# While

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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 block (or single statement) is executed. This is the main difference to a repeat … until-loop, where the block 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);
2. 
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 block 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);
2. 
3. var
4. 	x: integer;
5. begin
6. 	x := 1;
7. 
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.