http://qs321.pair.com?node_id=443218

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

Hello wise monks,

use warnings;
use strict;

my @l = qw/10 20 30/;
#my $t += $_ foreach @l;
my $t;
$t += $_ foreach @l;
print $t;

[download]
I know - commented code doesn't works, but can You tell my why?

stay in peace,
Lukasz

Replies are listed 'Best First'.
Re: Counting variable initialization
by dragonchild (Archbishop) on Mar 29, 2005 at 18:58 UTC
    You're asking the wrong question, which is the problem. You're asking "How do I simultaneously declare a variable and use it within a loop to sum a list?" You really want to ask "How do I sum the values in a loop and assign the result to a variable?" 
    use List::Util qw( reduce );

my $t = reduce { $a + $b } @list;

# Or ...

use List::Util qw( sum );

my $t = sum @list;

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    Being right, does not endow the right to be rude; politeness costs nothing.
    Being unknowing, is not the same as being stupid.
    Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence.
    Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.

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[d/l]
Re: Counting variable initialization
by tlm (Prior) on Mar 29, 2005 at 18:25 UTC

    Try -MO=Deparse to see what perl thinks of your code:

    $ perl -MO=Deparse 443218.pl
my(@l) = ('10', '20', '30');
foreach $_ (@l) {
    my $t += $_;
}
print $t;
443218.pl syntax OK

    [download]
    where file 443218.pl contains 
    $ cat 443218.pl
use warnings;
use strict;

my @l = qw/10 20 30/;
my $t += $_ foreach @l;
#my $t;
#$t += $_ foreach @l;
print $t;

    [download]

    the lowliest monk

    Update: Added the contents of the file given to -MO=Deparse for clarification.

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[d/l]
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      Deparse is wrong in this case, on two counts: 1) print($t) is not strict safe in the deparsed version, but it is in the original, and 2) print($t) prints the package variable in the deparsed version, but not in the original.

      To answer the OP, my is considered a function for syntax purposes, so my ... foreach ...; means (my ...) foreach ...;, which makes no sense.

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[d/l]
[select]

        I wondered about the fact that strict seemed to be violated. I defer to your interpretation, but it is a bummer to know that Deparse can be wrong, because I use it quite a bit. Sorry to the OP for the wrong info.

        the lowliest monk

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[d/l]

      Thats odd. That isn't the deparse I get at all. Before: 

      use strict;

my @l = qw/10 20 30/;
my $i += $_ foreach @l;
my $t;
$t += $_ foreach @l;
print '$t = ' . $t . "\n";
print '$i = ' . $i . "\n";

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      After: 
      use strict 'refs';
my(@l) = ('10', '20', '30');
;
my $i += $_ foreach (@l);
my $t;
;
$t += $_ foreach (@l);
print '$t = ' . $t . "\n";
print '$i = ' . $i . "\n";

      [download]
      Output for both scripts: 
      $t = 60
$i =

      [download]


      ___________
      Eric Hodges
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Re: Counting variable initialization
by ambs (Pilgrim) on Mar 29, 2005 at 18:24 UTC
    my receives a list of variables to initialize. Perl would first interpret the expression and then try to create a variable with the result. And that doesn't work that way.

    I think other monks can give wiser answers, but this is how I understand it.

    Alberto Simões

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