Section 5.3 - Simple Loops

Ada has a number of `looping constructs' for situations where something must repeat over and over again. The simplest looping construct just repeats `forever'. A simple loop simply begins with the phrase ``loop'', has statements to repeat, and ends with ``end loop;''. Here's an example of a procedure body that, when run, repeatedly prints the same message over and over again: 

with Text_IO; use Text_IO;
procedure Forever is
begin
 loop
  Put_Line("Hello again!");
 end loop;
end Forever;

A loop can be terminated through an ``exit'' or an ``exit when'' clause (similar to C's break statement). An `exit' causes an immediate exiting of the innermost loop. An `exit when' clause causes exiting of the innermost loop only if the following condition is true.

The `exit when' clause is particularly useful in circumstances where you must do some action(s) before you can figure out if the loop has to stop or not. These are called ``loop-and-a-half'' constructs - you start with "loop", perform calculations to determine if the loop should stop, use an `exit when' to exit on that condition, and then work on the result.

Here's an example of a loop-and-a-half. This program reads in a list of numbers and prints out their squares, but stops (without printing) when it reads in the number `0': 

with Text_IO, Ada.Integer_Text_IO;
use  Text_IO, Ada.Integer_Text_IO;
procedure Print_Squares is
 X : Integer;
begin
 loop
  Get(X);
 exit when X = 0;
  Put(X * X);
  New_Line;
 end loop;
end Print_Squares;

Quiz:

What will A's final value be? 

A := 1;
loop
 A := A + 1;
 exit when A > 3;
end loop;
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  3. 3
  4. 4
  5. 5
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David A. Wheeler (dwheeler@dwheeler.com)

The master copy of this file is at "http://www.adahome.com/Tutorials/Lovelace/s5s3.htm".