How to access an element deep in an array of arrays without getting 'undefined method' error [duplicate]

Question

This question already has an answer here:

• Ruby Style: How to check whether a nested hash element exists 16 answers
• How to avoid NoMethodError for nil elements when accessing nested hashes? [duplicate] 4 answers

When trying to access an element deep in an array of arrays, what is the best way to avoid getting the error 'undefined method `[]' for nil:NilClass' if an element doesn't exist?

For example I'm currently doing this, but it seems bad to me:

if @foursquare['response']['groups'][0].present? && @foursquare['response']['groups'][0]['items'].present?

Answer 1

Depending on your array content, you can omit the .present?. Ruby will also just take the last value in such a construct, so you can omit the if statement.

@foursquare['response']['groups'][0] &&
@foursquare['response']['groups'][0]['items'] &&
@foursquare['response']['groups'][0]['items'][42]

More elegant solutions for this problem are the egonil (blog post), the andand gem (blog post), or even Ruby 2.3's safe navigation operator.

Answer 2

Ruby 2.3.0 introduced a new method called dig on both Hash and Array that can be combined with the new safe navigation operator (&.) to solve your problem.

@foursquare.dig('response', 'groups')&.first&.dig('items')

This will return nil if a value is missing at any level.

Answer 3

if @foursquare['response']['groups'][0].to_a['items']
  . . .

It happens that NilClass implements a #to_a that returns []. This means that you can map every nil to [] and typically write a single expression without tests.