Ruby: check if all array elements are equal

Question

I'm having a bit of a 'problem' with Ruby code. I want to check if all elements of an array are equal.

For example, say I have an array of only 5s:

arr = [5, 5, 5, 5, 5]

I know I can do something like

arr[0] == arr[1] == arr[2] == arr[3] # == arr[4] == ...

but this is impossible for huge arrays and also not very Ruby-like in my opinion. We can improve it by doing something like this:

def all_equal?(arr)
  for i in 0..(arr.size-2)
    if arr[i] != arr[i+1] then
      return false
    end
  end
  true
end

But I also think this is pretty ugly. So is there any built-in/better/shorter (more Ruby-esque) way to do this?

TL;DR what is the shortest/most Ruby-esque way to check if an array contains only one distinct element (e.g. [5, 5, 5])?

Thanks.

If the elements of arr are all comparable with <=> you could do this: arr.min==arr.max, but this is clearly a job for uniq. — Cary Swoveland
– Cary Swoveland, Commented Dec 14, 2014 at 18:45
arr[0] == arr[1] == arr[2] Really? What Ruby version are you using? — tokland
– tokland, Commented Aug 9, 2017 at 13:49

Luan Nico · Accepted Answer · 2014-12-14 16:23:08Z

39

You could also use .uniq, that returns an array with no duplicates, and check the size:

def all_equal?(arr)
    arr.uniq.size <= 1
end

answered Dec 14, 2014 at 16:23

Luan Nico

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2 Comments

nathanvda Over a year ago

Imho it should be exactly 1. If it is smaller it depends on your semantics. Does an empty array have all elements equal? ;)

Luan Nico Over a year ago

Indeed it is open to interpretation - for me an empty array has all elements equal, because it has no elements :) And it is not very efficient indeed, the better way would be the for. But often we exchange speed with prettiness in ruby, as these kind of operations are rarely the bottleneck in most applications.

humza · Accepted Answer · 2014-12-14 16:51:11Z

20

Couple of ways.

The best one:

array.uniq.count <= 1 # or == 1 if it can't be an empty array

And:

array == ([array.first] * array.count)

And:

(array | array).count <= 1 # basically doing the same thing as uniq

Also:

array.reduce(:|) == array.first # but not very safe

And if it's a sortable array, then:

array.min == array.max

And, just for sake of variety:

!array.any?{ |element| element != array[0] } # or array.first instead of array[0]

Alternatively:

array.all?{ |element| element == array[0] } # or array.first instead of array[0]

edited Dec 14, 2014 at 16:51

answered Dec 14, 2014 at 16:31

humza

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Comments

tokland · Accepted Answer · 2018-08-08 10:20:12Z

6

Using Enumerable#each_cons:

def all_equal?(xs)
  xs.each_cons(2).all? { |x, y| x == y }
end

edited Aug 8, 2018 at 10:20

answered Jul 29, 2016 at 8:35

tokland

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1 Comment

elquimista Over a year ago

Never knew that this kind of method existed in standard library. Thanks!

ouflak · Accepted Answer · 2021-12-05 11:12:45Z

3

This works on Ruby 3.0+:

  [1,1,1,1].minmax.reduce(&:eql?)

Looks pretty. Might not be the fastest though.

edited Dec 5, 2021 at 11:12

ouflak

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answered Dec 5, 2021 at 10:13

sreedev

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Comments

Arvind singh · Accepted Answer · 2020-05-06 14:07:38Z

2

Try This ..

array = ["cars","cars","truck", nil]
array.uniq.compact.length == 1

answered May 6, 2020 at 14:07

Arvind singh

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Comments

Reinstate Monica -- notmaynard · Accepted Answer · 2014-12-14 16:19:58Z

1

Sort the array and compare the first value to the last.

answered Dec 14, 2014 at 16:19

Reinstate Monica -- notmaynard

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2 Comments

InputUsername Over a year ago

I like this solution. It does require that the array can be sorted/compared (which is not always the case), so I'm going to go with Luan Nico's answer.

m26a Over a year ago

Order an array just to check if all elements are unique is not efficient.

Joel · Accepted Answer · 2023-04-24 04:31:38Z

1

Looks like with later versions you can just pass matching argument to all? without a block

[5, 5, 5].all?(5) # => true

edited Apr 24, 2023 at 4:31

answered Apr 23, 2023 at 21:51

Joel

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Comments

esad · Accepted Answer · 2025-04-01 22:16:55Z

0

Apart from (generalized version of Joel's answer) arr.all?(arr.first) (which returns true for empty array), something like arr.reduce {|acc, x| x == acc ? x : (break false)} also works, returning the one value if all elements are equal or false if they are not equal. Only problem is if the array contains boolean values, then it's impossible to tell if they are all false or if they are different.

answered Apr 1 at 22:16

esad

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