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Re: opening a file destroys nulling entries in a list?!?!

by cfreak (Chaplain)
on Aug 07, 2003 at 02:56 UTC ( #281712=note: print w/ replies, xml ) Need Help??


in reply to opening a file destroys nulling entries in a list?!?!

$_ is a global variable. Your while is setting it to the last record of your filehandle and since your files end with a \n, $_ returns blank (I guess $_ gets set before the test is done to see if it has a value). Since this is Perl there is more than one way to get around this:

You can localize $_ in your sub like so:

sub screwed { my ($in) = @_; local $_; # basically a temporary $_ just for this sub open(IN, "<$in"); while(<IN>) { print "file $in contains $_"; } close(IN); }
Or you can set each line of the file to a variable in the while like so:
while(defined(my $foo = <IN>)) { print "file $in contains $foo"; }

Hope that helps

Lobster Aliens Are attacking the world!


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Re: Re: opening a file destroys nulling entries in a list?!?!
by sgifford (Prior) on Aug 07, 2003 at 03:32 UTC
    To elaborate a bit (it took me some experimenting to realize why cfreak is right), inside a map the variable $_ is magic. While each entry in the list is evaluated, $_ is an alias to that entry. Assigning a new value to $_ changes the value of the current list entry. Because you're calling screwed from within map without localizing $_, it's being assigned to each successive line in the file, and then set to undef to indicate end-of-file. When screwed returns, $_ is still set to undef, so all entries in the list are changed to undef.

    You can verify this by changing screwed to:

    sub screwed { $_ = 'hi'; }
    in which case you'll get:
    before a
    before b
    after hi
    after hi
    
Re: Re: opening a file destroys nulling entries in a list?!?!
by bobn (Chaplain) on Aug 07, 2003 at 03:38 UTC

    How come while(<IN>){}, but not for (@x) {}? why is while (<IN>) so special?

    I love perl, but this scares the hell out of me.

    --Bob Niederman, http://bob-n.com

      Nevermend. I think I get it - someone else posted that while(<>){} is the same as while($_=<>){} - it does an assignment to $_ and that's what does the damage.

      Somehwere, I think I remember someone (Tom Christiansen, I think) saying to always localize $_ in subroutines. Now I know why.

      --Bob Niederman, http://bob-n.com

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