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Re: Re: while(<>) { ... } considered harmful

by Ovid (Cardinal)
on Sep 08, 2002 at 00:58 UTC ( #195948=note: print w/ replies, xml ) Need Help??


in reply to Re: while(<>) { ... } considered harmful
in thread while(<>) { ... } considered harmful

Unless I totally misunderstood the above comments -- which is possible as I just briefly skimmed them -- the problem here is not that map and grep don't localize $_, but that they may execute code that may alter the value of $_, which in this case is aliased to a constant. Consider this code:

$_ = 'Ovid'; %hash = map { $_ => 1 } qw/ foo bar baz /; print;

That code will print 'Ovid' because $_ has been localized within the map statement. Now, if you try to alter the $_, your code fails because you're trying to alter a constant:

$_ = 'Ovid'; %hash = map { $_ .= 'asdf'; $_ => 1 } qw/ foo bar baz /; print;

To get around that, you can do this:

$_ = 'Ovid'; %hash = map { local $_ = $_; $_ .= 'asdf'; $_ => 1 } qw/ foo bar baz /; print;

That allows you to assign the aliased constant to a real variable, thus avoiding the problems outlined in this thread.

Cheers,
Ovid

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