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How do I localize $_ in while loops?

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Contributed by ajdelore on Jul 21, 2003 at 23:49 UTC
Q&A  > input and output


Answer: How do I localize $_ in while loops?
contributed by ajdelore

As discussed here, $_ is automatically localized in a foreach loop. So, for example:

foreach (qw/foo bar/) { print; print foreach (qw/one two three/); } # output: fooonetwothreebaronetwothree

However, a while loop does not do this automatically:

foreach (qw/foo bar/) { print; my @list = qw/one two three/; print while (shift @list); } # output: foofoofoofoobarbarbarbar

In order to do this, you need to explicitly localize $_ within the while loop:

foreach (qw/foo bar/) { print; my @list = qw/one two three/; print while (local $_ = shift @list); } # output: fooonetwothreebaronetwothree

Another way to do it:

{ local $_; print while (shift @list); }

Note that you must have the local declaration enclosed in a block with the while statement, as above. The following example will not work:

# will not localize $_ to the while loop local $_; print while (shift @list);

See perlop for more information.

Answer: How do I localize $_ in while loops?
contributed by Roy Johnson

Please note that shift does not populate $_ by default, so ajdelore's "Another way to do it" is not. The assignment still has to be explicit.

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