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Scoping of my with foreach loop

by uksza (Canon)
on Mar 07, 2005 at 19:05 UTC ( #437285=perlquestion: print w/replies, xml ) Need Help??

uksza has asked for the wisdom of the Perl Monks concerning the following question:

Hello wise monks,

Is any difrence between
my $item; foreach $item (@array) and foreach my $item (@array)

stay in peace, sisters and brothers


20050308 Edit by castaway: Changed title from 'my ()'

Replies are listed 'Best First'.
Re: Scoping of my with foreach loop
by BrowserUk (Pope) on Mar 07, 2005 at 19:29 UTC

    It doesn't make a lot of sense to use a loop variable that was previously declared, because the one that gets used within the loop is (temporarally), an entirely different variable, despite that it has the same name:

    my $i = -23; for $i ( 1 .. 10 ) { print $i; last if $i == 5; } print $i; 1 2 3 4 5 -23

    Doing so tends to make look as if $i will retain the last value used in the loop, but as you can see, it doesn't.

    It caught me out a couple of times and is something that I hope will not persist into Perl6.

    In fact, I think I would change it in perl 5.10. Any code that got bitten by the change in behaviour would be better changed anyway.

    Examine what is said, not who speaks.
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Re: Scoping of my with foreach loop
by dragonchild (Archbishop) on Mar 07, 2005 at 19:08 UTC
    Yes. In the second, $item is scoped only to the loop. In the first, $item is scoped outside the loop. You would use the second first if you want to access the value of $item outside the loop. As in
    my $item; foreach $item (@values) { last if $item =~ /Some Complicated Regex/; } do_something_with( $item );

    Update: Fixed typo as per Fletch's /msg.

    Being right, does not endow the right to be rude; politeness costs nothing.
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      Not quite.

      do_something_with( $item );
      will always be the same as
      do_something_with( undef );
      in your code. foreach restores the initial value when the loops exits (in effect), as seen in the following snippet:

      use strict; my @values = (1, 2, 3, 'a', 4); my $item = 'z'; foreach $item (@values) { last if $item =~ /[^\d]/; } print($item); # Prints 'z', not 'a'.

      I thought it might be using a localized global when my is omitted, but it's clearly not the case:

      This is perl, v5.6.1 built for MSWin32-x86-multi-thread
Re: Scoping of my with foreach loop
by ikegami (Pope) on Mar 07, 2005 at 19:24 UTC

    The difference can be observed by running the following two snippets.

    use strict; my @array = (1, 2, 3); my $item = 'z'; foreach $item (@array) {} print($item); # Prints 'z' (not '3'!!!).
    use strict; my @array = (1, 2, 3); foreach my $item (@array) {} print($item); # Compile error. $item only in scope for loop.
Re: Scoping of my with foreach loop
by Fletch (Chancellor) on Mar 07, 2005 at 19:09 UTC

    Missing $ sigils aside: In the first example $item is visible outside and after the for BLOCK; in the second $item is only visible within the for BLOCK.

Re: Scoping of my with foreach loop
by mikeB (Friar) on Mar 07, 2005 at 19:12 UTC
    Neither will compile? No, wait. That's a similarity. :)

    Assuming you meant $item in each case, the difference would be in the scope of $item. In the second case it would be limited to the foreach block.

Re: Scoping of my with foreach loop
by Popcorn Dave (Abbot) on Mar 07, 2005 at 19:17 UTC
    Other than it won't compile? You need to make that $item.

    And yes you could concievably run in to scoping issues. Someone else will correct me if I'm incorrect about this but I believe that if for example, you declare my $item at the top of your program, then it's got global scope. However if that foreach loop was placed in a subroutine then it's going to be local to that routine and you wouldn't have access to it outside the loop.

    If both of those are at the top of a script, then there shoudln't be any difference.

    Update:Fletch picked up what I failed to take in to account completely about being visible within the foreach loop.

    Useless trivia: In the 2004 Las Vegas phone book there are approximately 28 pages of ads for massage, but almost 200 for lawyers.
Re: Scoping of my with foreach loop
by cog (Parson) on Mar 07, 2005 at 19:50 UTC
    Also, the first version tends to be faster, because the second one has to declare a new variable for each element of @array (unless this has been optimized by now).

    Benchmark it with a big array.

      use Benchmark; my @array = (1..999999); timethis (200, sub { my $foo; for $foo ( @array ) { 1; } }); timethis (200, sub { for my $foo ( @array ) { 1; } }); #timethis 200: 33 wallclock secs (31.58 usr + 0.02 sys = 31.60 CPU) @ + 6.33/s (n=200) #timethis 200: 32 wallclock secs (31.65 usr + 0.04 sys = 31.69 CPU) @ + 6.31/s (n=200)
      seems to be optimized (perl 5.8.6)

      holli, /regexed monk/

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