Mastering Perl for Bioinformatics [Electronic resources] نسخه متنی

A.7 Hashes

A
hash
(also called an associative array) is a collection of zero or more
pairs of scalar values, called keys and values. The values are
indexed by the keys. An array variable begins with the
% sign followed by a legal variable name. For
instance, possible hash variable names are:

%hash1
%genes_by_name

You can assign a value to a key with a simple assignment statement.
For example, say you have a hash called
%baseball_stadiums and a key
Phillies to which you want to assign the value
Citizens Bank
Park. This statement accomplishes the assignment:

$baseball_stadiums{'Phillies'} = 'Citizens State Bank';

Note that a single hash value is referenced by a $
instead of a % at the beginning of the hash name;
this is similar to the way you reference individual array values
using a $ instead of a @.

You can assign several keys and values to a hash by placing their
scalar values in a list separated by commas and surrounded by a pair
of parentheses. Each successive pair of scalars becomes a key and a
value in the hash. For instance, you can assign a hash the empty
list:

%hash = (  );

You can also assign one or more scalar key/value pairs:

%genes_by_name = ('gene1', 'AACCCGGTTGGTT', 'gene2', 'CCTTTCGGAAGGTC');

There is an another way to do the same thing, which makes the
key/value pairs more readily apparent. This accomplishes the same
thing as the preceding example:

%genes_by_name = (
'gene1' => 'AACCCGGTTGGTT',
'gene2' => 'CCTTTCGGAAGGTC'
);

To get the value associated with a particular key, precede the hash
name with a $ and follow it with a pair of curly
braces containing the scalar value of the key:

$genes_by_name{'gene1'}

This returns the value 'AACCCGGTTGGTT', given the
value previously assigned to the key 'gene1' in
the hash %genes_by_name. Figure A-2 shows a hash with three keys.

You can get an array of all the keys in a hash with the operator
"keys", and you can get an array of all the values
in a hash with the operator "values".

The arrays you get won't be sorted.
Here's an example:

%h = ( 'one'         => 'for the money',
'two'         => 'for the show',
'three'       => 'to get ready'
);
@keys = keys %h;
@values = values %h;
print "@keys\n@values\n";

gives the output:

three one two

to get ready for the money for the show.

You can make a reference to a hash by preceding it with a backslash;
you dereference it by preceding the reference with a percent sign
% for the entire hash or with an extra dollar sign
$ for an individual value of some hash key:

%h = ( 'one'         => 'for the money',
'two'         => 'for the show',
'three'       => 'to get ready'
);
$href =  src="/image/library/english/10240_U2030.GIF" border=0>h;
print $href, "\n";
print $$href{'two'}, "\n";
foreach $key ( keys %$href ) {
print "key = $key         value = $$href{$key}\n";
}

gives the output:

HASH(0x811d1e0)
for the show
key = three    value = to get ready
key = one      value = for the money
key = two      value = for the show

You can also define a hash as an anonymous hash. An anonymous hash
isn't saved in a named hash variable;
it's a reference to hash data, and can only be saved
in a reference. It is initialized within curly brackets:

$anonhash = { 'one' => 'first',
'two' => 'second',
'three' => 'third'
};
foreach $key ( keys %$anonhash ) {
print "key = $key         value = $$anonhash{$key}\n";
}

gives the (unsorted) output:

key = three   value = third
key = one     value = first
key = two     value = second