Find a set of common elements from a multi-dimensional array recursively

Question

I have a multi-dimensional array:

a=[[2,3,4],[1,3,4],[1,2],[1,2,3,4]]

i've to compare all the 4 sub-arrays and get common elements.Next,take 3 subarrays at a time and get common elements.then take 2 sub arrays at a time and get common elements, in RUBY.

Answer 1

This should do the job in recent Ruby versions:

a.length.downto(1).map{|i| a.combination(i).map{|sub| sub.inject(&:&)}}
#=> [[[]], [[], [3, 4], [2], [1]], [[3, 4], [2], [2, 3, 4], [1], [1, 3, 4], [1, 2]], [[2, 3, 4], [1, 3, 4], [1, 2], [1, 2, 3, 4]]]

Here's a related question with similar solution. The "trick" is in method Array#&, which calculates the intersection (as a set operation) of the two arrays. It is associative operation, so we can apply it on each subarray in turn, keeping the accumulated result, hence inject is perfect for it. In short, array.inject(&:&) will result with a greatest common subset of elements in every member of array. &:& is just a Ruby shorthand for making a Proc out of method named & and submitting it as a block to inject, instead of writing:

array.inject{|a,e| a & e}