[Tucson-pm] while and until statements

[Paul Scott]

Day Irmiter wrote:
> In the third edition of "Programming Perl" (the "camel book", 2000), 
> page 115 reads as follows.
> 
> "The 'while' statement repeatedly executes the block as long as EXPR is 
> true. If the word 'while' is replaced by the word 'until', the sense of 
> the test is reversed; that is, it executes the block only as long as 
> EXPR remains false." Makes sense to me, except in practice the effect of 
> the two statements is not parallel.
> 
> ---
> CODE_1:
> $x = 1;
> while ($x != 4)
> {
>    print "$x\n";
>    $x++;
> }
> ---
> OUTPUT_1:
> H:\Perl>perl -w temp
> 1
> 2
> 3
> 
> ---
> CODE_2:
> $x = 1;
> until ($x = 4)
> {
>    print "$x\n";
>    $x++;
> }
> ---
> OUTPUT_2:
> H:\Perl>perl -w temp
> 
> H:\Perl>
> 
> In other words, with the until statement, the print statement does not 
> execute. And yet, if the second bit of code is run under the debugger, 
> it will be seen that $x is incremented to 4. Anybody have any comments 
> about this? I'm running the ActiveState 5.8.4 binary under Win98, by the 
> way. I'd be interested to know if running the above code on a Unix 
> platform yields the same result.

How about:

until ($x == 4)

?

Paul Scott