Find all max of elements of an array

Question

Suppose I have a array, namely arr: [1, 2, 3, 4, 8, 8], and I want to find all max elements in this array:

arr.allmax # => [8, 8]

Is there a built-in method combinations to solve this? I don't like to monkey patch as I am doing now:

class Array
  def allmax
    max = self.max
    self.select { |e| e == max }
  end
end

Monkey patch is not a good idea, I could just do:

some_array.select { |e| e == some_array.max }

and it will work as allmax. Thanks for all answers and comments for inspirations.

it also relieves you of the need to account for empty array... — Uri Agassi, May 13 '14 at 06:18
Yes, this question is a duplicate, but there are at least two answers here that are different than those given earlier. Should that not matter? — Cary Swoveland, May 13 '14 at 07:13
This is not a duplicate. The older question is very specific to the case where a modulus operating is being applied, whereas this question and its answer is more generic an so more useful. I found the older question first and then kept looking till I found this one, which was perfect for what I needed. Is there anyway of removing the duplicate mark/ — Obromios, Nov 28 '17 at 05:13

Arup Rakshit · Answer 1 · 2014-05-13T06:25:39.240

1

Here is one way :

2.1.0 :006 > arr = [1, 2, 3, 4, 8, 8]
 => [1, 2, 3, 4, 8, 8] 
2.1.0 :007 > arr.group_by { |i| i }.max.last
 => [8, 8] 
2.1.0 :008 >

Here is a method :-

  def all_max(arr)
    return [] if arr.empty?
    arr.group_by { |i| i }.max.last
  end

edited May 13 '14 at 06:25

answered May 13 '14 at 06:14

Arup Rakshit

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fails for empty array... – Uri Agassi May 13 '14 at 06:15
@UriAgassi I will wrap inside a method.. Give me a moment – Arup Rakshit May 13 '14 at 06:16
Great to see the `group_by`, my question is "do not monkey patch" the built-in `Array`, so plain `group_by` is okay, although it cannot handle the empty case. Thanks a lot, Arup! – Juanito Fatas May 13 '14 at 06:18
@juanitofatas Humm.. *monkey patch* not sounds good always.. But on empty array no need to do any operation. Just check for emptiness,if empty return it, or go for your other task to do.. I think this is a good way. – Arup Rakshit May 13 '14 at 06:20

score 1 · Accepted Answer · answered May 13 '14 at 06:20

1

Here's a fun way to do it.

arr.sort!.slice arr.index(arr[-1]) || 0..-1

Sort the array, then find the leftmost index of the array which matches the rightmost index of the array, and take the subslice that matches that range (or the range 0..-1 if the array is empty).

This one is kind of fun in that it requires no intermediate arrays, though it does mutate the input to achieve the one-liner.

answered May 13 '14 at 06:20

Chris Heald

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So cool. Never think of it this way. Thanks, Chris. – Juanito Fatas May 13 '14 at 06:24

Cary Swoveland · Answer 3 · 2014-05-13T07:06:40.123

1

Another way:

def all_max(arr)
  return [] if arr.empty?
  mx = arr.max
  [mx] * arr.count { |e| e == mx }
end

all_max([1, 2, 3, 4, 8, 8]) 
  #=> [8, 8]

To construct the array in a single pass, you could do this:

arr.each_with_object([]) do |e,a|
  if a.empty?
    a << e
  else
    case e <=> a.first
    when 0 then a << e
    when 1 then a.replace([e])
    end
  end
end

edited May 13 '14 at 07:06

answered May 13 '14 at 06:33

Cary Swoveland

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Thanks for inspiration, Cary! – Juanito Fatas May 13 '14 at 06:40

Find all max of elements of an array

3 Answers3