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:
0program whileFalse(input, output, stderr);
1
2begin
3 while false do
4 begin
5 writeLn('never gets printed');
6 end;
7end.
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).
0program whileDemo(input, output, stderr);
1
2var
3 x: integer;
4begin
5 x := 1;
6
7 // prints non-negative integer powers of two
8 while x < high(x) div 2 do
9 begin
10 writeLn(x);
11 inc(x, x); // x := x + x
12 end;
13end.
see also
Keywords: begin — do — else — end — for — if — repeat — then — until — while