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Confusion with context

by thinker (Parson)
on Mar 11, 2005 at 10:39 UTC ( #438593=perlquestion: print w/replies, xml ) Need Help??
thinker has asked for the wisdom of the Perl Monks concerning the following question:

Hello everyone

I was writing a function to extract enum values from a mysql table, when I encountered something which confused me. My first attempt was

sub get_enums { my ($field, $table) = @_; my $dbh = get_ka_dbh(); my $sql = qq/SHOW COLUMNS FROM $table LIKE "$field"/; my $sth = $dbh->prepare($sql) or die $!; $sth->execute(); my $ref = $sth->fetchrow_arrayref(); my ($f) = $ref -> [1] =~ /enum\('(.*)'\)/ ; return split "','", $f; }

This works fine, but I felt there should be no need for the temporary $f variable

My first attempts at removing it brought failure, as the $ref -> [1] =~ /enum\('(.*)'\)/ was returning in the wrong context (I think), and returning "1".

After some experimentation I discovered that what I wanted could be achieved by  return split  "','",   ( $ref->[1] =~ /enum\('(.*)'\)/ )[0];

I have decided that my original version, with temp variable, is clearest, and will be what i use, but I wonder if anyone could kindly explain what is happening here, and why I need the [0] at the end

Thank You


Replies are listed 'Best First'.
Re: Confusion with context
by Roy Johnson (Monsignor) on Mar 11, 2005 at 14:30 UTC
    You may or may not consider this clearer:
    my $ref = $sth->fetchrow_arrayref(); $ref->[1] =~ /enum\(/g or return (); return $ref->[1] =~ /\G'(.*?)',?/g;
    "Find the start of the enum, then extract whatever is between quotes and followed optionally by a comma."

    Caution: Contents may have been coded under pressure.
Re: Confusion with context
by davis (Vicar) on Mar 11, 2005 at 10:50 UTC
    (sidestepping your actual question...)
    what about:
    #!/usr/bin/perl use warnings; use strict; use Data::Dumper; my $array_ref = [qw/one two three four/]; $array_ref->[1] =~ s/w//g; print Dumper($array_ref);
    This avoids the temporary variable, and looks pretty readable to me.

    Ignore me — I didn't see the capturing parentheses. {sigh}. /me goes to find coffee.

    Update: I'd probably write it like this:

    #!/usr/bin/perl use warnings; use strict; use Data::Dumper; sub foo { my $array_ref = ["foo" , "enum('bar,baz')"]; if($array_ref->[1] =~ /enum\('(.*?)'\)/) { return split ",", $1; } } print Dumper(foo());
    The temporary variable you're talking about is automatically created (called "$1") containing the contents of the capturing parentheses. If you had more than one capturing set, they'd be returned as ($1, $2) (ie the match operation returns an arraylist containing the contents of each of the capturing parentheses, hence your needing the [0] in the second version).

    It wasn't easy to juggle a pregnant wife and a troubled child, but somehow I managed to fit in eight hours of TV a day.

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