Ruby Hash

A Hash is a collection of key-value pairs similar to "key" => "value". Hashes are similar to arrays, but their indices are not limited to integers.

The indices (or "keys") of a Hash can be almost any object.

Although Hashes are similar to arrays, there is a significant difference: the elements of a Hash have no specific order. If order is important, you should use an array.

Creating a Hash

Like arrays, there are various ways to create Hashes. You can create an empty Hash using the new class method:

months = Hash.new

You can also create a Hash with a default value using new, where a Hash without a default value returns nil:

months = Hash.new("month")

or

months = Hash.new "month"

When you access any key in a Hash with a default value, if the key or value does not exist, accessing the Hash will return the default value:

Example

#!/usr/bin/ruby

months = Hash.new("month")

puts "#{months[0]}"
puts "#{months[72]}"

The output of the above example is:

month
month

Example

#!/usr/bin/ruby

H = Hash["a" => 100, "b" => 200]

puts "#{H['a']}"
puts "#{H['b']}"

The output of the above example is:

100
200

You can use any Ruby object as a key or value, even an array, as shown in the following example:

[1,"jan"] => "January"

Hash Built-in Methods

To call Hash methods, you need to instantiate a Hash object. Here are the ways to create Hash object instances:

Hash[[key =>|, value]* ] or

Hash.new [or] Hash.new(obj) [or]

Hash.new { |hash, key| block }

This will return a new Hash filled with the given objects. Now, using the created object, we can call any available methods. For example:

Example

#!/usr/bin/ruby

$, = ", "
months = Hash.new("month")

months = {"1" => "January", "2" => "February"}

keys = months.keys

puts "#{keys}"

The output of the above example is:

["1", "2"]

Below are the public Hash methods (assuming hash is a Hash object):