Difference between revisions of "While"

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(Clean up text a little)
(Undo revision 140570 by Rfc1394 (talk): ensure reserved words are highlighted as such, remove incorrect information, ensure every sentence spans exactly one source code line for better diffs)
Tag: Undo
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{{while}}
 
{{while}}
  
The [[Reserved word|reserved word]] {{HL|while}} is used as a [[loop instruction]] where the test occurs before the loop is executed. It is a control construct that is similar to a [[Repeat|{{HL|repeat}}]]-[[Until|{{HL|until}}]] loop. Unlike {{HL|for}} or {{HL|repeat}} loops, {{HL|while}} is the only type of loop where the loop might not be executed at all.
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<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>]].
 
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The condition [[expression]] is evaluated prior each iteration, determining whether the following block (or single statement) is executed.
While shares a characteristic with {{HL|for}} loops, in that the {{HL|while ... do}} construct only covers a single statement. If more than one statement needs to be executed in the loop, a {{HL|begin ... end}} block is required.
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This is the main difference to a [[Repeat|<syntaxhighlight lang="pascal" enclose="none">repeat … until</syntaxhighlight>-loop]], where the [[Block|block]] is executed at any rate, but succeeding iterations do not necessarily happen, though.
 
 
The {{HL|while}} statement, in conjunction with [[Do|{{HL|do}}]] repeats a statement as long as a condition evaluates to [[True|{{HL|true}}]].
 
The condition [[expression]] is evaluated prior to each iteration, determining whether the following block (or single statement) is to be executed. If the condition is false to begin with, the loop is never executed. This is the main difference from a {{HL|repeat … until}}-loop, where the [[Block|block]] is executed at least once, but succeeding iterations do not necessarily happen, though.
 
  
 
The following example contains unreachable code:
 
The following example contains unreachable code:
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</syntaxhighlight>
 
</syntaxhighlight>
  
You usually use {{HL|while}}-loops where, in contrast to [[For|{{HL|for}}]], a running index [[Variable|variable]] is not required, the block executed can't be deduced from an index that's incremented by one, or to avoid a [[Break|{{HL|break}} statement]] (which usually indicates bad programming style).
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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|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 highlight="9">
 
<syntaxhighlight lang="pascal" line highlight="9">
 
program whileDemo(input, output, stderr);
 
program whileDemo(input, output, stderr);
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{{Keywords}}
 
{{Keywords}}
 
[[Category:Code]]
 
[[Category:Code]]
[[Category:Pascal]]
 

Revision as of 15:52, 20 November 2020

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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:

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 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).

 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.

see also


Keywords: begindoelseendforifrepeatthenuntilwhile