Perl Hash Operation

A Perl Hash Operation is a Perl-based associative array operation for a Perl hash.

• Context:
  • It can (typically) involve a pandas.DataFrame Method.
• Example(s)
  • create an empty array:
    • …
  • create a prepopulated array:
    • …
  • populate an array:
    • …
  • update an array (or add rows to an array):
    • …
  • add columns to an array:
    • …
  • array query:
    • …
  • array metadata query:
    • …
  • array modification.
    • …
  • array iteration.
    • …
• Counter-Example(s)
  • a pandas.DataFrame Operation.
  • a Perl Array Operation.
  • a SQL Table Operation.
  • a pandas.Series Operation.
  • an R Array Operation (R DataFrame Operation).
• See: pandas.DataFrame Attribute.

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References

2011

• (Melli, 2011-09-01) ⇒ Gabor Melli. (2011). “Perl Hash Examples.".

Define a hash

   my %coins = (
      "Quarter", 25,
      "Dime", 10,
      "Nickel", 5) ;

    my %planetMass= (
        mercury ⇒ 0.055,
        venus   ⇒ 0.86,
        earth   ⇒ 1.0,
        mars    ⇒ 0.11,
        jupiter ⇒ 318,
    );

Number of members

    my $size = scalar(keys %planetMass);

Iterate through

   while (my ($key, $value) = each(%coins)){
     ...
   }

   while (my ($key, $value) = each(%$hash_ref) ) {
     ...
   }

Sort a hash

• using an in-line function

    # largest to smallest
    @array = reverse sort { $hash{$a} <=> $hash{$a} } keys %hash;

Sort a hash - using a subroutine

    foreach $key (sort by_value keys %ary) {
        print $key, '=', $ary{$key}, "\n";
    } 
    sub by_value { $ary{$a} cmp $ary{$b}; }

Sort a hash - descending value through subroutine

    sub by_value { $ary{$b} <=> $ary{$a}; }

Union of Two Hashes

    sub union { return { map { %$_ } @_ } }

• or

    sub union { +{ map { %$_ } @_ } }

Print a hash

   print %coins;
   while (my ($key, $value) = each(%coins)){
      print "$key, $value\n";
   }
   print map { "$_ ⇒ $hash{$_}\n" } keys %hash;

print by interpolating the hash into an array

   print "@{[ %hash ]}\n";

print a temporary array variable

   my @temp = %hash;
   print "@temp";

print a hash reference

 
   print "the keys... “, sort keys %$hash_ref, "...\n";

Delete an element pair.

   delete($coins{Penny});
   delete($coins{HalfDollar});

Copy a hash

   my %hash_copy = %hash;  # copy a hash

   my $href_copy = $href;  # copy a hash ref

   # pick the keys/values to copy
   my %newRecord_ ;
   @newRecord_{@OntRecordAttrs} = $RKBRecord[$i]{@OntRecordAttrs} ;

Use hash references

    sub foo
    {
        my $hash_ref;
 
        $hash_ref->{ 'key1' } = 'value1';
        $hash_ref->{ 'key2' } = 'value2';
        $hash_ref->{ 'key3' } = 'value3';
 
        return $hash_ref;
    }
 
    my $hash_ref = foo();

Maximum key

 my $max_ = (sort {$h{$b} <=> $h{$a}} keys %h)[0]
 print $max_ ;

 my %hashName = (a ⇒ -1, b ⇒ -2, c ⇒ -3);
 my $max_ = $hash{each %hashName};
 $_ > $max and $max = $_ for values %hashName;
 print $max_ ;

 use List::Util;
 $max_ = $h{ max values %h };
 print $max_ ;

All keys having max value:

  my $max = [sort {$b<=>$a} values %h]->[0] ;
  my @List = map { ($h{$_} == $max) ? $_ : () } keys %h ;

or

sub largest_value_mem (\%) {
    my $hash   = shift;
    my ($key, @keys) = keys   %$hash;
    my ($big, @vals) = values %$hash;

    for (0 .. $#keys) {
        if ($vals [$_] > $big) {
            $big = $vals [$_];
            $key = $keys [$_];
        }
    }
    $key
}

print largest_value_mem %hash; # prints 'largest'

REF: http://stackoverflow.com/questions/2886872/what-is-the-easiest-way-to-get-a-key-with-the-highest-value-from-a-hash-in-perl

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