reverse order still does nothing. What you probably want instead is: 
  for J in reverse 1 .. 10 loop

Quiz:

If you wanted to repeat something exactly ten times, which iteration construct would be the most straightforward?

  1. While loop.
  2. For loop.
  3. A loop without an iteration scheme.
You may also:

PREVIOUS Go back to the previous section

NEXT Skip to the next section

OUTLINE Go up to lesson 5 outline

David A. Wheeler (dwheeler@dwheeler.com)

The master copy of this file is at "http://www.adahome.com/Tutorials/Lovelace/s5sf.htm".