grep and $

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

Hi, can someone please explain the $_ in the bottom?
I am trying to get a better understanding of grep and
below $_ seems to be bit hard to understand
Does $_ iterate over and over for @skipper so that it will evauluate like below

preserver eq blue_shirt
preserver eq hat
preserver eq jacket
preserver eq perserver ==> returns 1

I was thinking since it was iterating over @required , I thought $_ would be from @required.
but I am guessing since grep does expression, $_ becomes from @skipper?

my @required = qw(preserver sunscreen water_bottle jacket);
my @skipper  = qw(blue_shirt hat jacket preserver sunscreen);

for my $item (@required) {
  unless (grep $item eq $_, @skipper) { # not found in list?
    print "skipper is missing $item.\n";
  }
}
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Re: grep and $_ by grep (Monsignor) on Aug 11, 2007 at 23:26 UTC
In your case, `$_` is an alias (think pronoun) for the current list value being iterated in the closest loop. I think the main problem you are having is the implicit loop that grep creates. grep actually creates another loop which sets $_. Also your outside loop over `@required` never sets $_ since you explicitly create a variable. ex. `my @required = qw(preserver sunscreen water_bottle jacket); foreach my $item (@required) { print "$_\n"; ## You just get a warning }` [download] grep	[reply] [d/l] [select]
Re^2: grep and $_ by convenientstore (Pilgrim) on Aug 11, 2007 at 23:38 UTC
no wonder I coudn't print $_ thanks guys	[reply]
Re^3: grep and $_ by mreece (Friar) on Aug 11, 2007 at 23:58 UTC
you can if you use the BLOCK form of grep.. `unless ( grep { print "$item eq $_? ", $item eq $_, "\n"; $item eq $_; } @skipper ) { # not found in list? print "skipper is missing $item.\n"; }` [download]	[reply] [d/l]
Re: grep and $_ by bruceb3 (Pilgrim) on Aug 11, 2007 at 23:24 UTC
Basically you are right. $_ iterates over @skipper, which happens for each element of the @required list. Adding some print statements help show what is happening. `my @required = qw(preserver sunscreen water_bottle jacket); my @skipper = qw(blue_shirt hat jacket preserver sunscreen); for my $item (@required) { print "$item\n"; unless (grep $item eq $_, @skipper) { # not found in list? print "skipper is missing $item.\n"; } }` [download] The variable $item has each value from the @required list. In scalar context grep will return the number of times that the expression was true. $_ is an aliases to the element in the @skipper list.	[reply] [d/l]