fetch elements that occurred multiple times in Ruby Array [duplicate]

Question

Given you have a Ruby Array like a = [1,2,3,2,4,4,2,5] how can you select elements that occurred multiple times in the given array ?

so return value == [2,4]

sawa · Accepted Answer · 2015-07-10 13:53:56Z

3

a.group_by(&:itself).select{|_, a| a[1]}.keys

answered Jul 10, 2015 at 13:53

sawa

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2 Comments

Although a[1] is shorter, I'd use the more explicit a.length > 1. Nice side effect: it works for nil and false values.

yep it's clearly the fastest, here are some benchmarks gist.github.com/equivalent/3c9a4c9d07fff79062a3

equivalent8 · Accepted Answer · 2015-07-10 13:49:46Z

2

My colleague suggested this :

a = [1,2,3,2,4,4,2,5]
a.select{ |el| a.count(el) > 1 }.uniq
# => [2,4]

answered Jul 10, 2015 at 13:49

equivalent8

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