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Re: Inverting a hash
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From:
Shawn H Corey
Date:
May 9, 2010 07:50
Subject:
Re: Inverting a hash
Message ID:
4BE6CBC2.5020207@gmail.com
Harry Putnam wrote:
> I have an example from Shawn C. from another thread, presented here
> out of context. The code does just what he meant it to do.
> It inverts a hash.
>
> I'm trying to understand what is going on inside the sub function
> `invert()'.
>
> ------- --------- ---=--- --------- --------
>
> my %hash = (
> './b/fb' => 'fb',
> './b/c/fd' => 'fd',
> './b/l/c/f2' => 'f2',
> './b/g/f/r/fc' => 'fc',
> './b/g/h/r/fb' => 'fb'
>
> );
>
> ## %hash is sent to an inversion sub function
> my %inv_hash = invert( \%hash );
>
> sub invert {
> my $h = shift @_;
> my %inv = ();
>
> while( my ( $k, $v ) = each %{ $h } ){
> push @{ $inv{$v} }, $k;
> }
> return %inv;
> }
>
> ------- --------- ---=--- --------- --------
>
> What is actually being sent at
> my %inverse_hash = invert( \%hash );
A reference to the hash %hash is being sent.
>
> Inside the sub function:
> incoming data is seen as array (@_)
> What happened to the hash? Is it flattened or what?
It's still there. The array @_ has a reference to the hash as its first
value.
>
> What is actually being shifted off here
> my $h = shift @_; (Just one element?)
The reference is stored in the variable $h.
>
> Here it appears $h is expected to have two parts.
> So apparently sending \%hash to this sub function has flattened it
> into an array where each elements is made up of the Key and value
> of the former array...
>
> while( my ( $k, $v ) = each %{ $h } ){
The function each only works on hashes. $h is dereference with %{ $h }
See `perldoc -f each`
>
> It appears above that $h has all the former parts of the hash
> else how can it be looped?
>
> but at `my $h = shift @_;'
> It looks like only one is shifted.
Yes, only one argument is sent to the function, the reference to the hash.
>
> Then at:
>
> push @{ $inv{$v} }, $k;
>
> It appears the parts to an array are again flattened into
> an array
If $inv{$v} does not exists, then this statement create an empty,
anonymous array and pushes the value of $k onto it. If $inv{$v} exists,
it must be a reference to an array or this will cause a fatal error.
>
> Then here:
> return %inv;
>
> It magically becomes a hash again only upside down.
> and of course same name keys are zapped.
>
> %inv_hash is now:
> 'fc' => './b/g/f/r/fc'
> 'f2' => './b/l/c/f2'
> 'fb' => './b/fb' (might be './b/g/h/r/fb'. One was deleted)
> 'fd' => './b/c/fd'
>
> or seen through Data::Dumper:
>
> Key fc | value ARRAY(0x8bb29c0)
> Key f2 | value ARRAY(0x8bb2a60)
> Key fb | value ARRAY(0x8bb29f0)
> Key fd | value ARRAY(0x8bb2a20)
>
> There is a lot happening in there... but I'm not following how it
> happens.
>
>
my %inv_hash = invert( \%hash );
print '%inv_hash: ', Dumper \%inv_hash;
This will output:
%inv_hash: $VAR1 = {
'f2' => [
'./b/l/c/f2'
],
'fb' => [
'./b/fb',
'./b/g/h/r/fb'
],
'fc' => [
'./b/g/f/r/fc'
],
'fd' => [
'./b/c/fd'
]
};
Note that both values for fb are preserved.
--
Just my 0.00000002 million dollars worth,
Shawn
Programming is as much about organization and communication
as it is about coding.
I like Perl; it's the only language where you can bless your
thingy.
Eliminate software piracy: use only FLOSS.
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