How can I find unique values in an array and assign them to variables in ruby

Question

How can I find unique values in an array.

For instance:

my_array = [2,4,4,5,5,7]

In the above array are two unique values 2 and 7 and I would like to know either a self defined or other method to assign these unique values to variables.

running count on the array every time will be super slow if you're working with a large array, as it grows with O(n^2) — sakurashinken
– sakurashinken, Commented Oct 31, 2017 at 22:22
arr.uniq.select { |n| arr.count(n) == 1 } would be an improvement to my earlier suggestion, though still not as performant as the answers given so far. — Sagar Pandya
– Sagar Pandya, Commented Oct 31, 2017 at 23:03

sakurashinken · Accepted Answer · 2017-11-03 05:02:20Z

3

Use array.group_by.

my_array.group_by{|v| v}.delete_if{|k,v| v.length > 1}.keys

or alternatively

  my_array.group_by{|v| v}.select{|k,v| v.length == 1}.keys

edited Nov 3, 2017 at 5:02

answered Oct 31, 2017 at 21:38

sakurashinken

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3 Comments

sakurashinken Over a year ago

not sure why this is being downvoted. I just benchmarked it with a 2 million element array and it seems relatively performant, and correct.

David Aldridge Over a year ago

Have you got the benchmark results? It would be good to be able to compare all the methods.

sakurashinken Over a year ago

I just generated an array in irb with 2 million elements, called shuffle, then called Benchmark.measure. The each with object measure was about 1.14s real time vs group_by at 1.66s

David Aldridge · Accepted Answer · 2017-10-31 21:32:42Z

2

This is a bit leggy, perhaps, but gets the job done

my_array.each_with_object(Hash.new(0)){|x,h| h[x] += 1}.select{|k,v| v == 1}.keys

answered Oct 31, 2017 at 21:32

David Aldridge

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4 Comments

Noel Vock Over a year ago

This returns an array with the unique values of the original array. Is it in terms of result in anyway different to or better then myarray.select { |n| myarray.count(n) == 1 }? I'm assuming it might be faster?

sakurashinken Over a year ago

much faster. myarray.count will hang on huge shuffled arrays.

Noel Vock Over a year ago

which of these two answers is faster and better practice?

sakurashinken Over a year ago

Id say Davids is slightly faster, but mine is a bit more readable. If you need performance, go with davids, readability, mine.

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