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Think about Loose Coupling

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the $_ inside the grep is an alias of the hash-values, i.e. any assignment would be mirrored.

For reasons that I don't understand yet (under-coffeination perhaps) this triggers an autovivification.

compare Re^2: unexpected modify hash in a distance with grep { $_ }

This can easily circumvented with a more common approach to only pass the keys.

#!/usr/bin/perl use strict; use warnings; use Data::Dumper; my $h = { 'a' => 1, 'b' => 2, 'c' => 3, }; warn Dumper [ $$h{a}, $$h{x}, $$h{b}, $$h{c} ]; warn Dumper [ keys %$h ]; warn Dumper [ grep {$$h{$_}} qw/a b c x/ ]; # <-- no side effect warn Dumper [ keys %$h ];

C:/Perl_524/bin\perl.exe d:/tmp/pm/ $VAR1 = [ 1, undef, 2, 3 ]; $VAR1 = [ 'a', 'c', 'b' ]; $VAR1 = [ 'a', 'b', 'c' ]; $VAR1 = [ 'a', 'c', 'b' ];

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

) my guess is it's an implementation detail/bug

In reply to Re: unexpected modify hash in a distance with grep { $_ } by LanX
in thread unexpected modify hash in a distance with grep { $_ } by leszekdubiel

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