Wanted further clarification on Data Usage

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

In my talk with davorg he mentioned that i may have had
a bug in my program... here's the setup:

I use a variable named $thisDataset as a pointer
to a Hash table. i then address %thisDataset
at a later time. Let's clear this up a bit...

    
    my $filehandle = &magic_box(@ARGV);  
    my $thisDataset = &doStuff($filehandle);
{
my (%HASH);  #this wasn't here till after talking to dave :)
             #if you'd like to comment on the effects with 
             #and without i'd be very interested :)
sub doStuff {
    while(<>) {
           my @array = split(/,/);
           my $key = shift @array;
           $HASH{$key} = [@array];
    }                             
    return(\%HASH); # this is the same as $thisDataset now
}                   # at least, $thisDataset is the same as 
}                   # this... :)   
&someThingElse($thisDataset);
sub someThingElse {
   foreach my $KEY (sort keys %{$_[0]}) {
              print "$KEY\n"; 
              foreach my $VALUE (@{$_{$KEY}}) {
                     print "$VALUE,";
              }
              print "\n";
   }  # in the real world, i do more with this stuff but i 
}     # think this illustrates the point adequately
[download]

notice how it's listed as %{$_[0]} in the second function?
that's the same as saying %thisDataset (IIUC)
now that's aparently ALSO an error or potential error and i don't understand
exactly why

Comment on Wanted further clarification on Data Usage Select or Download Code

Replies are listed 'Best First'.
Re: Wanted further clarification on Data Usage by davorg (Chancellor) on Jul 11, 2000 at 19:05 UTC
To summarise even further :) The problem I saw was this... You have a function which returns a reference to a hash like this: `sub make_hash { my %hash; # Do clever stuff to fill in values in hash return \%hash; }` [download] which you call to get a reference to a hash which you store in a scalar called `$thisDataset` like this: `my $thisDataset = make_hash();` [download] Later on you pass this hash to another function which assumes that its first parameter is a hash reference like this: `sub do_stuff_to_hash { foreach (keys %{$_[0]}) { # do something with each key } } do_something_with_hash($thisDataset);` [download] All this is fine, but later still you suddenly start to treat `$thisDataset` as if it's a hash by accesing it like this: `$thisDataset{$curr_month};` [download] This is the error that `use strict` picked up as it realised that you were accessing both the scalar `$thisDataset` and the hash `%thisDataset`. I suspect that want you really wanted to do was something like: `$thisDataset->{$curr_month};` [download] `use strict` is your friend. Use `use strict`. -- <http://www.dave.org.uk> European Perl Conference - Sept 22/24 2000, ICA, London <http://www.yapc.org/Europe/>	[reply] [d/l] [select]
Re: Wanted further clarification on Data Usage by ZZamboni (Curate) on Jul 11, 2000 at 18:57 UTC
There's no real question in your post, so I'll just comment on it :-) I don't see any real problems with it (other than the fact that I prefer to group function definitions and not mix them with the rest of the code). The `my (%HASH)` has the effect of localizing the %HASH variable to the block where the `doStuff` function is declared, effectively making it invisible to everyone else and accessible only through that function. In this sense it is a good thing to have the my statement. If you remove it, the %HASH variable will be accessible from other parts in the program, which may or may not be what you want. You mention `%thisDataset` a couple of times, but there is no such variable in your program. You have `%HASH`, and you have `$thisDataset`, which is a pointer to `%HASH`. The use of `%{$_[0]}` in the second function looks good to me, but the `@{$_{$KEY}}` does not. I think it should be `@{$_[0]->{$KEY}}`. It may make things clearer to assign `$_[0]` to another variable, like this: `sub someThingElse { my $data=shift; # then you can use %{$data} and @{$data->{$KEY}} }` [download] --ZZamboni	[reply] [d/l]


We don't bite newbies here... much
	PerlMonks