1. Hashes a “hash” is another fundamental data structure, like scalars and arrays. Hashes are sometimes called “associative arrays”. Basically, a hash associates a key with a value. A hash is composed of a set of key-value pairs. A key is a string: any collection of characters, generally enclosed in quotes. Any scalar can be a key, but they are all converted to strings. A value can be almost anything: the values are just scalar variables. One hash oddity: neither the keys nor the values is sorted or stored in a useful order. The order you enter hash items is not related to the order with which you retrieve them.

2. Why Use Hashes? The C language doesn’t have anything like a hash in it, and clearly C can do just about anything you need to do in programming. The point of Perl is to make your life easier, to include useful tools, even if they are messy and clutter up the language. Examples of hash usage: --keeping count of the number of times a word is used in a text, or that a particular sub-sequence appears in DNA. Use the word as the key and the number of appearances as the value. --Associating ID numbers with people’s names --Associating protein names with their properties. And lots more. The hash is a tool that gets used very frequently once you understand them.

3. Hash Basics The punctuation mark used to denote a hash is % (percent sign). Note that hashes, arrays, and scalars are completely different variables. The variables $cat, @cat, and %cat are all different and independent variables. I don’t recommend using the same names for different variables, but it is legal. Hash elements are accessed by enclosing the key in curly braces. For example, the hash %stoplight is populated as follows: $stoplight{red} = “stop”; $stoplight{yellow} = “caution”; $stoplight{green} = “go”; In this hash, “red”, “yellow”, and “green” are the keys, and “stop”, “caution”, and “go” are the values. Each key can refer to only a single value. You can’t have duplicate keys. If you try, the first value will be lost and only the second will work: $stoplight{yellow} = “speed up”; print “$stoplight{yellow}\n”; # prints “go faster” However, different keys can refer to the same value without any problem.

4. Alternative Way of Loading Hashes A hash really is an array with alternating keys and values. You could load a hash by simply writing the keys and values the same way as you would load an array: %stoplight = (“red”, “stop”, “yellow”, “caution”, “green”, “go”); This method is a bit annoying, because it is easy to lose track of keys and values. A better way is to use the => operator (“big arrow”), which is really just a synonym for a comma: %stoplight = ( “red” => “stop”, “yellow” => “caution”, “green” => “go” ); Note the use of newlines here--makes reading the code easier.

5. Hash Operators “keys” gives a list of all the keys used in the hash. Here’s a common use: foreach $key (keys %stoplight) { print “$key stands for $stoplight{$key}\n”; } Note that the keys are not returned in a useful order. If you want them sorted you could write: foreach (sort keys %stoplight) { or foreach (sort {$a $b} keys %stoplight) { Similarly, “values” gives a list of all the values, in some unusual order. “each” is an operator that returns a 2 element list consisting of the key and the value. It needs to do this in tandem with “while”: while ( ($key, $value) = each %stoplight) { print “$key : $value\n”; }

6. More Hash Operators Removing elements in a hash is done with “delete”: delete $stoplight{“red”}; # both key and value are removed Testing for existence with “exists”: exists $stoplight{“red”) returns true if that key-value pair exists, and “false” if it doesn’t.

7. Counting Here’s a simple use of hashes to get word counts. Input is a file of words, one per line. while ($word = ) { $chomp $word; $word_hash{$word}++; } Note that %word_hash was created implicitly, without ever being explicitly declared. This is a standard Perl feature, but I will shortly discourage its use. Also, a nice feature of using hashes for counting is that each value is automatically set to 1 the first time a new key is encountered. That is, you don’t have to initialize each key-value pair; Perl does it for you automatically.