Weird grep behaviour

chrism01 has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Weird grep behaviour by shmem (Chancellor) on Oct 17, 2007 at 06:16 UTC
If you want a match, do a match. Use `if( grep(/$var1/, @arr1) )` [download] or `if( grep {/$var1/} @arr1 )` [download] or `if( grep { $_ eq $var1} @arr1 )` [download] Your usage of grep evaluates `$var1` for every instance of `@arr1`, then happily returns the element for which the evaluation of `$var1` succeeded. See grep. --shmem _($_=" "x(1<<5)."?\n".q·/)Oo. G°\ / /\_¯/(q / ---------------------------- \__(m.====·.(_("always off the crowd"))."· ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}	[reply] [d/l] [select]
Re: Weird grep behaviour by Krambambuli (Curate) on Oct 17, 2007 at 08:08 UTC
The following might surprise you: `use strict; use warnings; use Data::Dumper; my @arr1 = ('a'); my $var1 = 'b'; my (@surprise) = grep($var1, @arr1); print Dumper( \@surprise );` [download] Just re-read grep. Remember that grep uses a boolean filter - the first argument - to select values out of a list. Your 'filter' - $var1 - always evals to TRUE. If seems that what you are looking for might be something like `use strict; use warnings; my @arr2 =('a', 'b'); my @arr1 = ('a'); my %reference = map { $_ => 1 } @arr1; for my $var1 (@arr2) { print "match $var1\n" if exists $reference{$var1}; }` [download] Hth.	[reply] [d/l] [select]
Re: Weird grep behaviour by moritz (Cardinal) on Oct 17, 2007 at 05:56 UTC
Try `grep {$var1 eq $_ }` instead. My guess is that grep uses numerical comparison if called with an EXPR (and not a BLOCK), so "a" == "b".	[reply] [d/l]
Re^2: Weird grep behaviour by shmem (Chancellor) on Oct 17, 2007 at 06:08 UTC
No, it's just that `$var1` is "true" for every element in `@arr1`. There's no comparison in the OP. --shmem _($_=" "x(1<<5)."?\n".q·/)Oo. G°\ / /\_¯/(q / ---------------------------- \__(m.====·.(_("always off the crowd"))."· ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}	[reply] [d/l] [select]
Re^2: Weird grep behaviour by naikonta (Curate) on Oct 17, 2007 at 07:36 UTC
It's not about comparison, BLOCK and EXPR are evaluated in boolean context. From OP's sample code, in `grep($var1, @arr1)`, `$var1` is always true. Open source softwares? Share and enjoy. Make profit from them if you can. Yet, share and enjoy!	[reply] [d/l] [select]
Re: Weird grep behaviour by chrism01 (Friar) on Oct 17, 2007 at 06:55 UTC
According to the Perl manpage: "In particular, it is not limited to using regular expressions" Changing it to use a regex ( /$var1/ ) worked, but I still don't understand why it didn't previously. Chris	[reply]
Re^2: Weird grep behaviour by snoopy (Curate) on Oct 17, 2007 at 07:57 UTC
You grep expression is in the form:`grep EXPR,LIST`. But EXPR is fixed as $var1 - not a regexp or code block. grep will return the whole list or an empty list depending on whether $var evaluates to true or false. Consider: `my @arr1 = ('a'); foreach my $var1 (0, 1) { if( grep($var1, @arr1) ) { print "match $var1\n"; } else { print "fail $var1\n"; } }` [download] nothing is returned when $var1 is 0 (false) the entire list is returned when $var1 is 1 (true) Update: Here's a sample usage: `#!/usr/bin/perl use warnings;use strict; my $paid = 0; # :-( my @shopping = ( 'twinkies','coffee', (grep !$paid => 'gruel', 'cabbage', 'cordial'), (grep $paid => 'caviar', 'truffles', 'champagne'), ); print "@shopping";` [download]	[reply] [d/l] [select]
Re^2: Weird grep behaviour by naikonta (Curate) on Oct 17, 2007 at 07:44 UTC
Well, I think because your filter was so loose everything gets through. With `/$var1/`, the filter is pretty strict. Open source softwares? Share and enjoy. Make profit from them if you can. Yet, share and enjoy!	[reply] [d/l]
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