Re: Problem with while loop inside a foreach loop

There were answers provided but I was missing the all critical, 'why does it happen this way?' One quick test shows that it probably should not:

 perl -e '$_ = 100; for(1..5){};print;'
[download]

Produces '100', not '5'. Obviously for localizes $_, but apparently while does not...

From Programming Perl Section 4.4.1: Unlike the foreach loop we'll see in a moment, a while loop never implicitly localizes any variables in its test condition. This can have "interesting" consequences when while loops use globals for loop variables.

So you don't really need to add new variables at all. You need to force Perl to localize $_, like so:

#...
    open FLOORFILE, $floorfile or die $!;
    {
          local($_);
          while (<FLOORFILE>)
          {
               print "$_\n";
          }
    }
    
#...
[download]

Now the output is this:

...
building2

floor1
aldfja;jd;af

floor2
aldfja;jd;af

floor3
aldfja;jd;af
[download]

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Re^2: Problem with while loop inside a foreach loop by Laurent_R (Canon) on May 03, 2013 at 22:06 UTC
Produces '100', not '5'. Obviously for localizes $_, but apparently while does not... I do not know for sure how the compiler does this with $_, but I have noticed that for or foreach is safer than while, and that map or grep are much much safer than for or foreach. I was able to do deeply nested map instructions and get it right (almost) immediately, while the equivalent constructs with foreach did not work because of $_ corruption. As an example, consider this code, which I gave as an example of functional programming under Perl on another forum: `while (my $line = <$FILE_IN>) { my ($start, $end) = split /##/, $line; my $out_line = $start . "##" . join ";", map {$_->[0]} sort {$a->[1] <=> $b->[1]} map { my ($id, @values) = split /;/, $_; @values = map { (split /-/, $_)[0]} @values; map {[$id, $_]} @values;} split /@/, $end; }` [download] I know that this is quite far-fetched LISP-like Perl code (I proposed this code just for the fun, not for serious consideration), but it does work perfectly on the data provided. Trying to replace some of the map instructions with some foreach instructions simply breaks every thing, presumably because the program gets confused about $_.	[reply] [d/l]
Re^3: Problem with while loop inside a foreach loop by Anonymous Monk on May 03, 2013 at 22:25 UTC
map or grep are much much safer than for or foreach. No they aren't :) They alias just like for/foreach. I know that this is quite far-fetched LISP-like Perl code Not really :)	[reply]
Re^4: Problem with while loop inside a foreach loop by Laurent_R (Canon) on May 04, 2013 at 11:02 UTC
No they aren't :) They alias just like for/foreach. I know that they alias the same way. But for some reason, I found that nesting foreach statements with the default $_ variable often seems to lead to confusions between the internal and the external values of $_, while I do not have this problem with nested map statements. But it is also possible that I simply don't do it right with the nesting of foreach statement. I don't do that too often anyway, I usually prefer to use explicit variables when nesting such statements in production code.	[reply]
Re^2: Problem with while loop inside a foreach loop by andylevel2 (Initiate) on May 07, 2013 at 10:42 UTC
Thanks for that, it does make it clearer.	[reply]
Re^2: Problem with while loop inside a foreach loop by andylevel2 (Initiate) on May 07, 2013 at 10:47 UTC
Thanks, that makes it clearer.	[reply]


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