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

Question

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?

Closely related question: http://stackoverflow.com/questions/6224875/equivalent-of-try-for-a-hash/6224980#6224980 — Andrew Grimm, Jun 15 '11 at 00:47

score 5 · Answer 1 · answered Jan 06 '16 at 00:34

5

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.

answered Jan 06 '16 at 00:34

user513951

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now that's a way to leverage an answer a few times :P – Drew Jan 06 '16 at 00:40

J-_-L · Accepted Answer · 2019-11-11T14:29:53.013

3

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.

Update: Recent Rubies include the #dig method, which might be helpful in this case. See user513951's answer for more details.

edited Nov 11 '19 at 14:29

answered Jun 13 '11 at 18:51

J-_-L

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I've wound up using this for now: `if @foursquare['response']['groups'].try(:[], 0).try(:[], 'items').present?`. Thanks! – Michael Irwin Jun 13 '11 at 19:16

score 2 · Answer 3 · answered Jun 13 '11 at 18:50

2

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.

answered Jun 13 '11 at 18:50

DigitalRoss

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

3 Answers3