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 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:
1program whileFalse(input, output, stderr);
2
3begin
4 while false do
5 begin
6 writeLn('never gets printed');
7 end;
8end.
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).
1program whileDemo(input, output, stderr);
2
3var
4 x: integer;
5begin
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;
14end.
You can use Continue to jump to the end of the loop.
In the example below all values from 1 to 10 are printed, except 5.
1program whileDemo(input, output, stderr);
2var
3 i:integer=0;
4begin
5 while true do
6 begin
7 inc(i);
8 if i>10 then break;
9 if i=5 then continue;
10 writeLn(i);
11 end;
12end;
see also
Keywords: begin — do — else — end — for — if — repeat — then — until — while
