strat has asked for the wisdom of the Perl Monks concerning the following question:

Dear monks,

I've got a problem with reverse in map: 

C:\ce3\bin>perl -w
my @list = qw(abc2 abc3 abc1 abc0);
my @list2 = map { my $r = reverse($_); $r } @list; # scalar context
print "@list2";
^Z
2cba 3cba 1cba 0cba
C:\ce3\bin>

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The code above works as I want it to work: it reverses every element in the list as string. But this code below doesn't reverse the strings: 
C:\ce3\bin>perl -w
my @list = qw(abc2 abc3 abc1 abc0);
my @list2 = map { reverse($_) } @list;
print "@list2";
^Z
abc2 abc3 abc1 abc0

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I think, in the second example, the reverse-function is somehow used in list-context, not in scalar context. Is this a perl bug, or am I the bug (as usual)?

I've run these codes under Activestateperl 631 (Win2k) and Perl 5.005_03 under Solaris.

Best regards,
perl -le "s==*F=e=>y~\*martinF~stronat~=>s~[^\w]~~g=>chop,print"

Replies are listed 'Best First'.
Re: Bug? with reverse in map
by ariels (Curate) on May 14, 2002 at 13:29 UTC

    map does, indeed, evaluate its BLOCK in list context. This is so you can say stuff like 

    my $n=0;
my %position = map {$_ => $n++} @list

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    The simple fix is to make your block evaluate reverse in scalar context: map { scalar reverse($_) } @list.

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      I'm appalled that the first thing I thought of to scalarify the return from reverse was: 
      my @list2 = map { ~~reverse } @list;

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      Too much golf!
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Re: Bug? with reverse in map
by Joost (Canon) on May 14, 2002 at 13:31 UTC
    The interior subroutine / block in a map is called in list context, this is not a bug, as it allows for things like: 
    %seen = map { $_ => 1 } qw( some list );

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    and other (maybe more useful) things... </code> 
    -- 
Joost       downtime n. The period during which a system
            is error-free and immune from user input.

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