Ruby Examples > Arrays 

#---Arrays---

Arrays are one of the most commonly used data structures . Ruby provides powerful and versatile arrays for all your needs.

Mastering them means you can get a lot done out of them.

Creating Arrays

There are many ways to create arrays.

Create an empty array:

 
arr = []

An array of items of same type:

 
nums = [11, 22, 33]

An array of arrays:

 
matrix_2_by_2 = [[1, 11], [2, 22], [3, 33]]
## Now create a 3 by 3 array. Go ahead. Make my day.

A nested array:

 
nested = [11, :a, [:alpha, :beta, [:matt, :joe]]]

An array of items of different types:

 
junkyard = [11, 'alpha', ['x', 'y'], nil, Object.new]

Creating with the Array.new constructor (rarely used):

 
p Array.new(3) # => [nil, nil, nil]
p Array.new(3, 'a') # => ["a", "a", "a"]

Using a block to dynamically create items:

 
p Array.new(3) { |i| i * 2 } # => [0, 2, 4]

If you don’t like quoting strings, use %w() to create string arrays and %i() to create symbol arrays:

 
p %w(one two three) # => ["one", "two", "three"]
p %i(one two three) # => [:one, :two, :three]

Accessing Array items

The operator method [] is used to access one or more items from the array:

 
arr = ['a', 'b', 'c', 'd', 'e']

Array index starts from 0:

 
p arr[0]      # => "a" (first item)
p arr[4]      # => "e" (last item)

To count backwards (from last to first), use negative indices:

 
p arr[-1]     # => "e" (last item)
p arr[-2]     # => "d" (second-to-last item)

Extract a portion of an array:

 
p arr[1, 3]   # => ["b", "c", "d"]

Same thing with a range:

 
p arr[1..3]   # => ["b", "c", "d"]
p arr[1...3]  # => ["b", "c"]

There are other ways to access items too:

 
p arr.first   # => "a"
p arr.last    # => "e"
p arr.first(2) # => ["a", "b"]
p arr.last(2)  # => ["d", "e"]
p arr.take(2)  # => ["a", "b"]

Accessing beyond range returns nil. It doesn’t throw any error:

 
p arr[5]      # => nil

In some situations, this behavior can result in unexpected bugs. If you want to know if the index is out of range, use fetch instead of [].

This will blow up with an exception if the index is out of range:

 
p arr.fetch(5) rescue nil
## 💥 without `rescue nil` blows up with IndexError

You can provide a default value with fetch if there’s nothing in that index:

 
p arr.fetch(5, 'FIVE!') # => "FIVE!"

Accessing Array Items By Digging

If you have a nested array, and you want to access a specific item, you can use [] like so:

 
arr = [11, :a, [:alpha, :beta, [:matt, :joe]]]
p arr[2][2][1] # => :joe

But a better, safer way is to use dig. Why?

Assume arr is user input and you expect it to have 3 items, with the third one being another array whose third item is yet another array of 2 items.

But what if the user only supplied 2 items for arr? What if the 3rd item is not present, or is nil? How would the code above work?

It’ll blow up with a NoMethodError exception:

 
arr[2] = nil
p arr[2][2][1] rescue nil
## 💥 without `rescue nil` blows up with:
## => undefined method '[]' for nil (NoMethodError)

But with dig, you get nil if there’s no item in any of the intermediate indices:

 
p arr.dig(2, 2, 1) # => nil

And you get the correct value if the item exists:

 
arr[2] = [:alpha, :beta, [:matt, :joe]]
p arr.dig(2, 2, 1) # => :joe

This means you can use this in conditionals like so:

 
if arr.dig(2, 2, 1)
  ## do something
else
  ## inform user that input format is incorrect
end

Without dig this is how that would’ve looked like:

 
if arr[2] && arr[2][2] && arr[2][2][1]
else
end

Modifying Array Items

 
arr = [11, 22]

Adding Items
  • Use << or push to append an item to the array’s end:
 
arr << 33
arr.push(44)
p arr # => [11, 22, 33, 44]
  • Use unshift to prepend an item to the array’s beginning:
 
arr.unshift(55)
p arr # => [55, 11, 22, 33, 44]
  • Use insert to place an item at any given index:
 
arr.insert(3, 7)
p arr # => [55, 11, 22, 7, 33, 44]
  • Use concat to add arrays of items:
 
arr.concat([:a, :b], [:c])
p arr # => [55, 11, 22, 7, 33, 44, :a, :b, :c]

Note: All these methods modify the original array, and also return the modified array as the return value.

Removing Items
  • Use pop to remote the last item from the array, and also return that last item:
 
popped = arr.pop
p popped # => 55
p arr # => [66, 11, 22, 7, 33, 44]
  • Use delete_at to delete an item at a specified index:
 
deleted = arr.delete_at(3)
p deleted # => 7
p arr # => [66, 11, 22, 33, 44]
  • You can also delete all occurrences of a specific item with delete simply:
 
10.times { arr.push(44) } # check arr's value
arr.delete(44)
p arr # => [66, 11, 22, 33]

Replacing Items

Use the []= operator method to replace an item in a given index:

 
arr[2] = 222
p arr # => [66, 11, 222, 33]

You can replace consecutive items if you pass two indices, or a range to []=:

 
arr[1, 2] = [44, 55]
p arr # => => [66, 44, 55, 33]
arr[1..2] = [66, 77]
p arr # => => [66, 66, 77, 33]

Common Array Methods
  • size gives you the count of array items:
 
p junkyard.size # => 5
  • empty? returns true if array has no items:
 
p junkyard.empty?, [].empty? # => [false, true]
  • map takes a block and returns a new array of items that’s the result of calling the block for each item:
 
p [1, 2, 3, 4].map { |e| e * 2 } # => [2, 4, 6, 8]
  • compact removes all nils from an array:
 
p [11, 22, nil, 33, nil].compact # => [11, 22, 33]
  • flatten returns a single-dimensional array from all the items flattened in a multi-dimensional array:
 
p [1, 2, [3, 4], [[5, [6]]]].flatten
## => [1, 2, 3, 4, 5, 6]
  • uniq removes duplicates:
 
p [11, 22, 11].uniq # => [11, 22]
  • sort does its thing:
 
p [22, 11].sort # => [11, 22]
  • join joins the items with the given string:
 
p %w(cat mat bat).join # => "catmatbat"
p %w(cat mat bat).join(',') # => "cat,mat,bat"
  • sample returns one or more random items:
 
p arr.sample # => "a"
p arr.sample(3) # => ["d", "c", "b"]
  • shuffle returns a new array with items shuffled:
 
p arr.shuffle # => ["e", "a", "c", "b", "d"]

Bang Methods

All the methods above don’t modify the array they were called on. They just return a new modified array.

 
arr = [11, nil, 'a', 11, ['x', 'y']]
p arr.compact # => [11, "a", 11, ["x", "y"]]
p arr.uniq # => [11, nil, "a", ["x", "y"]]
p arr.flatten # => [11, nil, "a", 11, "x", "y"]
## `arr` hasn't changed one bit!
p arr
## => [11, nil, 'a', 11, ['x', 'y']]

To modify the original array, use the “bang” versions of these methods: compact!, flatten!, uniq!, sort!, reverse! etc.

 
arr.compact! # => [11, "a", 11, ["x", "y"]]
arr.uniq! # => [11, "a", ["x", "y"]]
arr.flatten! # => [11, "a", "x", "y"]
## Now `arr` is a changed man.
p arr
## => [11, "a", "x", "y"]

In general, prefer non-bang methods because they’re easier to debug as they don’t modify the original array.

Array Set Operations

 
a = [11, 22]
b = [22, 33]
c = [33, 44]
p (a + b + c) # => [11, 22, 22, 33, 33, 44]
  • Use union or | to return combined items from all given arrays:
 
p a.union(b, c) # => [11, 22, 33, 44]
p (a | b | c) # => [11, 22, 33, 44]
  • Use intersection or & to return common items from all given arrays:
 
p a.intersection(b) # => [22]
p a.intersection(b, c) # => []
p (a & b) # => [22]
p (a & b & c) # => []
  • Use difference or - to return items that are not common among all given arrays:
 
p a.difference(b, c) # => [11]
p (a - b - c) # => [11]

Note: union, intersection difference all can take any number of arrays as arguments, whereas the |, & and - operator methods can only take one array.

Useful Enumerable Array Methods

The Array class includes the Enumerable module. That’s where it gets a lot of its superpowers from.

In fact, the compact, sort, map and uniq methods mentioned above… they all come from Enumerable.

There’s a lot more. Read the docs for full list. But I’ll list some of my favorite and most useful methods:
  • include? returns true if the given item is included in the array:
 
p %w(11 22 33).include?(22) # => false
p %w(11 22 33).include?('22') # => true
  • find returns the first item that makes the block return a truthy value:
 
p [11, 22, 33].find { |e| e.odd? } # => 11
  • find_all returns an array of all items that make the block return a truthy value:
 
p [11, 22, 33].find_all { |e| e.odd? } # => [11, 33]
  • all? returns true only if all items make the block return true:
 
p [2, 4, 6].all? { |e| e.even? } # => true
  • any? returns true if even one item makes the block return true:
 
p [2, 4, 6].any? { |e| e.odd? } # => false
  • none? does the opposite. Returns true only if none of the items make the block return true:
 
p [2, 4, 6].none? { |e| e.odd? } # => true
  • grep? is a fun method. It returns only items that match a given pattern. But it can also grep items within a range, or items that are only of a particular class:
 
p %w(duck buck nick).grep(/uck$/) # => ["duck", "buck"]
p [11, '22', 33].grep(Integer) # => [11, 33]
p [11, 22, 33].grep(20..30) # => [22]
  • each_with_index is there just so you don’t have to do this: i = 0 and then i += 1 for each iteration:
 
[11, 22].each_with_index { |e, i| p "idx of #{e}: #{i}" }
## => "idx of 11: 0"
## => "idx of 22: 1"
  • each_with_object is elegant. It saves you an extra step when you’re building an object based on each array item:
 
hash_obj = {}
%w(a b c).each_with_object(hash_obj) do |ele, hash_obj|
  hash_obj[ele.to_sym] = ele.upcase
end
p hash_obj # => {a: "A", b: "B", c: "C"}
  • reduce is used when you want to reduce a whole array into a single value based on a starting value, a function and the individual array items one at a time:
 
## sum of all items
p [10, 20, 30].reduce { |mem, e| mem + e } # => 60
## product of all items
p [10, 20, 30].reduce { |mem, e| mem * e } # => 6000
## 100, plus sum of all items
p [10, 20, 30].reduce(100) { |mem, e| mem + e } # => 160

In reduce, mem is just a block variable and it can be named anything. But it’s customary to name it mem or memo with reduce because it represents “memory”. It is the value Ruby “keeps in memory” during each iteration.

Aliases

Many methods we discussed above have other names as well. These are method aliases.
  • map is the same as collect
  • find is detect
  • find_all is filter. It’s also select
  • reduce is inject
  • include? is member?
  • reduce is inject
  • [] is slice

Which one to use?
As with all things in Ruby related to style… it’s a matter of preference. (I prefer the left ones in the list above.)

Simplify with Block Argument it

From Ruby 3.4, instead of this:

 
p [11, 22].map { |e| e.even? }

You can do this:

 
p [11, 22].map { it.even? } # => [false, true]

But be careful. If there’s already an it variable in the scope of the block’s definition, then only that will be used inside the block. This is called closure.

 
it = 10
p [11, 22].map { it.even? } # => [true, true]

So only use this feature:
  • if the block needs only one argument,
  • and if you’re sure there isn’t any it variable defined already

Official Docs

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