Ruby how to delete arrays if they have matching element?

Question

I have this array:

[[4, 50], [2, 28], [1, 4], [4, 41], [1, 9], [2, 25]]

I wish to construct an array consisting of those tuples [f,l] of this array such that there is no tuple [g,h] preceding [f,l] for which g==f. The selected tuples are to be in the same order as in the original array.

[[4, 50], [2, 28], [1, 4]]

Example if there are any more arrays that begin with 4 or 1 it should be deleted. It could be any integer, 4 and 1 is only an example.

group by first element and select only the first item of the group. — Sergio Tulentsev
– Sergio Tulentsev, Commented Jun 25, 2014 at 14:39
@sawa - Because there have already been a array starting with 2 ´[2, 28]´ — Rails beginner
– Rails beginner, Commented Jun 25, 2014 at 14:42
Rails, you've got two downvotes and two votes to close. Let's rescue the question. It's a good one and @Stefan provided a very good answer. What you need to do is edit and insert something like the following, right before "I want the result to be:": "I wish to construct an array consisting of those tuples [f,l] of this array such that there is no tuple [g,h] preceding [f,l] for which g==f. The selected tuples are to be in the same order as in the original array." Yes, that's a bit of a mouthful, but I think it states unambiguously what you are trying to accomplish. — Cary Swoveland
– Cary Swoveland, Commented Jun 25, 2014 at 16:52

Stefan · Accepted Answer · 2014-06-25 15:02:40Z

7

Array#uniq seems to work:

[[4, 50], [2, 28], [1, 4], [4, 41], [1, 9], [2, 25]].uniq(&:first)
#=> [[4, 50], [2, 28], [1, 4]]

answered Jun 25, 2014 at 15:02

Stefan

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4 Comments

sawa Over a year ago

This is better than mine.

Stefan Over a year ago

@CarySwoveland the documentation doesn't specify which duplicate is preserved and which is removed. The behavior could be implementation specific.

Cary Swoveland Over a year ago

I expect that's a reflection on the incompleteness of the documentation, rather than evidence that maintenance of order was intentionally omitted from the spec. It would make no sense for uniq with a parameter to return a value that is preceded in the array by a matching value. Also, it is part of the spec ("yields items in order") for Ruby MRI.

Stefan Over a year ago

@CarySwoveland thanks for the RubySpec pointer. The failing example would be "compares elements based on the value returned from the block", returning [1, 4] instead of [1, 2] if the implementation would keep the last item

Cary Swoveland · Accepted Answer · 2014-06-25 18:52:37Z

0

I can't improve on @Stefan's answer, but here's another way that merely steps through the elements of the array:

a = [[4, 50], [2, 28], [1, 4], [4, 41], [1, 9], [2, 25]]

a.each_with_object([]) { |(f,l),a| a<<[f,l] unless a.any? { |g,_| g==f } }
  #=> [[4, 50], [2, 28], [1, 4]]

answered Jun 25, 2014 at 18:52

Cary Swoveland

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