What is an array in Ruby?
An array is a way to store a collection of objects in the same place.
You can put any object inside an Array…
Including other arrays!
You can access the elements inside an array using their index, which starts at 0.
If you had an array with the words “cat”, “dog” and “tiger” it would like this:
Here’s a code example that represents the data in the picture:
["cat", "dog", "tiger"]
Now:
Let’s learn what you can do with arrays!
Ruby Array Methods
To work with an array object you will have to create one first, there are many ways to do it.
Initialize an empty array:
# Both of these do the same thing, but the second form is preferred
users = Array.new
users = []
Initialize an array with data:
users = ["john", "david", "peter"]
You can avoid having to type the quotes for every string by creating a ruby array with %w:
# Same effect as last time, but much faster to type
users = %w(john david peter)
Now that we have an array you can access the elements it contains.
Here’s how:
users[0] # First element of the array
users[1] # Second element of the array
users[2] # Third element of the array
You can also use the first and last methods:
users.first # First element of the array
users.last # Last element of the array
Here’s how to add items to your array:
# Both of these have the same effect, << is preferred.
users.push "andrew"
users << "andrew"
Here's how you delete elements from your array:
last_user = users.pop # Removes the last element from the array and returns it
user.delete_at(0) # Removes the first element of the array
There is also the shift & unshift methods, which are similar to pop/push but take or add elements in front of the array.
users.unshift "robert" # Adds an element in front of the array
users.shift # Removes the first element of the array and returns it
Does this array contain or include a certain item?
Check if a value exists in an array in Ruby:
numbers = [1,2,3]
numbers.include?(2)
# true
numbers.include?(10)
# false
What is the length of the array? (In other words, how many elements it contains)
words = ["cat", "coconut", "bacon"]
words.size
# 3
Here is a small cheatsheet for you:
init -> Array.new, [], %w
read -> [0], first, last
add -> push, <<, unshift
remove -> pop, delete_at, shift
How to Convert An Array into a String
You can convert any array into a string by using the Array#join method.
Here's an example:
letters = %w(a b c d)
letters.join
# "abcd"
Notice that this method takes 1 optional parameter so you can define a separator between the array elements when they are joined together.
letters = %w(a b c d)
letters.join(" ")
# "a b c d"
If you want the reverse operation, from string to array, you want to use the split method:
"a b c".split
Which results in:
["a", "b", "c"]
Multidimensional Arrays
An array can be composed of other arrays, we call that a multi-dimensional array.
Example of 2d array:
users = [
[1, 'Peter'],
[2, 'Steven']
]
To access the first element from the first sub-array you can use this syntax:
users[0][0]
# 1
And for the second element of the first sub-array:
users[0][1]
# 'Peter'
There are some cases where you want to convert a multi-dimensional array into a regular array.
You can do that using the flatten method:
users.flatten
# [1, "Peter", 2, "Steven"]
Iterating Over Ruby Arrays
Now that you have an array wouldn't it be nice if you could enumerate its contents and print them?
The good news is that you can!
Example: Print your array using each
users.each { |item| puts item }
If you need to work with both the value and the index then you can use array's each with index method:
users.each_with_index { |item, idx| puts "#{item} with index #{idx}" }
Note: Most of these looping operations are available thanks to the Enumerable module, which is mixed into the Array class by default.
Example: Capitalize every word in your Array using map.
users = users.map { |user| user.capitalize }
users = users.map(&:capitalize)
The map method doesn't modify the array in-place, it just returns a new array with the modified elements, so we need to assign the results back to a variable. There is also a map! (notice the exclamation point) method which will modify the array directly, but in general the simpler version is preferred.
Another thing you may want to do is find all the items in your array that fit certain criteria.
Example: Find all the numbers greater than 10:
numbers = [3, 7, 12, 2, 49]
numbers.select { |n| n > 10 }
# => 12, 49
Negative Indexing
You learned that if you want to access a certain element from a Ruby array you need to use indexing (or methods like first & last).
But what about negative indexing?
I'm talking about using something like -1 as your array index.
What Ruby will do is start from the end of the array. So -1 will get you the last item, -2 will get you second to last, etc.
Example:
letters = ["a", "b", "c"]
letters[-1]
# "c"
letters[-2]
# "b"
More Array Operations
There are a lot of things you can do using arrays, like sorting them or picking random elements.
You can use the sort method to sort an array, this will work fine if all you have is strings or numbers in your array. For more advanced sorting check out sort_by.
numbers = numbers.sort
You can also remove the duplicate elements from an array, if you find yourself doing this often you may want to consider using a Set instead.
numbers = [1, 3, 3, 5, 5]
numbers = numbers.uniq
# => [1, 3, 5]
Notice that this doesn't permanently change your array. Most Ruby methods will create a new array with the changes you requested. So if you want to save the results you need to use a variable or in this example, use the uniq! method.
If you want to pick one random element from your array you can use the sample method:
numbers.sample
You may also want to "slice" your array, taking a portion of it instead of the whole thing.
Example: Take the first 3 elements from the array, without changing it:
numbers.take(3)
numbers[0,3]
You can do more advanced Ruby array slicing. For example, you may want to get all the elements but the first one:
numbers[1..-1]
Notice the difference between using a comma (0,3) & a range (1..-1). The first one says "get me 3 elements starting at index 0", while the last one says "get me the characters in this range".
Get the size of an array:
numbers.size
# 5
Check if an array is empty:
numbers.empty?
# false
Remove nil values:
numbers << nil
# [1, 3, 3, 5, 5, nil]
numbers.compact
# [1, 3, 3, 5, 5]
Operations With Multiple Arrays
If you have two arrays and want to join or merge them into one you can do it like this:
# Faster, because this changes the users array
users.concat(new_users)
# Slower, because this creates a new array
users += new_users
You can also remove elements from one array like this, where users_to_delete is also an array:
users = users - users_to_delete
Finally, you can get the elements that appear in two arrays at the same time:
users & new_users
Conclusion
Ruby arrays are very useful and they will be a powerful ally by your side.
Make sure to practice creating an array, adding elements to it, accessing elements by index, etc.
Practice makes perfect!
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