uninitialized $_ sneaks past warnings in grep/map block or expression

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

Update: I'd downvote this if I could. My error was thinking $a and $b when $_ was appropriate.

The recent thread, Better algorithm or data structure?, shows that this behavior of $_ is not well known. It surprised me. It took prompting for this aspect to register (thanks, BrowserUk).

Update: reduced code to the point. This expresses that to which I was referring. Map and grep, like for can, localize $_; this makes this post nonsensical;

## perl 5.10.1
use strict;
use warnings;

my $oo;
my @ary = ( 0, 1, 2, 3);

grep $ary[$_ ], @ary;  # no warning

grep $ary[$oo], @ary;  # warns
[download]

Be well,
rir

Original verbose code:

Comment on uninitialized $_ sneaks past warnings in grep/map block or expression Select or Download Code

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Re: uninitialized $_ sneaks past warnings in grep/map block or expression by merlyn (Sage) on Sep 01, 2010 at 13:39 UTC
If you understand the rule that it's ok to copy undef to another variable, but not use it in an operation, all of these make perfect sense. -- Randal L. Schwartz, Perl hacker The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.	[reply]
Re^2: uninitialized $_ sneaks past warnings in grep/map block or expression by ikegami (Patriarch) on Sep 01, 2010 at 14:05 UTC
That explains the first, but `undef` isn't being copied in the `map` and `grep` statements. The OP doesn't know that `map` and `grep` set `$_`.	[reply] [d/l] [select]
Re: uninitialized $_ sneaks past warnings in grep/map block or expression by AnomalousMonk (Archbishop) on Sep 01, 2010 at 14:01 UTC
rir: Do you understand that `$_` is 'topicalized' within the body of `grep map for` (and maybe one or two more I've forgotten) statements? `>perl -wMstrict -le "print $_; my @ra = (0, 1, 2, 3); print grep { print qq{topicalized \$_: '$_'}; $ra[$_] } @ra; print $_; " Use of uninitialized value $_ in print at -e line 1. topicalized $_: '0' topicalized $_: '1' topicalized $_: '2' topicalized $_: '3' 123 Use of uninitialized value $_ in print at -e line 1.` [download]	[reply] [d/l] [select]
Re: uninitialized $_ sneaks past warnings in grep/map block or expression by mr_mischief (Monsignor) on Sep 01, 2010 at 14:09 UTC
Are you sure it's not documented in perlvar? The entry for $_ states when it is used as an iterator or the destination of an input record for a while loop. If it's not defined, it's not defined. If it is being given a value by a for loop, a while loop with a readline operator, given, grep, or map, then it is defined.	[reply]
Re^2: uninitialized $_ sneaks past warnings in grep/map block or expression by JavaFan (Canon) on Sep 01, 2010 at 16:00 UTC
If it is being given a value by a for loop, a while loop with a readline operator, given, grep, or map, then it is defined. Not if the value given isn't defined. `$ perl -wE 'say for 1, undef, 3' 1 Use of uninitialized value in say at -e line 1. 3` [download]	[reply] [d/l]
Re^3: uninitialized $_ sneaks past warnings in grep/map block or expression by mr_mischief (Monsignor) on Sep 01, 2010 at 16:05 UTC
That's a trick. You've very specifically given it a non-value. ;-)	[reply]


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