Flatten an array of hashes in ruby

Question

I have some simple code that looks like this:

fruit.each do |c|
  c.each do |key, value|
    puts value
  end
end

This works fine, but it feels un-ruby like. My goal is to take this array:

[{"fruit_id"=>"1"}, {"fruit_id"=>"2"}, {"fruit_id"=>"3"}]

And convert it to this:

[ "1", "2", "3" ]

Thoughts?

Solution 1

If it's in Rails or if you have to_proc defined, you can go a little bit shorter than @toholio's solution:

arr = [{"fruit_id"=>"1"}, {"fruit_id"=>"2"}, {"fruit_id"=>"3"}]
out = arr.map(&:values).flatten # => ["1", "2", "3"]

Solution 2

So you want each hash to be converted to its values then joined with the values from the other hashes?

in = [{"fruit_id"=>"1"}, {"fruit_id"=>"2"}, {"fruit_id"=>"3"}]
out = in.map{ |item| item.values }.flatten

out will then be ["1","2","3"].

Solution 3

@wombleton @aronchick I suggest to use "flat_map(&:values)" instead "map(&:values).flatten" for better performance.

arr = [{"fruit_id"=>"1"}, {"fruit_id"=>"2"}, {"fruit_id"=>"3"}]
out = arr.flat_map(&:values)

Take a look here for more info: http://gistflow.com/posts/578-use-flat_map-instead-of-map-flatten

Solution 4

if you use map!, you don't create another array in memory

a.map! {|h| h.values}.flatten

this can be also written as

a.map!(&:values).flatten!

also looking at the comments, theres a method in ruby 1.9.3 called flat_map (http://ruby-doc.org/core-1.9.3/Enumerable.html#method-i-flat_map) that will do the map and the flatten in one call.

a.flat_map(&:values)

Solution 5

a.inject([]) { |mem,obj| mem += obj.values }

View more solutions

Comments

[aronchick]

I wish I could approve both - this is great too!

[toholio]

Well, they're actually the same solution really. :)

[glenn jackman]

injecting into an empty array is also known as collect (or map). see other answers here.

[glenn jackman]

Use << instead of += to append to arrays. It will be slightly faster as you won't be creating all those new array objects.

[Hemant Kumar]

<< and += are not the same. If you want to use "<<" you will have to flatten again. Anyways its just nitpick.

[jtzero]

for 1.9.3 there is arr.flat_map(&:values)

[Taryn East]

...or at least not use a local variable.