Replace from one array values of another array

Question

I have two arrays:

array1 = [3, 4, 4, 5, 6, 7, 8, 8]
array2 = [4, 5, 8, 8]

I want to remove those elements of array1, which are found in array2, but only in one instance. The resulting array, array3, must be like this:

array3 = [3, 4, 6, 7]

I tried:

array3 = array1 - array2

but the result was unsatisfactory:

array3 -> [3, 6, 7]

Andorbal · Accepted Answer · 2013-02-14 14:07:07Z

1

This may not be the most efficient way of doing what you want, but it works:

array1 = [3, 4, 4, 5, 6, 7, 8, 8]
array2 = [4, 5, 8, 8]
array2.each do |item|
    index = array1.index item
    array1.delete_at index if index
end

answered Feb 14, 2013 at 14:07

Andorbal

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1 Comment

tokland Over a year ago

An imperative solution is ok because it's simple, but note this is O(n^2) time while the problem is O(n). It can be solved using an auxiliar hash.

tokland · Accepted Answer · 2013-02-14 14:17:02Z

0

A non-imperative just to show other ways of doing things. Using Facets (just for convenience to get the histogram), I'd write this. O(n):

require 'facets'

array3 = array1.reduce([array2.frequency, []]) do |(h, output), x|
  if h[x] && h[x] > 0
    [h.update(x => h[x] - 1), output]
  else
    [h, output << x] 
  end
end[1]
#=> [3, 4, 6, 7]

To make the snippet purely functional you would use Hash#merge/Array#+ instead of Hash#update/Array:<<, but due to the nature of these data structures it would be terribly inefficient.

edited Feb 14, 2013 at 14:17

answered Feb 14, 2013 at 14:04

tokland

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