readdir and while - magic?

[bjelli]

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 ;-)

p.s.
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
http://www.horus.com/~bjelli/         http://perlwelt.horus.at

[jmcnamara]

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.

--
John.

[broquaint]

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.
HTH

_________
broquaint