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readdir and while - magic?

by bjelli (Pilgrim)
on Jan 03, 2003 at 15:16 UTC ( #224035=perlquestion: print w/replies, xml ) Need Help??
bjelli has asked for the wisdom of the Perl Monks concerning the following question:

dear fellow monks,

why does ...

opendir(D, "test"); while ( $file = readdir(D) ) { print "$file is "; print ($file ? "true" : "false"); print "\n"; }

... work correctly, even for a filename like "0" that evaluates to false in boolean context? (see here for full code + output)

This seems to be an extreme case of "do-what-i-mean". I find it a little scary, because I don't know how it does what I mean ;-)

I hit on this while doing research for my talk on php an perl at this years german perl workshop. the php counterpart looks appalingly complicated:

# this is php, not perl! $d = opendir('test); while (false !== ($file = readdir($d))) { print "$file is "; print ($file ? "true" : "false"); print "\n"; }

see also the php documentation.

Brigitte    'I never met a chocolate I didnt like'    Jellinek

Replies are listed 'Best First'.
Re: readdir and while - magic?
by jmcnamara (Monsignor) on Jan 03, 2003 at 15:28 UTC

    perl adds a defined to the while so you get something like this:
    $ perl -MO=Deparse -e 'while($file = readdir(D)) { print $file; }' while (defined($file = readdir D)) { print $file; } -e syntax OK

    This is explained in more detail at A most obscure bug.


Re: readdir and while - magic?
by broquaint (Abbot) on Jan 03, 2003 at 15:22 UTC
    why does ... work correctly, even for a filename like "0" that evaluates to false in boolean context?
    Because it really looks like this under the hood
    opendir(D, "test"); while ( defined( $file = readdir(D) ) ) ...
    This is a DWIM perlism, nifty huh? I think it applies to any assignment in a while statement where the result of the assignment is evaluated for the condition, or something along those lines.


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