PERL HASH

 

REVISED: Sunday, March 3, 2013

You will learn how to use the Perl hash.

I.  PERL HASH

A Perl hash is a Perl array that is indexed by a string instead of a number. Hashes are like arrays except they link a key to a value.  To define a hash, use the percent (%) symbol before the name.

A.  PERL HASH INDEXING

Hashes can be key-value pair indexed using the two scalar variables $key and $value.  The table below contains USA currency portrait and dollar amounts.  $key is the last name of the person whose portrait is on the bill and $value is the face value of the bill in dollars.

The hash %bills is declared in bill amount order $value.

$key	$value
Washington	1
Jefferson	2
Lincoln	5
Hamilton	10
Jackson	20
Grant	50
Franklin	100
McKinley	500
Cleveland	1000
Madison	5000
Chase	10000
Wilson	100000

II. PERL HASH EXAMPLE SOURCE CODE

Copy paste the following into your text editor.

#Provides undefined value warnings.
use warnings;

#enables perl's -w flag.
use diagnostics;

#Helps you catch typos.
#Forces you to use my() function to declare all variables.
#The my() function makes the variables local to the main package.
use strict;

#stack trace
use Carp ( );
  local $SIG{__WARN__} = \&Carp::confess;

#Specify the file.
my($file) = "C:/Strawberry/hash.pl";

#Open the file and read data.
#Die with grace if it fails.
open (FILE, "<$file") or die "Can't open $file: $!\n";

#DEFINE HASH %bills

my(%bills) = (
   Washington => 1,
   Jefferson => 2,
   Lincoln => 5,
   Hamilton => 10,
   Jackson => 20,
   Grant => 50,
   Franklin => 100,
   McKinley => 500,
   Cleveland => 1000,
   Madison => 5000,
   Chase => 10000,
   Wilson => 100000,
);

print "\n\tBILLS SORTED BY PORTRAIT NAME\n";
foreach my $key (sort (keys(%bills))) {
   if (length($key) <= 6){
      printf ("\t%s \t\t%.6s\n", $key, $bills{$key});
 }
   else{
      printf ("\t%s \t%.6s\n", $key, $bills{$key});
   }

}

#Close the file.
close FILE;

From your text editor, "File Save As" hash.pl using the path to the folder of your Perl download.

From the Perl prompt type the following:

C:\WINDOWS\system32>C:\Strawberry\hash.pl

Press Enter and the following will display on your screen:

C:\WINDOWS\system32>C:\Strawberry\hash.pl

                     BILLS SORTED BY PORTRAIT NAME
                     Chase                            10000
                     Cleveland                     1000
                     Franklin                        100
                     Grant                             50
                     Hamilton                       10
                     Jackson                        20
                     Jefferson                      2
                     Lincoln                         5
                     Madison                       5000
                     McKinley                      500
                     Washington                 1
                     Wilson                          100000
              
C:\WINDOWS\system32>

III. PERL HASH SUMMARY

The hash %bills is sorted by bill portrait last name $key.

$key	$value
Chase	10000
Cleveland	1000
Franklin	100
Grant	50
Hamilton	10
Jackson	20
Jefferson	2
Lincoln	5
Madison	5000
McKinley	500
Washington	1
Wilson	100000

The each( ) function removes the topmost key-value pair from the hash and stores them in the variables $key and $value.  Each time the loop iterates, the statement ($key, $value) = each(%bills) executes and the hash pops off the top key-value pair and continues doing so until it has gone through every key-value pair in the hash.  When it is done, each( ) returns false and the loop stops running.

You have learned how to use the Perl hash.

Elcric Otto Circle

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