How to avoid $

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

Is there any way to name the $_ variable in grep?

For instance, in a foreach loop, we can use $_ like this:

foreach (@array) {
   dosomething($_);
}
[download]

I can avoid by "naming" it, just like this:

foreach my $element (@array) {
   dosomething($element);
}
[download]

Let's go to the point. In a grep statement, I have something like this:

my @array_2 = grep {$_->func eq 'TEST'} @array_1;

I would really like to name the $_ because I do scripts for biologists and sometimes they modify scripts and they always get lost with $_. Is there any simple way to accomplish this?

Thanks in advance.

Comment on How to avoid $_ in grep Select or Download Code

Replies are listed 'Best First'.
Re: How to avoid $_ in grep by GrandFather (Saint) on Nov 13, 2007 at 21:41 UTC
The bottom line is $_ is Perl magic. No amount of waving wands, shaking them vigorously or banging them on the table top will dispel the underlying magic that is $_. Much subtle stuff happens with $_ in map, foreach and grep. A comment that describes any relevant and important magic in each case may be more useful to newcomers to the language than trying to paper over the good magic that $_ provides. Perl is environmentally friendly - it saves trees	[reply]
Re: How to avoid $_ in grep by gamache (Friar) on Nov 13, 2007 at 20:47 UTC
If you want to name it `$ARG`, then stick `use English;` somewhere at the top of your code. Update: As mentioned below, `use English qw( -no_match_vars );` is even better. The English docs explain why.	[reply] [d/l] [select]
Re^2: How to avoid $_ in grep by agianni (Hermit) on Nov 13, 2007 at 20:54 UTC
Great advice, and might I add: `use English qw( -no_match_vars );` is an even better idea. `perl -e 'split//,q{john hurl, pest caretaker}and(map{print @_[$_]}(joi +n(q{},map{sprintf(qq{%010u},$_)}(2*23074993,51016415261,75979 +36997,13177145131,3*26789167*181))=~/\d{2}/g));'` [download]	[reply] [d/l] [select]
Re^3: How to avoid $_ in grep by gamache (Friar) on Nov 13, 2007 at 21:13 UTC
Ah, I didn't know this... those match variables are more trouble than they're worth! Thanks for the tip!	[reply]
Re: How to avoid $_ in grep by blahblahblah (Priest) on Nov 14, 2007 at 03:06 UTC
I feel that all of the above suggestions, though good, missed your original point: "I do scripts for biologists ... and they always get lost with $_." If biologists can't understand $_, then keep your code simple for them. Don't use $_, and while you're at it avoid map & grep. It's not that much harder to write out a loop, but it can make a world of difference to someone who is only a novice in perl. Joe	[reply]
Re^2: How to avoid $_ in grep by Krambambuli (Curate) on Nov 14, 2007 at 09:39 UTC
I'd agree with the above; also, maybe it is possible to put a thicker line between things that 'custom users' need or are allowed to change ? Maybe offering a distinct Config file, or separating the allowed/easy to change code in front of the scripts or even distinct modules ? If the users are really meant to tamper the very hearth of underlying algorithms and so, $_ would probably be the smallest difficulty to overcome. Maybe you can do yet another thing, as I'm just learning from the book I mention in the signature below: can you offer them some placeholder subs and use callbacks, so that they could easily manage their own particular code snippets within the otherwise bigger code machinery ? Krambambuli --- enjoying Mark Jason Dominus' Higher-Order Perl	[reply]
Re^2: How to avoid $_ in grep by felipe (Initiate) on Nov 14, 2007 at 14:27 UTC
Thanks all for your replies. I have to say that, although blahblahblah didn't give me the solution I was looking for, he understood me perfectly and he came up with a nice solution. Thanks all. Problem solved: I won't use grep.	[reply]
Re: How to avoid $_ in grep by lupey (Monk) on Nov 13, 2007 at 21:00 UTC
Would this work for you? Just take a reference to $_. `my @array_2 = grep {my $element = \$_; $$element =~ /foo/} @array_1;` [download] lupey	[reply] [d/l]
Re^2: How to avoid $_ in grep by ikegami (Patriarch) on Nov 13, 2007 at 21:10 UTC
If you don't mind the usually small cost of copying, `my @array_2 = grep { my $element = $_; $element->func() eq 'TEST' } @array_1;` [download] It's basically the same as `my ($arg1, $arg2) = @_;` in subs. If you're dealing with very long strings and want to avoid the cost of copying without having to deal with references, you can create an alias, `my @array_2 = grep { our $element; local *element = \$_; $element->func() eq 'TEST' } @array_1;` [download] or `use Data::Alias qw( alias ); my @array_2 = grep { alias my $element = $_; $element->func() eq 'TEST' } @array_1;` [download]	[reply] [d/l] [select]