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

Hi Monks, I am trying to get a simple subroutine to work, whereby I am passing it two hashes by reference and returning two arrays by reference. Anyway, its not behaving as I hoped and I wondered if anyone could explain to me why/ give me a few pointers!? Cheers. 
#The code not working

my ($common_ref, $total_ref) = calc_enthalpy (\%hash, \%enthalpy_hash)
+;

@total = @$total_ref;
@common = @$common_ref;

print "TEST: @total  @common<P>";


  sub calc_enthalpy   {

                      my ($hash, $enthalpy_hash) = @_;
                      my (@common, @total);

                             foreach (keys %enthalpy_hash)      {


                                      if (exists $hash{$_})    {
                                              push @common, "$_ = ", $
+enthalpy_hash{$_} * $hash{$_}, "\n";
                                              push @total, $enthalpy_h
+ash{$_} * $hash{$_}, "\n";
                                       }

                                    return (\@common, \@total);

                             }

   }

[download]
The next snippet is the code working perfectly not in a sub-routine. 

foreach (keys %enthalpy_hash)      {


        if (exists $hash{$_})    {
                push @common, "$_ = ", $enthalpy_hash{$_} * $hash{$_},
+ "\n";
                push @total, $enthalpy_hash{$_} * $hash{$_}, "\n";
        }
#  etc 

}

[download]

Replies are listed 'Best First'.
Re: subroutines - passing hashes by reference
by perlplexer (Hermit) on May 01, 2003 at 15:12 UTC
    Not using strict, eh?
    If you did, Perl would compain right here: 
    foreach (keys %enthalpy_hash) { ...

    [download]
    and here 
    if (exists $hash{$_})    { ...

    [download]
    Since you're passing hash refs, you need to treat them accordingly;i.e, 
    foreach (keys %$enthalpy_hash) { ...

    [download]
    and 
    if (exists $hash->{$_})    { ...

    [download]
    Those are just a couple of examples, of course. There's a few other places where you used hash refs improperly.

    Time to read perlref. :)

    --perlplexer
[reply]
[d/l]
[select]
      Hi, thanks for your advice but I am using strict! Is this any better?? It atill doesn't work right ;-( 
       sub calc_enthalpy   {

                      my ($hash, $enthalpy_hash) = @_;
                      my (@common, @total);

                             foreach (keys %$enthalpy_hash)      {


                                      if (exists $hash->{$_})    {
                                              push @common, "$_ = ", $
+enthalpy_hash->{$_} * $hash->{$_}, "\n";
                                              push @total, $enthalpy_h
+ash->{$_} * $hash->{$_}, "\n";
                                       }

                                    return (\@common, \@total);

                             }

   }

      [download]
[reply]
[d/l]
        OK, then Perl doesn't complain because you have global variables with the same names.
        Please see other comments as well. Your return() is most likely misplaced.

        --perlplexer
[reply]
Re: subroutines - passing hashes by reference
by broquaint (Abbot) on May 01, 2003 at 15:14 UTC
    Aha, your problem there is that you are treating a scalar has reference as a hash, however they are two distinct things. What you need to be doing is dereferencing your hash reference when using it e.g 
    foreach (keys %$enthalpy_hash) {
  if(exists $hash->{$_}) {
    push @common, "$_ = ", $enthalpy_hash->{$_} * $hash->{$_}, "\n";
    push @total, $enthalpy_hash->{$_} * $hash->{$_}, "\n";
  }
}

    [download]
    See. tye's References quick reference and the venerable perlref for more info on references and dereferencing in perl. On a side note the likes of strict will catch this sort of error upon a simple perl -c.
    HTH

    _________
    broquaint

[reply]
[d/l]
Re: subroutines - passing hashes by reference
by jasonk (Parson) on May 01, 2003 at 15:14 UTC

    That return(\@common, \@total) probably needs to be closer to the end of the subroutine, the way you have it written now, you are returning after checking only one of the keys of %enthalpy_hash.

    We're not surrounded, we're in a target-rich environment!
[reply]
[d/l]
Re: subroutines - passing hashes by reference
by Mr. Muskrat (Canon) on May 01, 2003 at 15:18 UTC

    In addition to what perlplexer said, try moving your return outside the foreach loop :-)

    What happened? I thought I wrote this right after perlplexer posted his reply. /me shrugs.

[reply]
      many many thanks - moved the return and its perfect!
[reply]
        A side note - really just a personal preference of mine that might help you. Name variables what they are - if a scalar is a reference to a hash, name it as such. For example, if you pass a reference to a 'user' hash in to subroutine print_user, then do it this way: 
           sub print_user {
      my $user_hashref = shift;
      print "user name = $user_hashref->{'name'}\n";
      print "user age  = $user_hashref->{'age'}\n";
   }

   ### main part of code ###
   my %user = { "name" => "John Doe",
                "age"  => 12 }
   print_user(\%user);

        [download]
        That way, you always know that scalar $user_hashref is really just a *reference* to the user hash, and it needs to be treated like a reference to a hash, instead of like an actual hash.

        HTH.
[reply]
[d/l]

Back to Seekers of Perl Wisdom