8.1
Hashes
(associative arrays)
8.2
Hash Motivation
Let's say we want to create a phone book . . .
Enter a name that will be added to the phone book:
Dudi
Enter a phone number:
6409245
Enter a name that will be added to the phone book:
Dudu
Enter a phone number:
6407693
8.3
Hash – an associative array
An associative array (or simply – a hash) is an unordered set of
pairs of keys and values. Each key is associated with a value.
A hash variable name always start with a “%”:
my %hash;
Initialization:
%hash = ("a"=>5, "bob"=>"zzz", 50=>"John");
Accessing:
you can access a value by its key:
print $hash{50};
John
Tip you can reset the hash (to an empty one) by %hash = ();
%hash
"a"
=>
5
"bob" => "zzz"
50
=> "John"
8.4
Hash – an associative array
modifying :
$hash{bob} = "aaa";
adding :
$hash{555} = "z";
(modifying an existing value)
%hash
(adding a new key-value pair)
You can ask whether a certain key exists in a hash:
if (exists $hash{50} )...
You can delete a certain key-value pair in a hash:
delete($hash{50});
"a"
=>
5
"bob" => "aaa"
"zzz"
555
50
=> "John"
"z"
555
=>
"z"
8.5
Variable types in PERL
Scalar
Array
Hash
$number
-3.54
@array
%hash
=>
$string
"hi\n"
=>
$array[0]
=>
$hash{key}
8.6
Hash – an associative array
An associative array of the phone book suggested in the first slide
(we will se a more elaborated version later on):
Declare
my %phoneBook;
Updating
$phoneBook{"Dudi"} = 9245;
$phoneBook{"Dudu"} = 7693;
Fetching
print $phoneBook{"Dudi"};
%hash
"Dudi" =>
9245
"Dudu" =>
7693
8.7
Iterating over hash elements
It is possible to get a list of all the keys in %hash
my @hashKeys = keys(%hash);
Similarly you can get an array of the values in %hash
my @hashVals = values(%hash);
@hashKeys
"a"
"bob"
@hashVals
50
5
"zzz" "John"
%hash
"a"
=>
5
"bob" => "zzz"
50
=> "John"
8.8
Iterating over hash elements
To iterate over all the values in %hash
my @hashVals = values(%hash);
foreach my $value (@hashVals)...
%hash
"a"
=>
5
"bob" => "zzz"
To iterate over the keys in %hash
my @hashKeys = keys(%hash);
foreach my $key (@hashKeys)...
50
=> "John"
@hashVals
5
"zzz" "John"
@hashKeys
"a"
"bob"
50
8.9
Iterating over hash elements
For example, iterating over the keys in %hash :
my @hashKeys = keys(%hash);
foreach my $key (@hashKeys) {
print "The key is $key\n";
print "The value is $hash{$key}\n";
}
The key is a
The value is 5
The key is bob
The value is zzz
The key is 50
The value is John
%hash
"a"
=>
5
"bob" => "zzz"
50
=> "John"
@hashVals
5
"zzz" "John"
@hashKeys
"a"
"bob"
50
8.10
Iterating over hash elements
Notably: The elements are given in an arbitrary order,
so if you want a certain order use sort:
my @hashKeys = keys(%hash);
my @sortedHashKeys = sort(@hashKeys);
foreach $key (@sortedHashKeys) {
print "The key is $key\n";
print "The value is $hash{$key}\n";
}
%hash
"a"
=>
5
"bob" => "zzz"
50
=> "John"
@hashVals
5
"zzz" "John"
@hashKeys
"a"
"bob"
50
8.11
Example – phoneBook.pl #1
######################################
# Purpose: Store names and phone numbers in a hash,
# and allow the user to ask for the number of a certain name.
# Input: Enter name-number pairs, enter "END" as a name to stop,
# then enter a name to get his/her number
#
use strict;
my %phoneNumbers = ();
my $number;
8.12
Example – phoneBook.pl #2
# Ask user for names and numbers and store in a hash
my $name = "";
while ($name ne "END") {
print "Enter a name that will be added to the phone book:\n";
$name = <STDIN>;
chomp $name;
if ($name eq "END") {
last;
}
print "Enter a phone number: \n";
$number = <STDIN>;
chomp $number;
$phoneNumbers{$name} = $number;
}
8.13
Example – phoneBook.pl #3
# Ask for a name and print the corresponding number
$name = "";
while ($name ne "END") {
print "Enter a name to search for in the phone book:\n";
$name = <STDIN>;
chomp $name;
if (exists($phoneNumbers{$name})) {
print "The phone number of $name is: $phoneNumbers{$name}\n";
}
elsif ($name eq "END") {
last;
}
else {
print "Name not found in the book\n";
}
}
8.14
Class exercise 8
1.
Write a script that reads a file with a list of protein names and lengths
(proteinLengths ):
AP_000081 181
AP_000174 104
AP_000138 145
stores the names of the sequences as hash keys, with the length of the sequence as the
value. Print the keys of the hash.
2.
Add to Q1: Read another file, and print the names that appeared in both files with the
same length. Print a warning if the name is the same but the length is different.
3.
Write a script that reads a GenPept file (you may use the preproinsulin record), finds all
JOURNAL lines, and stores in a hash the journal name (as key) and year of publication
(as value):
a. Store only one year value for each journal name
b*. Store all years for each journal name
Then print the names and years, sorted by the journal name (no need to sort the years
for the same journal in b*, unless you really want to do so…)