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while in conjunction with do repeats a statement as long as a condition evaluates to true. The condition expression is evaluated prior each iteration, determining whether the following statement is executed. This is the main difference to a repeat  until-loop, where the loop body is executed at any rate, but succeeding iterations do not necessarily happen, though.

The following example contains unreachable code:

1 program whileFalse(input, output, stderr);
3 begin
4 	while false do
5 	begin
6 		writeLn('never gets printed');
7 	end;
8 end.

You usually use while-loops where, in contrast to for-loops, a running index variable is not required, the statement executed can't be deduced from an index that's incremented by one, or to avoid a break-statement (which usually indicates bad programming style).

 1 program whileDemo(input, output, stderr);
 3 var
 4 	x: integer;
 5 begin
 6 	x := 1;
 8 	// prints non-negative integer powers of two
 9 	while x < high(x) div 2 do
10 	begin
11 		writeLn(x);
12 		inc(x, x); // x := x + x
13 	end;
14 end.

see also

Keywords: begindoelseendforifrepeatthenuntilwhile