Confusion with context

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;
}
[download]

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

thinker

Comment on Confusion with context Select or Download Code

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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;` [download] "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.	[reply] [d/l]
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);` [download] 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());` [download] 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 ~~array~~list containing the contents of each of the capturing parentheses, hence your needing the [0] in the second version). davis 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.	[reply] [d/l] [select]


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