Remove duplicates from nested array

Question

I have an Array of Arrays that contains numbers in a particular order. I want to remove the duplicates out of the nested arrays, but there is a hierarchy: If a number occurs in a lower-index of the array, remove all duplicates down the Array chain.

Example: nums = [[10, 6, 14], [6], [10, 6, 9], [10, 13, 6], [10, 13, 6, 9, 16], [10, 13]]

nums[0] contains [10,6,14] so any subsequent mention of 10,6,14 should be removed from the other arrays in the chain, meaning nums[2] should have 10,6 removed and only 9 should remain.

I'm having trouble doing this with nested loops, can any Ruby wizards help please?

Theo · Accepted Answer · 2010-12-31 18:07:23Z

10

This should do it:

input = [[10, 6, 14], [6], [10, 6, 9], [10, 13, 6], [10, 13, 6, 9, 16], [10, 13]]
seen = []
output = input.map do |numbers|
  new = numbers.uniq - seen
  seen += new
  new
end
# => output is [[10, 6, 14], [], [9], [13], [16], []]

If you want to remove the empty lists in the output, simply

output.reject!(&:empty?)

answered Dec 31, 2010 at 18:07

Theo

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2 Comments

beeudoublez Over a year ago

Wow, that is beautiful and elegant. Thanks!

beeudoublez Over a year ago

Accepted as answer now, site made me wait 2 minutes.

Theo · Accepted Answer · 2010-12-31 18:11:12Z

3

require 'set'

nums = [[10, 6, 14], [6], [10, 6, 9], [10, 13, 6], [10, 13, 6, 9, 16], [10, 13]]
found = Set.new
new_nums = []

for subarray in nums do
    sub_new = []
    for i in subarray do
        if not found.member? i
            sub_new << i
        end
        found << i
    end
    new_nums << sub_new
end

puts(nums.inspect)
puts(new_nums.inspect)

edited Dec 31, 2010 at 18:11

Theo

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answered Dec 31, 2010 at 18:08

MFreck

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Comments

tokland · Accepted Answer · 2010-12-31 18:16:22Z

1

Yet another way. It keeps original order of elements in arrays:

require 'set'
nums = [[10, 6, 14], [6], [10, 6, 9], [10, 13, 6], [10, 13, 6, 9, 16], [10, 13]]
nums2 = nums.inject([[], Set.new]) do |(output, seen), ary|  
  [output << ary.reject { |a| seen.include?(a) }, seen.union(ary)]
end[0]
p nums2
# [[10, 6, 14], [], [9], [13], [16], []]

answered Dec 31, 2010 at 18:16

tokland

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Comments

Phrogz · Accepted Answer · 2010-12-31 18:13:24Z

0

Is the following incorrect? Should the [6] be removed or not?

nums = [[10, 6, 14], [6], [10, 6, 9], [10, 13, 6], [10, 13, 6, 9, 16], [10, 13]]

def remove_duplicate_numbers( array )
  seen = []
  array.map{ |sub_array|
    result = sub_array - seen
    seen += sub_array
    result
  }
end

p remove_duplicate_numbers( nums )
#=> [[10, 6, 14], [], [9], [13], [16], []]

If this is not what you want, please post the actual output you expect for your array.

answered Dec 31, 2010 at 18:13

Phrogz

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