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Re: while nested in foreach ($_ pitfalls)

by Russ (Deacon)
on Dec 01, 2003 at 17:54 UTC ( #311333=note: print w/replies, xml ) Need Help??

in reply to while nested in foreach

When I read the subject ("while nested in foreach"), I thought this would be a different problem. Just for grins, here is a "gotcha" to watch for: resetting $_.
for (1..10){ print "Before nested \$_ setter: $_\n"; open JUNK, 'junk' or die "Nope: $!"; while(<JUNK>){ # Do nothing, just let $_ get set } close JUNK; print "After nested \$_ setter: $_\n"; }
In this example (perl 5.8.0), the second print statement shows an empty $_ because the diamond operator (in the while loop) overwrote $_ (as we told it to do). So, after the nested while loop, we no longer have the old value in $_ (the counter in the for loop).

Exercises left for the reader: replace the while loop with other $_-using constructs and see how they behave...

Most perl built-ins, and many CPAN modules, are "smart" enough to localize $_ to prevent confusion, but not all.

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