Working with Collections

A collection is a grouping (or organizing) of zero or more elements used to solve the problem at hand. Often, collection are looped through using the loop keyword or a method to that effect.


An array is a data object that stores ordered list of values. In other programming language like Python, arrays are referred to as “lists”.

They are written with square brackets ([]) and can be filled with any kind of data object.

Arrays in Ruby are zero-based, meaning that when accessing their elements with their numerical place, one should start at 0. The second item is at 1, and so on. The address location in the array is accessed using integers.

arr = ['one', 2, ['three']]

p arr[2] # => ["three"]

See also Sorting of Arrays.


A hash is data structure consisting of a key-value pairs. It is like an array that can use any data type as its key (the hash equivalent of an index). In other programming language like Python, hashes are referred t oas “dictionary”.

Hashes are written using curly brackets ({}) but its notation is a little bit more complicated than that of arrays because of hashes’ need for key and value pair (note also the older syntax until Ruby 1.9, called hash rocket, but still used nowaday):

hash_syntax_old = {:attack_titan => 'Eren Yeager'}
hash_syntax_new = {female_titan: 'Annie Leonhart'}

Although a key can be any data types, Hash are commonly created using symbols as key and any data types as values. Note also that when not using symbols as keys, we are forced to use the old syntax (i.e., =>).

Hashes vs. Arrays

Deciding whether to use hash or array can be dauting at first. As a general rule, one should use a hash when the data need to be associated with a specific label. Otherwise, an array should be fine.

Also note that as of Ruby 1.9, hashes maintain order but arrays are still prefered for ordered items.


Range is a kind of collection of ordered values. It is created on the go with a low and high value separated by two or three dots (.., ...) and surrounded by parenthesis (()).

(1..3).sum  # => 6
(1...3).sum # => 3

A range with two dots is inclusive, meaning it includes the high number. In the example abobe: for every number in 1 to 3, do something.

A range with three dots is excluding: for every number in 1 to 2, do something.

A range’s beginning and end doesn’t need to be numbers. It can be letters:

('a'..'c').to_a # => ["a", "b", "c"]

Common Collection Methods

Common Collection Methods