Compare two array for missing elements in Ruby

Question

I have two arrays that have thousands of elements. I need to find what elements are missing in one array by comparing it to another. Is there a way to get the missing elements without iterating through the entire array? Or is there something faster than what I am doing?

Here is what I am using now:

def find_missing(array1, array2)
    missing_elements = []
    array1.each { |e|  
        unless array2.include? e
            missing_elements << e
        end
    }

    return missing_elements
end

array1 = [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
array2 = [1, 2, 4, 5, 6, 7, 9]

puts find_missing(array1, array2)

Yu Hao · Accepted Answer · 2016-05-06 18:06:00Z

9

You want a copy of the first array, but removing any elements that appears in the second array? That's what Array#- (array difference) does:

array1 - array2
# => [10, 8, 3]

answered May 6, 2016 at 18:06

Yu Hao

123k50 gold badges253 silver badges305 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

tadman Over a year ago

@MrAutoIt The Array class has a bunch of these handy methods. If you ever want to know what tools are there, check the documentation as there's a lot of things to enjoy.

Hamms · Accepted Answer · 2016-05-06 18:05:22Z

2

You can just use the difference operator

@irb(main):001:0> array1 = [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
=> [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
@irb(main):002:0> array2 = [1, 2, 4, 5, 6, 7, 9]
=> [1, 2, 4, 5, 6, 7, 9]
@irb(main):003:0> array1 - array2
=> [10, 8, 3]

answered May 6, 2016 at 18:05

Hamms

5,11724 silver badges30 bronze badges

Comments

Cary Swoveland · Accepted Answer · 2016-05-06 21:17:12Z

I doubt that you can beat array1 - array2, Array#- being coded in C, but you might want to benchmark it against:

require 'set'

def use_sets(array1, array2)
  a1 = array1.uniq
  s2 = array2.to_set
  a1.reject { |e| s2.include?(e) }
end

added..

require 'fruity'

def compare_em(array1, array2)
  compare do 
    _minus { array1 - array2 }
    _sets  { use_sets(array1, array2) }
  end
end

n = 100_000
array1 = (1..n).to_a.shuffle
array2 = (1..n-2).to_a.shuffle
compare_em(array1, array2)
  #_minus is faster than _sets by 2x ± 1.0

n = 1_000_000
array1 = (1..n).to_a.shuffle
array2 = (1..n-2).to_a.shuffle
compare_em(array1, array2)
  #_minus is faster than _sets by 2x ± 1.0

n = 100_000
array1 = (([1]*n).concat [1,2]).shuffle
array2 = [1]*n
compare_em(array1, array2)
  #_minus is faster than _sets by 5x ± 1.0

Alexander Swann · Accepted Answer · 2020-12-05 11:52:15Z

To add to the original accepted answer, it is worth noting that the below solution would ensure finding the difference between two arrays, irrelevant of the order of comparison.

array1 - array2 | array2 - array1
# => [10, 8, 3]

If for some reason you were to compare the two arrays the other way round, it would return empty array and give the illusion nothing is missing. This may be obvious for small arrays but not for much larger ones.

array2 - array1
# => []

However a caveat to this is if you have differences in both arrays, this would capture the collective difference between them.

array1 = [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
array2 = [1, 2, 4, 5, 6, 7, 9, 11]

array1 - array2 | array2 - array1
# => [11, 10, 8, 3]

Collectives™ on Stack Overflow

Compare two array for missing elements in Ruby

4 Answers 4

1 Comment

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related