subroutine list return

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

Why doesn't $ret_val (scalar) in the following code get the value 4: the number of elements in the list returned by the test subroutine? What's happening here? ...

perl -e '
    sub test
    {
        my @array = qw{ fish, chicken, pork };
        return @array,1;
    }

    # $ret_val gets the value 1;
    $ret_val = test();

    # @ret_val get each item in @array and the last
    # index in @ret_val is 1
    @ret_val = test();

    print $ret_val . "\n";
    print @ret_val . "\n";
 
    for ( @ret_val ) { print $_ . "\n" };
'
1
4
fish,
chicken,
pork
1
[download]

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•Re: subroutine list return by merlyn (Sage) on Jun 24, 2003 at 18:03 UTC
First, your qw{} is wrong. Don't use commas there. Warnings would have told you otherwise. Second, the subroutine's last expression (the return value) is evaluated in the context of the caller. The expression: `my $ret_val = (@array, 1);` [download] doesn't assign 4 to $ret_val. It uses the comma operator to calcuate the elements of the list one by one, and then discards all but the last, thus returning 1 there. I have a column on context that you might find useful. -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply] [d/l]
Re: subroutine list return by Tomte (Priest) on Jun 24, 2003 at 18:04 UTC
Because a list, as oposed to an array, in scalar context assignment returns the last value of the list. perldoc perldata: If you evaluate an array in scalar context, it returns the length of the array. (Note that this is not true of lists, which return the last value, like the C comma operator, nor of built-in functions, which return whatever they feel like returning.) regards, tomte Hlade's Law: If you have a difficult task, give it to a lazy person -- they will find an easier way to do it.	[reply]
Re: subroutine list return by Mr. Muskrat (Canon) on Jun 24, 2003 at 18:10 UTC
To add to what's already been said (fix your `qw()`), this can be fixed easily by pushing the 1 onto the array and then returning only the array. Something like: `sub test { my @array = qw{ fish chicken pork }; # I assume it's not this simple + in your real script push @array, 1; return @array; }` [download]	[reply] [d/l]
Re: subroutine list return by NetWallah (Canon) on Jun 25, 2003 at 02:45 UTC
Agreed, chromatic - that is much cleaner. Thanks - always something to learn here.	[reply]
Re: subroutine list return by NetWallah (Canon) on Jun 25, 2003 at 01:06 UTC
I don't believe Mr Muskrat's suggestion would work. You are still in a scalar context. If you really wanted to get the count of items in the array returned, you could do it thus: `sub x{return qw/e f g h 31/}; $a= @{[x()]}; # Force array context when calling sub print $a This prints the value 5` [download] The trick with the "@{[]}" is described in perfunc, under the function "scalar".	[reply] [d/l]
Re: Re: subroutine list return by chromatic (Archbishop) on Jun 25, 2003 at 01:25 UTC
Ugh. How about something a little cleaner? `my $count = () = x(); print "$count\n";` [download]	[reply] [d/l]