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Re: When is $_ local and when is it not?

by Abigail (Deacon)
on Jul 13, 2001 at 17:17 UTC ( #96378=note: print w/ replies, xml ) Need Help??


in reply to When is $_ local and when is it not?

The answers are:

  • Yes
  • Yes
  • Just because.... ;-)
In a few cases, $_ is implicitely localized: in the block/expression of map and grep and in the block of a foreach that doesn't have its own iterator mentioned. (If the iterator is mentioned, with no my preceeding it, the iterator is localized too). This is all documented where foreach, map and grep are documented: perlsyn and perlfunc.

There is a difference between foreach and while because they are two totally different things. foreach always assigns to a variable when looping over a list, while while normally doesn't. It's just that while (<>) is an exception and only when there's a single diamond operator there's an implicite assignment to $_.

-- Abigail


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Re: Re: When is $_ local and when is it not?
by Hofmator (Curate) on Jul 13, 2001 at 17:42 UTC

    Good explanation, Abigail. I'm just curious, is there a good reason why while (<>) doesn't implicitely localize $_?

    -- Hofmator

      One possible reason for why while(<>) does not implicitly localize $_ as part of its magic is that sometimes you want to access the last value of $_ outside the loop. For example:
      while (<>) { chomp; last if /\S/; } print "You said: $_\n";
      If $_ were localized, you would have to copy the value to another variable:
      my $in; while (local $_ = <>) { chomp; $in = $_, last if /\S/; } print "You said: $in\n";
      Although it may just be as Abigail said, that foreach always assigns to a variable as part of the loop, whereas while can have anything in the conditional and (<>) is just a special case.
Re: Re: When is $_ local and when is it not?
by iakobski (Pilgrim) on Jul 13, 2001 at 17:55 UTC
    Thanks for the explanation. I guess a more thorough reading of the docs should have sorted this out for me.

    However, do you think this is the way it should work? After all you don't expect your variables to suddenly change as you go through a function. I know $_ is a global, but I imagine a lot of people write code like the stuff I was maintaining where there is a call to a sub in the middle of using $_. And then maybe that calls a sub and that calls another one, etc. Then someone comes along one day and puts while(<INPUT>) in one of those subs, all careful with use strict and warnings, but suddenly some bit of code miles away just breaks!

    Anyway, I know I'm getting a bit hot under the collar over this, perhaps because I spent about an hour not knowing why my change had broken the whole program. But shouldn't there be some kind of health warning at least over the use of while(<>) since it must be one of the first idioms beginners learn.

    I know I'm going to write

    local $_; while(<>){ }
    from now on!

    update No I'm not I'm going to follow jeffa's example! Doh! I might go through and put local in the old "code"/plate of spaghetti that I'm supposed to be maintaining.

    -- iakobski

      ... or better yet, just don't use $_ in such situations. You hit the nail on the head when you mentioned someone coming along and breaking it.
      my $line; while ($line = <>) { # stuff }
      i gonna wash that ambiguity outta my hair . . .

      Jeff

      R-R-R--R-R-R--R-R-R--R-R-R--R-R-R--
      L-L--L-L--L-L--L-L--L-L--L-L--L-L--
      
      Yes, I think it should work that way. Then you can write code like:
      sub next_foo { while (<>) { return $. if /^FOO:/ } } while (next_foo) { do_something # with $_ }
      Some people won't like this style. That's fine, you don't have to. Other will like it, and Perl let's you.

      -- Abigail

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