Beefy Boxes and Bandwidth Generously Provided by pair Networks
more useful options

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??
Fellow Monks,

I have just spent an hour debugging a very strange problem, only to discover that I have been bitten by the behaviour of each. While all this is properly documented (each), I was not aware of it, so I thought I'd bring it up here as a caveat to remember.

The subroutine in the following snippet works only once (or rather every other time).

use strict; use warnings; our %hash = ( 1=> 2, 3=>4 ); sub find_2{ while (my ($key,$val) = each(%hash)) { return "The key for 2 is $key\n" if $val == 2; } die "could not find 2"; } print find_2; print find_2;
After the first call, it dies:
The key for 2 is 1 could not find 2 at line 11.
What happens here is that the subroutine does not search through the whole hash (it short-cicuits for performance reasons by returning the solution as soon as it has found it), and as a result, the next call to each on the same hash (even in completely different parts of the code!) will not start at the beginning again, but pick up where the first call left off.

Solution: Either go through the whole hash, or rewind it after using each:

if ($val == 2){; #rewind each my $a = scalar keys %hash; return "The key for 2 is $key\n" }

In reply to Ouch! Each! Reentrant it is not by Thilosophy

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?

    What's my password?
    Create A New User
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others imbibing at the Monastery: (6)
    As of 2020-05-31 11:29 GMT
    Find Nodes?
      Voting Booth?
      If programming languages were movie genres, Perl would be:

      Results (173 votes). Check out past polls.