While on hashes doesn't work?

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

#!/usr/sbin/perl -w
use strict;
use diagnostics;

# QUESTION:
# Why does the for loop execute and the while loop does not?
# Obviously the while conditional is not evaluating true, but why? The
+re are still elements in the list...?
# Also, in the case of longer hashes, wouldn't the while be more effic
+ient?

my %data = (    a01    => "two",
        a02    => "peas",
        a03    => "in",
        a04    => "a",
        a05    => "pod."
);

for (sort (keys %data)) {
    print "for: $data{$_}\n";
}

while (sort (keys %data)) {
    print "while: $data{$_}\n";
}
[download]

Thanks,
Alex

Edit by tye to replace PRE tags around long lines with CODE tags

Comment on While on hashes doesn't work? Download Code

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Re: While on hashes doesn't work? by gellyfish (Monsignor) on Feb 04, 2002 at 22:27 UTC
Simply put because `while` doesn't work like that :) Explicitly, `while` doesn't iterate over the items of a list in the way that `for` does and it doesn't set `$_` unless the expression is the diamond operator. To get your example to work with `while` you will need to do something like : `my @foo = sort (keys %data); while( $_ = shift @foo ) { print "while: $data{$_}\n"; }` [download] /J\	[reply] [d/l] [select]
Re: While on hashes doesn't work? by mkmcconn (Chaplain) on Feb 04, 2002 at 23:26 UTC
Not recommending it, but perhaps it gives insight into how while works (in addition to what gellyfish said above) `#!/usr/bin/perl -w use strict; my %data = ( a01 => "two", a02 => "peas", a03 => "in", a04 => "a", a05 => "pod." ); while (($_) = sort keys %data){ print "while: $data{$_}\n"; delete $data{$_}; } print "while loop terminates because", "%data is now empty >", %data,"< \n"; __END__` [download] Without those parens around $_, I would expect it to try to pass '5' into the block (which it would do if you took away the sort function), but it doesn't do that. I suppose that might qualify as an insignificant bug. update: thanks to merlyn for fixing the bug in my understanding of how sort behaves. mkmcconn </code>	[reply] [d/l]
Re: Re: While on hashes doesn't work? by merlyn (Sage) on Feb 04, 2002 at 23:38 UTC
`while (($_) = sort keys %data){ print "while: $data{$_}\n"; delete $data{$_}; }` [download] This is a pretty expensive loop. You'd be much better doing: `while (my ($k, $v) = each %data) { print "key is $k, value is $v\n"; }` [download] Without those parens around $_, I would expect it to try to pass '5' into the block (which it would do) if you took away the sort function, but it doesn't do that. I suppose that might qualify as an insignificant bug. No, sort in a scalar context always returns undef. Maybe someday, it'll return something useful. But not yet. No bug, except in perhaps your understanding. {grin} -- Randal L. Schwartz, Perl hacker	[reply] [d/l] [select]