Re^2: Doubt on defined-ness

Actually I was about to answer just that, but then I tested it - and it turned out I was wrong.

Consider this:

$ hexdump -C foo
00000000  61 0a 30                                          |a.0|
00000003
$ perl -wle 'print "read line" while (my $x  = <>)' foo
read line
read line
$
[download]

I have a file foo here with an 'a', a newline, and a '0' - no trailing newline.

I'd expect just one line of output, because 0 is false. But why am I getting two lines? That doesn't seem to be logical to me.

So I modified my test scrpt a bit:

$ perl -MDevel::Peek -wle 'while (my $x  = <>){ print "read line"; pri
+nt "and its true" if $x }' foo
read line
and its true
read line
[download]

So, the second line evaluates to true in the while-condition, but not in the following if-condition. Is there some kind of special magic? If yes, where is it documented?

Update ok, got it. Thank you all. I didn't realize it works for arbitrary variable names, and thought that $_ was special cased.

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Re^3: Doubt on defined-ness by FunkyMonk (Bishop) on Jun 18, 2008 at 19:35 UTC
It's because perl applies its magic to your while. Deparse shows it as `while (defined(my $x = <ARGV>))` [download] Unless I state otherwise, all my code runs with strict and warnings	[reply] [d/l]
Re^3: Doubt on defined-ness by betterworld (Curate) on Jun 18, 2008 at 19:38 UTC
It's just syntactic sugar. Perlop says: The following lines are equivalent: while (defined($_ = <STDIN>)) { print; } while ($_ = <STDIN>) { print; } while (<STDIN>) { print; }	[reply]
Re^3: Doubt on defined-ness by tinita (Parson) on Jun 18, 2008 at 19:39 UTC
So, the second line evaluates to true in the while-condition, but not in the following if-condition. Is there some kind of special magic? If yes, where is it documented? the only location where i found it mentioned after a quick search is in perlvar $_: `$ARG $_ The default input and pattern-searching space. The following pairs are equivalent: while (<>) {...} # equivalent only in while! while (defined($_ = <>)) {...}` [download] edit: and in perlsyn edit: ah, it's in perlop, like betterworld mentioned. overlooked it.	[reply] [d/l]