#!/usr/bin/perl -w

use 5.010;
#############################################
# This document explains Perl's 
# Hash facilities
# AAK: Last Modification
# Tue Sep 11 22:10:27 PDT 2012
# ###########################################

#Hash is like array but unsorted, and index could be 
#a string as well as numbers:
#it is defined basically like : $hask{key} = value
#where key is the same as index for array
#and value is the correspoding value of the key:

$student_num{"arman"} = "s2932832";
$student_num{"daoyan"} = "s8473272";

@people = qw/ arman daoyan /;

foreach $person (@people) {
say "${person}'s student ID is: $student_num{$person}";
}

#you can refer to the hash as a whole via %hash:
#another way to define a hash is:

my %IPdatabase = (
   "google" => '74.128.23.93',
   "ubc" => '137.32.42.154',
   "yahoo" => '67.32.45.190',
);
#using reverse, you can swithc the keys with values and values with keys
%RIPdatabase = reverse %IPdatabase;

#using key operator you can list the keys in a hash
#and operator values returns values in a hash

@IPs = keys %RIPdatabase;
@Sites = keys %IPdatabase;

for $i (@IPs) {
say "$i : $RIPdatabase{$i}";
}
for $j (@Sites)
{
say "$j : $IPdatabase{$j}";
}

#using each operator:

while ( ($k, $v) = each %IPdatabase ) {
   say " $k , $v";
}

$nc = keys %IPdatabase;

#note the context dependency of perl, since $nc is a scalar
# return of keys will be the number of keys which is size of database

say "data base includes $nc data";

#using exist you can chech if a key exists in a hash or not:

if ( ! exists $IPdatabase{"microsoft"} ) {
   say "Sorry! no microsoft available, why do you even need that?";
}

#note that ! is the "NOT" operator

#using delete you can delete an entry from a hash:

delete $IPdatabase{yahoo};
@newkeys = keys %IPdatabase;
say "you dont even need yahoo: @newkeys";

#one important hash is the enviroumental variable stored in %ENV:
#it is similar to @ARGV, which stores the argument passed to the script
#by user, when they run the program

print "YOUR PATH IS: \n $ENV{PATH} \n";