using map to replace text without using a foreach....

dark314 has asked for the wisdom of the Perl Monks concerning the following question:

Question: Both of these methods seem to be the same. The map method is, of course, shorter...Why doesn't it work is my question.

here is my map strategy:

$MAIL_ROOT = '/var/mail';

my @users = (map  s/$MAIL_ROOT// , `find $MAIL_ROOT -type d -maxdepth 
+1`);
[download]

and here is the foreach that does what I want, I want to do a map instead of this foreach.

my @users = `find $MAIL_ROOT -type d -maxdepth 1`;
foreach (@users) {
    chomp;


    s/$MAIL_ROOT//;
    print $_ . "\n" unless $_ eq '';


}
[download]

is there some reason map is not working correctly?

-----------------------------------------------Update --------------------------------------------

What it might be perhaps...

$_ is undefined because its finding $_ for @users on that same line??? Is this my problem? In other words, we make sure to return $_ and we're in good shape.

-------------------Solution---------------

Ah, I see, I was assigning the value to s/// and mapping incorrectly: correct way:
chomp( my @users = map { s/$MAIL_ROOT//; $_ } `find $MAIL_ROOT -type d
+ -maxdepth 1` );
[download]
from jwkrahn is prob the least effort for a fix and the one i went with :)

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Re: using map to replace text without using a foreach.... by ikegami (Patriarch) on Aug 17, 2006 at 19:57 UTC
Check up on the return value of `s///`. In this case, your `s///` always returns `1` (the number of substititions made in that iteration). `map` takes all those `1`s and assigns them to `@users`. You need to return the changed value to `map`. Also, your usage of `map` clobbers the value passed to `map`. True, it doesn't matter in this instance, but it's a bad habit. my @users = map { my $s = $_; # Protect the caller. chomp($s); $s =~ s/^\Q$MAIL_ROOT//; $s } `find ...`; # Return the changed value. [download] Both List::MoreUtils and Algorithm::Loops provide a function that does this magic for you. use List::MoreUtils qw( apply ); my @users = apply { chomp; s/^\Q$MAIL_ROOT//; } `find ...`; [download] use Algorithm::Loops qw( Filter ); my @users = Filter { chomp; s/^\Q$MAIL_ROOT//; } `find ...`; [download] Update: I fixed your regexp.	[reply] [d/l] [select]
Re: using map to replace text without using a foreach.... by jwkrahn (Abbot) on Aug 17, 2006 at 20:25 UTC
map returns the result of its expression and `s///` returns either TRUE or FALSE so your array is receiving these true or false values. You need to return `$_` after `s///` has modified it: chomp( my @users = map { s/$MAIL_ROOT//; $_ } `find $MAIL_ROOT -type d + -maxdepth 1` ); [download] However what you really should be doing is: `$MAIL_ROOT = '/var/mail'; opendir my $dh, $MAIL_ROOT or die "Cannot open '$MAIL_ROOT' $!"; my @users = grep -d && !/\A\.\.?\z/, readdir $dh; closedir $dh;` [download]	[reply] [d/l] [select]