How do I see if a multi-dimensional hash has a value in ruby?

Question

Currently I am doing the following, but I am sure there must be a better way:

def birthday_defined?(map)
    map && map[:extra] && map[:extra][:raw_info] && map[:extra][:raw_info][:birthday]
end

There may be cases where only map[:extra] is defined, and then I will end up getting Nil exception errors cause map[:extra][:raw_info] doesn't exist if I dont use my checked code above.

If `map[:extra][:raw_info][:birthday]` is defined then `map[:extra]` must also be defined. — Austin Henley, Oct 22 '12 at 17:11
@AustinHenley But if you only test for `map[:extra][:raw_info][:birthday]` and it doesn't exist, you get a `NoMethodError` for `NilClass`.... hence the long expression (I'm guessing) — Charles Caldwell, Oct 22 '12 at 17:13
duplicate of [Is there a clean way to avoid calling a method on nil in a nested params hash?](http://stackoverflow.com/questions/5429790/is-there-a-clean-way-to-avoid-calling-a-method-on-nil-in-a-nested-params-hash) (and many others) — tokland, Oct 22 '12 at 17:57
To me, this is a sign that your data is structured incorrectly. We can propose a million ways of "hiding" this complexity, but the method probably just shouldn't need to dig through three hash levels in the first place. — pje, Oct 22 '12 at 18:02
@pje Unless the hash is coming from an outside API like Facebook (as this one appears to be) and hence such complexity can't be avoided. — Charles Caldwell, Oct 22 '12 at 18:06
Possible duplicate of [How to avoid NoMethodError for missing elements in nested hashes, without repeated nil checks?](http://stackoverflow.com/questions/4371716/how-to-avoid-nomethoderror-for-missing-elements-in-nested-hashes-without-repeat) — user513951, Jan 06 '16 at 05:24

score 1 · Answer 1 · answered Oct 22 '12 at 17:17

I have two ways. Both have the same code but subtly different:

# Method 1

def birthday_defined?(map)     
    map[:extra][:raw_info][:birthday] rescue nil # rescues current line
end

# Method 2

def birthday_defined?(map)     
    map[:extra][:raw_info][:birthday]
rescue # rescues whole method
    nil
end

score 1 · Accepted Answer · answered Oct 22 '12 at 17:40

If you're using Rails, then you can use try (and NilClass#try):

value = map.try(:[], :extra).try(:[], :raw_info).try(:[], :birthday)

That looks a bit repetitive: it is just doing the same thing over and over again while feeding the result of one step into the next step. That code pattern means that we have a hidden injection:

value = [:extra, :raw_info, :birthday].inject(map) { |h, k| h.try(:[], k) }

This approach nicely generalizes to any path into map that you have in mind:

path  = [ :some, :path, :of, :keys, :we, :care, :about ]
value = path.inject(map) { |h, k| h.try(:[], k) }

Then you can look at value.nil?.

Of course, if you're not using Rails then you'll need a replacement for try but that's not difficult.

score 0 · Answer 3 · answered Oct 22 '12 at 17:12

0

Use a begin/rescue block.

begin
  map[:extra][:raw_info][:birthday]
rescue Exception => e
  'No birthday! =('
end

answered Oct 22 '12 at 17:12

MurifoX

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-1 for using exceptions for control flow and for over-rescuing. Don't ever rescue Exception: http://www.mikeperham.com/2012/03/03/the-perils-of-rescue-exception/ – pje Oct 22 '12 at 17:30
It works, so i don't know why the downvote. There are many ways to solving the problem, and this is one of them. – MurifoX Oct 22 '12 at 17:35

score 0 · Answer 4 · edited May 23 '17 at 11:51

0

That's idiomatic why to do it. And yes it can be a little cumbersome.

If you want to extend Hash a bit though, you can do some cool stuff with something like a key path. See Access Ruby Hash Using Dotted Path Key String

def birthday_defined?
  map.dig('extra.raw_info.birthday')
end

edited May 23 '17 at 11:51

Community

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answered Oct 22 '12 at 17:13

Alex Wayne

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Charles Caldwell · Answer 5 · 2012-10-22T18:37:05.780

0

This is a little hacky but it will work:

def birthday_defined?(map)
    map.to_s[":birthday"]
end

If map contains :birthday then it will return the string which will evaluate to true in a conditional statement while if it doesn't contain :birthday, it will return nil.

Note: This assumes the key :birthday does not appear at potentially multiple locations in map.

edited Oct 22 '12 at 18:37

answered Oct 22 '12 at 18:29

Charles Caldwell

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score 0 · Answer 6 · edited Oct 22 '12 at 20:38

0

This should work for you:

def birthday_defined?(map)
  map
  .tap{|x| (x[:extra] if x)
  .tap{|x| (x[:raw_info] if x)
  .tap{|x| (x[:birthday] if x)
  .tap{|x| return x}}}}
end

edited Oct 22 '12 at 20:38

Chris Gerken

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answered Oct 22 '12 at 20:18

sawa

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How do I see if a multi-dimensional hash has a value in ruby?

6 Answers6