How to find the list of pairs in an array using ruby?

Question

Input:

a = [4, 5, 5, 5, 6, 6, 4, 1, 4, 4, 3, 6, 6, 3, 6, 1, 4, 5, 5, 5]

How to list out no of pairs in an array.

Output:

9

Description

#no 1(1 pair)
#no 3(1 pair)
#no 4(2 pairs)
#no 5(3 pairs)
#no 6(2 pairs)
#so total 9 pairs

What pairs are you considering? All combinations of 2 elements? Or just adjacent pairs? Also, your subject says "uniq pairs" but your question doesn't mention unique. Which is it? — lurker, May 27 '19 at 13:19
If you find it helpful. https://stackoverflow.com/questions/35585399/frequency-of-pairs-in-an-array-ruby — Junaid Ali, May 27 '19 at 13:29
The title asks for the *number* of unique pairs. In the body you ask for a *list* of such pairs. Which is it? Please show the 9 unique pairs that is the desired result for your example. For example, you say "#no 5(3 pairs)". What are those 3 pairs? — Cary Swoveland, May 27 '19 at 15:23
[1, 1, 3, 3, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6] in this array you can see the pairs list — Rajkumar Ulaganadhan, May 28 '19 at 05:45

score 4 · Accepted Answer · answered May 27 '19 at 17:43

Here is another option:

a.group_by(&:itself).transform_values{ |v| v.size / 2 }.values.sum
#=> 9

How it works.

First group the elements by value:

a.group_by(&:itself) #=> {4=>[4, 4, 4, 4, 4], 5=>[5, 5, 5, 5, 5, 5], 6=>[6, 6, 6, 6, 6], 1=>[1, 1], 3=>[3, 3]}

Then transforming the keys to the pair count:

a.group_by(&:itself).transform_values{ |v| v.size / 2 } #=> {4=>2, 5=>3, 6=>2, 1=>1, 3=>1}

So, get the values of the hash:

a.group_by(&:itself).transform_values{ |v| v.size / 2 }.values #=> [2, 3, 2, 1, 1]

Finally, sum the values, which is the first line of code posted above.

claasic · Answer 2 · 2019-05-27T14:40:20.373

1

arr = [4, 5, 5, 5, 6, 6, 4, 1, 4, 4, 3, 6, 6, 3, 6, 1, 4, 5, 5, 5]
hash = Hash.new(0)
arr.each { |e| hash[e] += 1 }
hash.values.reduce(0) { |s, n| s += n / 2 } // => 9

Since from what I can gather you are basically removing integers the moment they got paired once so technically it's just an integer division by two.

[1] How to count identical string elements in a Ruby array

[2] Reduce Hash Values

edited May 27 '19 at 14:40

answered May 27 '19 at 13:45

claasic

1,050
6
14

1

Good answer. OP has edited title so the total number (9) is no longer asked for, so you might write `arr.each_with_object(Hash.new(0)) { |e,hash| hash[e] += 1 }.transform_values { |v| v/2 } #=> {4=>2, 5=>3, 6=>2, 1=>1, 3=>1}`. – Cary Swoveland May 28 '19 at 07:31

Rajkumar Ulaganadhan · Answer 3 · 2019-05-28T10:31:16.620

1

I have done like this, It works

  b = []
  a.uniq.each { |i| b.push(a.count(i)/2)}
  b.sum

edited May 28 '19 at 10:31

answered May 27 '19 at 13:56

Rajkumar Ulaganadhan

698
5
12

1

Note that `count` has to traverse the entire array each time which makes this solution rather inefficient. – Stefan May 27 '19 at 14:30

How to find the list of pairs in an array using ruby?

3 Answers3