Ruby Array find_first object?

Question

Am I missing something in the Array documentation? I have an array which contains up to one object satisfying a certain criterion. I'd like to efficiently find that object. The best idea I have from the docs is this:

candidates = my_array.select { |e| e.satisfies_condition? }
found_it = candidates.first if !candidates.empty?

But I am unsatisfied for two reasons:

That select made me traverse the whole array, even though we could have bailed after the first hit.
I needed a line of code (with a condition) to flatten the candidates.

Both operations are wasteful with foreknowledge that there's 0 or 1 satisfying objects.

What I'd like is something like:

array.find_first(block)

which returns nil or the first object for which the block evaluates to true, ending the traversal at that object.

Must I write this myself? All those other great methods in Array make me think it's there and I'm just not seeing it.

score 253 · Accepted Answer · edited May 14 '17 at 19:37

253

Either I don't understand your question, or Enumerable#find is the thing you were looking for.

edited May 14 '17 at 19:37

esilver

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answered Mar 04 '10 at 17:13

Mladen Jablanović

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No problem, I also often find annoying that ruby documentation doesn't list methods of all inherited or included classes and modules. – Mladen Jablanović Mar 04 '10 at 17:33
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more info: `[1,2,3,5,2].find { |n| puts n; n == 2 } ` iteration stops once it finds the first element it matches. – rajuGT Apr 04 '17 at 13:23

score 112 · Answer 2 · answered Apr 17 '12 at 10:14

112

use array detect method if you wanted to return first value where block returns true

[1,2,3,11,34].detect(&:even?) #=> 2

OR

[1,2,3,11,34].detect{|i| i.even?} #=> 2

If you wanted to return all values where block returns true then use select

[1,2,3,11,34].select(&:even?)  #=> [2, 34]

answered Apr 17 '12 at 10:14

Sandip Ransing

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`.detect` is exactly what I need. But what's the difference with `.find` then? – Augustin Riedinger Jul 14 '14 at 10:35
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@AugustinRiedinger no difference, both are same.`detect` is just an alias to `find` http://ruby-doc.org/core-2.1.2/Enumerable.html#method-i-find – Sandip Ransing Jul 15 '14 at 11:16
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for some consistency I like to follow [The Ruby Style Guide](https://rubystyle.guide/#map-find-select-reduce-size), which favors find – pjvleeuwen Dec 30 '19 at 17:30

score 29 · Answer 3 · answered Mar 04 '10 at 17:13

29

Guess you just missed the find method in the docs:

my_array.find {|e| e.satisfies_condition? }

answered Mar 04 '10 at 17:13

sepp2k

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Or if you like code golfing, `my_array.find(&:satisfies_condition?)` – Andrew Grimm Oct 12 '11 at 06:22

score 17 · Answer 4 · answered Mar 04 '10 at 17:16

Do you need the object itself or do you just need to know if there is an object that satisfies. If the former then yes: use find:

found_object = my_array.find { |e| e.satisfies_condition? }

otherwise you can use any?

found_it = my_array.any?  { |e| e.satisfies_condition? }

The latter will bail with "true" when it finds one that satisfies the condition. The former will do the same, but return the object.

Ruby Array find_first object?

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