Search an two dimensional array by another array

Question

I have two arrays. One Mapper and one with my ID's.

My Array with the external ID's:

genres_array = [12,28,16]

The Mapper Array (Internal-ID, External-ID)

mapper = [
   [1,12],
   [2,18],
   [3,19],
   [4,28],
   [5,16],
   [6,90],
]

As Result i would like to have now a new array, with only the internal values of the genres_array (the genres_array had the external values first). In this case the result would be [1,4,5]

I tried a lot of ways but i really have no idea how to solve this simple problem in a clean way. Im pretty sure it will be something like

genres_array.map { |genre_id| get_internal_id_from_mapper }

PS: It could also happen that a ID won't be found in the mapper. In that i case i just want to remove it from the array. Any idea?

tckmn · Accepted Answer · 2013-11-08 22:44:14Z

1

You're looking for rassoc:

genres_array.map { |genre_id| mapper.rassoc(genre_id)[0] }

Which results in

[1, 4, 5]

EDIT: Just read the PS - try something like this:

genres_array.map { |genre_id|
    subarr = mapper.rassoc genre_id
    subarr[0] if subarr
}.compact

Then for an input of

genres_array = [12,28,100,16]

You would still get the output

[1, 4, 5]

edited Nov 8, 2013 at 22:44

answered Nov 8, 2013 at 22:38

tckmn

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1 Comment

Philip Giuliani Over a year ago

Thank you thats a very clean solution! Only one problem: If i have an id in my genres_array, that doesn't exists in the mapper array, i'm getting this exception: NoMethodError: undefined method []' for nil:NilClass`

rorra · Accepted Answer · 2013-11-08 22:41:47Z

1

Another way that won't throw an exception if the external id is not found:

genres_array = [12,28,16]

mapper = [
   [1,12],
   [2,18],
   [3,19],
   [4,28],
   [5,16],
   [6,90],
]

internal_ids = genres_array.map do |genre_id|
  element = mapper.detect { |m| m[1] == genre_id }
  element ? element[0] : nil
end.compact

answered Nov 8, 2013 at 22:41

rorra

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Comments

hirolau · Accepted Answer · 2013-11-09 09:41:56Z

1

Another solution involving a hash:

Hash[mapper].invert.values_at(*genres_array)

Asking for values that does not exist will return nil, if you do not want the nil just add .compact at the end.

answered Nov 9, 2013 at 9:41

hirolau

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