Checking If An Array Is Empty In Ruby

Published: December 3, 2017

Tags:

Ruby

Recently, I was working on some Ruby code where I had to check if an array is empty. It turns out there are many ways to skin this cat. Here I’ll document my learnings…

Evaluating The Array As A Boolean

As a developer mainly working in PHP my first instinct was simply to evaluate the array as a boolean (empty arrays are false-y in PHP). However I quickly learned that similar to JavaScript, empty arrays are truth-y in Ruby…

irb(main):001:0> !![]
=> true

As such, doing something like this won’t work…

if some_array
  do_something
  # More code...
end

Checking The Length Of The Array

One option that does work is to check the length of the array…

irb(main):001:0> [].length
=> 0
irb(main):002:0> [1].length
=> 1
irb(main):003:0> [1, 'foo'].length
=> 2

As such the following will work if you want to do something if the array has elements…

if some_array.length > 0
  do_something
  # More code..
end

This is how one would typically check this in JavaScript.

Avoiding Code Smells

On a pull request I was working on I started out checking the length, however when I ran Rubocop I saw it didn’t like that…

$ rubocop
Inspecting 1 file
C

Offenses:

test.rb:2:4: C: Use !empty? instead of length > 0.
if some_array.length > 0
   ^^^^^^^^^^^^^^^^^^^^^

Aha! So a cleaner approach is call the empty? method on the array…

irb(main):001:0> [].empty?
=> true
irb(main):002:0> ['foo'].empty?
=> false

Now we can rewrite our code as…

if !some_array.empty?
  do_something
  # More code
end

Avoiding Code Smells - Take 2

Rubocop is still not happy…

$ rubocop
Inspecting 1 file
C

Offenses:

test.rb:2:1: C: Favor unless over if for negative conditions.
if !some_array.empty? ...
^^^^^^^^^^^^^^^^^^^^^

It would like us to switch from if ! to unless. In my use case, however, unless wouldn’t work because I was checking two conditions.

if some_condition && !some_array.empty?
  do_something
  # More code
end

However another alternative, I subsequently learned is to use any?

if some_condition && some_array.any?
  do_something
  # More code
end

Gotchas with `any?` vs `!empty?`

One important thing to note about any? is documented in the Stack Overflow questions “Check for array not empty: any?”

any? will return false array that contains only nil elements…

irb(main):009:0> [nil].any?
=> false

However !empty? will return true…

irb(main):011:0> ![nil].empty?
=> true