Difference between revisions of "While"
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{{while}} | {{while}} | ||
| − | < | + | |
| − | ''' | + | <syntaxhighlight lang="pascal" enclose="none">while</syntaxhighlight> in conjunction with [[Do|<syntaxhighlight lang="pascal" enclose="none">do</syntaxhighlight>]] repeats a statement as long as a condition evaluates to [[True|<syntaxhighlight lang="pascal" enclose="none">true</syntaxhighlight>]]. |
| − | + | 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|<syntaxhighlight lang="pascal" enclose="none">repeat … until</syntaxhighlight>-loop]], where the block is executed at any rate, but succeeding iterations do not necessarily happen, though. | |
| + | |||
| + | The following example contains unreachable code: | ||
| + | <syntaxhighlight lang="pascal" line start="0"> | ||
| + | program whileFalse(input, output, stderr); | ||
| + | |||
| + | begin | ||
| + | while false do | ||
| + | begin | ||
| + | writeLn('never gets printed'); | ||
| + | end; | ||
| + | end. | ||
| + | </syntaxhighlight> | ||
| + | |||
| + | You usually use <syntaxhighlight lang="pascal" enclose="none">while</syntaxhighlight>-loops where, in contrast to [[For|<syntaxhighlight lang="pascal" enclose="none">for</syntaxhighlight>-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|<syntaxhighlight lang="pascal" enclose="none">break</syntaxhighlight>-statement]] (which usually indicates bad programming style). | ||
| + | <syntaxhighlight lang="pascal" line start="0"> | ||
| + | program whileDemo(input, output, stderr); | ||
| + | |||
| + | var | ||
| + | x: integer; | ||
| + | begin | ||
| + | x := 1; | ||
| + | |||
| + | // prints non-negative integer powers of two | ||
| + | while x < high(x) div 2 do | ||
| + | begin | ||
| + | writeLn(x); | ||
| + | inc(x, x); | ||
| + | end; | ||
| + | end. | ||
| + | </syntaxhighlight> | ||
| + | |||
| + | == see also == | ||
| + | * [[Infinite loop]] | ||
{{Keywords}} | {{Keywords}} | ||
| + | [[Category:Code]] | ||
Revision as of 00:11, 14 February 2018
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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);
12 end;
13end.
see also
Keywords: begin — do — else — end — for — if — repeat — then — until — while
