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Modify hash

zapzap
User

Oct 6, 2013, 1:12 PM

Post #1 of 9 (3396 views)

Modify hash	Can't Post

Greetings. I'm trying to reverse a hash so that the keys become values and values become keys. But...... I want the keys that have matching values with other keys to become an anonymous array. For example:

%h1 = { k => "dog", j => "cat", l => "dog" }

would yield

%h2 = { "dog" => [k,l], "cat" => j}

For the most part, I've already completed the code, so what I'm asking is how you would 'clean' my attempt up, or offer a different approach. Thank you very much all.

Code

sub uniq { 
  my %seen; 
  return grep { !$seen{$_}++ } @_; 
} 
 
&uniq(values %os); 
my %hash; 
my $temp; 
 
for my $val ( &uniq(values %os) ) { 
  $temp = []; 
  for my $k ( keys %os ) { 
    if( $os{$k} eq $val ) { 
      push @$temp, $k; 
    } 
  } 
  $hash{$val} = $temp; 
}

Note: I had to use my own uniq function because I am using version 5.10

BillKSmith
Veteran

Oct 6, 2013, 3:24 PM

Post #2 of 9 (3393 views)

Re: [zapzap] Modify hash [In reply to]	Can't Post

Checkout the module Hash::MoreUtils
Good Luck,
Bill

zapzap
User

Oct 6, 2013, 3:31 PM

Post #3 of 9 (3392 views)

Re: [BillKSmith] Modify hash [In reply to]	Can't Post

Thanx for the advice. I will check it out. But I still would like to improve this without a module. Maybe using grep?

BillKSmith
Veteran

Oct 6, 2013, 8:32 PM

Post #4 of 9 (3386 views)

Re: [zapzap] Modify hash [In reply to]	Can't Post

Your first call to uniq does not do anything useful.

The & on a subroutine call is seldom needed. Do not use it where it is not needed. In this case you really do not need uniq at all.

I like your suggestion of grep.

Code

use strict; 
use warnings; 
use Data::Dumper; 
my %h1 = ( 
    k => "dog", 
    j => "cat", 
    l => "dog", 
);  
 
my %h2 = %{invert_hash(\%h1)}; 
print Dumper \%h2; 
 
sub invert_hash { 
    my %os = %{$_[0]}; 
    my %hash; 
    for my $val (  values %os ) { 
        next if exists $hash{$val}; 
        $hash{$val} = [grep {$os{$_} eq $val} keys %os] 
    } 
    return \%hash; 
}

Good Luck,
Bill

2teez
Novice

Oct 7, 2013, 12:44 AM

Post #5 of 9 (3381 views)

Re: [zapzap] Modify hash [In reply to]	Can't Post

I would rather do this instead.

Code

use warnings; 
use strict; 
use Data::Dumper; 
 
my %hash1 = ( 
    i => 'dog', 
    j => 'cat', 
    k => 'dog', 
); 
 
my %hash2; 
 
for ( keys %hash1 ) { 
    if ( exists $hash2{ $hash1{$_} } ) { 
        if ( !ref $hash2{ $hash1{$_} } ) { 
            $hash2{ $hash1{$_} } = [ $hash2{ $hash1{$_} } ]; 
        } 
        push @{ $hash2{ $hash1{$_} } }, $_; 
    } 
    else { 
        $hash2{ $hash1{$_} } = $_; 
    } 
} 
 
print Dumper \%hash2;

BillKSmith
Veteran

Oct 7, 2013, 10:19 AM

Post #6 of 9 (3372 views)

Re: [2teez] Modify hash [In reply to]	Can't Post

Great idea. There is little doubt that your algorithm is much faster than either of the others. Even more important, it raises the issue of what to do when the original value is unique. Should the inverse be the value of the corresponding key or a reference to an array containing only one element, the value of that key.

Can anyone suggest a good reason to favor either one?
Good Luck,
Bill

Zhris
Enthusiast

Oct 7, 2013, 10:42 AM

Post #7 of 9 (3368 views)

Re: [BillKSmith] Modify hash [In reply to]	Can't Post

I favour using an array reference containing only one element. My reason is that we can treat every value as an array when processing the new hash, and not have to ref test each value to decide how to handle (again).

Although, it depends what the data is for. If serializing via i.e. XML::Simple XMLout(), the resultant structure may vary as per which choice was made (untested).

Chris

2teez
Novice

Oct 7, 2013, 11:52 AM

Post #8 of 9 (3360 views)

Re: [BillKSmith] Modify hash [In reply to]	Can't Post

Quote

There is little doubt that your algorithm is much faster than either of the others

I have not Benchmark any of the algorithm to tell which is faster, but why would I want to "grep" for every value ?

Quote

Should the inverse be the value of the corresponding key or a reference to an array containing only one element, the value of that key.

Bill, there is no one size fit all solution as you know. The solution given was in respect to the dataset shown by the OP.
Thanks.

Zhris
Enthusiast

Oct 7, 2013, 3:01 PM

Post #9 of 9 (3349 views)

Re: [2teez] Modify hash [In reply to]	Can't Post

Quote

2teez
The solution given was in respect to the dataset shown by the OP.

Although the OP described yielding { "dog" => [k,l], "cat" => j}, their code actually yields { "dog" => [k,l], "cat" => [j]}. This led to which output is best as oppose to which is right, after all the OP didn't suggest their code doesn't work the way they would like.

I found this of interest, since I commonly face this "dilemma". If there is alot of data and mostly single values / only one or two multiple values then I might use a combination of arrays and strings, since in this case it would be inefficient to treat every value as an array. If the other way round, then I would probably use all arrays, since it wouldn't be much harm to treat the one or two single values as an array. With a small dataset, this isn't of concern.

Quote

2teez
but why would I want to "grep" for every value

You are right, Bill's approach is not the best, but fulfills the OP's suggestion "maybe using grep" and provides "a different approach".

Your approach is very nicely thought out, and is probably the most ideal. My personal preference would be; a while loop is more suitable over a for loop in this case; I prefer to read if statements at the end of lines where possible; I love to use loop controls...

Code

my %hash2;  
while (my ($key, $val) = each %hash1) 
{  
    if (exists $hash2{$val}) 
    {  
        $hash2{$val} = [ $hash2{$val} ] if (!ref $hash2{$val}); 
        push @{$hash2{$val}}, $key; 
        next;  
    }  
    $hash2{$val} = $key; 
} 
print Dumper \%hash2;

If all arrays are acceptable, it can be done in 1 line...

Code

my %hash2;  
push @{$hash2{$hash1{$_}}}, $_ for (keys %hash1); 
print Dumper \%hash2;

Chris

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