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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.

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Hashes
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.


Why use hashes
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.


Hash basics
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.


Alternative way of loading hashes
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.


Hash operators
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”;

    }


More hash operators
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.


Counting
Counting

  • Here’s a simple use of hashes to get word counts. Input is a file of words, one per line.

    while ($word = <STDIN>) {

    $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.