How do I detect duplicate values within an array in Ruby?

Question

Say I have an array that looks like:

a = [cat, dog, cat, mouse, rat, dog, cat]

How do I cycle through that, and do something with duplicates - e.g. say delete them?

In other words, if I did a.each do |i|, how do I evaluate a[0], against a[1], a[2], a[3]...and then when I find the one I want, say a[2] in this case has the first duplicate, I then push it to a stack or remove it or something.

I know how to evaluate keys, versus values...but how do I evaluate values against each other within the same array?

Thanks.

waldyr.ar · Accepted Answer · 2013-01-17 18:47:25Z

13

You can create a hash to store number of times any element is repeated. Thus iterating over array just once.

h = Hash.new(0)
['a','b','b','c'].each{ |e| h[e] += 1 }

Should result

 {"a"=>1, "b"=>2, "c"=>1}

edited Jan 17, 2013 at 18:47

waldyr.ar

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answered Mar 7, 2012 at 10:54

ch4nd4n

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4 Comments

Aleksander Pohl Over a year ago

Why not h = Hash.new(0) and h[e] += 1?

ch4nd4n Over a year ago

Matter of syntax. It's at programmers discretion.

marcamillion Over a year ago

This is actually what I was trying to do....but...I couldn't figure out how to use the nil? and increment methods just like this. Thanks!

waldyr.ar Over a year ago

Just improving the current answer

Aleksander Pohl · Accepted Answer · 2012-03-07 11:30:52Z

6

This works efficiently and is rather simple:

require 'set'

visited = Set.new
array.each do |element|
  if visited.include?(element)
    # duplicated item
  else
    # first appearance
    visited << element
  end
end

answered Mar 7, 2012 at 11:30

Aleksander Pohl

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Comments

MMM · Accepted Answer · 2012-03-07 10:46:10Z

4

Try this:

class Array
    def find_dups
        uniq.map {|v| (self - [v]).size < (self.size - 1) ? v : nil}.compact
    end
end

a = ['cat', 'dog', 'cat', 'mouse', 'rat', 'dog', 'cat']

print a - a.find_dups # Removes duplicates

find_dups will return elements that have duplicates

answered Mar 7, 2012 at 10:46

MMM

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Comments

spas · Accepted Answer · 2012-03-07 19:13:06Z

3

Try this:

array.inject({}){|h, e| h[e] = h[e].to_i + 1; h}

edited Mar 7, 2012 at 19:13

answered Mar 7, 2012 at 17:02

spas

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1 Comment

Aleksander Pohl Over a year ago

or array.inject(Hash.new(0)){|h, e| h[e] += 1; h}

lesce · Accepted Answer · 2012-03-07 10:36:22Z

2

Use a.uniq! to remove duplicates .

also checkout the ruby-doc.org where you can find more info on ruby's class methods .

answered Mar 7, 2012 at 10:36

lesce

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2 Comments

Sergio Tulentsev Over a year ago

compact removes nils from the array. How is it helpful in this situation?

MMM Over a year ago

Your solution removes duplicates, it doesn't look for them as OP asked

Sergio Tulentsev · Accepted Answer · 2012-03-07 10:56:56Z

1

A simple solution is to run a double loop:

a.each_with_index do |a1, idx1|
  a.each_with_index do |a2, idx2|
    next if idx1 >= idx2 # Don't compare element to itself 
                         # and don't repeat comparisons already made

    # do something with a pair of elements (a1, a2)
  end
end

If you just want to eliminate duplicates, there's a method: Array#uniq.

edited Mar 7, 2012 at 10:56

answered Mar 7, 2012 at 10:35

Sergio Tulentsev

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4 Comments

marcamillion Over a year ago

Thought about this, but it seems so messy. There a more elegant, 'ruby-ish' solution?

Sergio Tulentsev Over a year ago

For eliminating duplicates, there's a method. For comparing all elements to each other, there's a double loop. I personally don't see any mess in it. It's plain simple code that reads well.

MMM Over a year ago

Sergio this method is inefficient as you're making unnecessary comparisons that have been made in the past. Your second inner loop should start it's iteration later (i.e. further in the array) on each loop.

Sergio Tulentsev Over a year ago

@MMM I didn't say it's efficient. I said it's simple :)

jorgenj · Accepted Answer · 2014-08-07 01:02:50Z

1

This will print all the duplicates in an array:

array.inject(Hash.new(0)) { |hash,val| 
  hash[val] += 1; 
  hash 
}.each_pair { |val,count| 
  puts "#{val} -> #{count}" if count > 1 
}

answered Aug 7, 2014 at 1:02

jorgenj

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Comments

Callan A. Carr · Accepted Answer · 2017-09-11 15:23:20Z

1

The best way to do it is to compare it with a unique version of itself. If its the same then it has no duplicates, if not then duplicates exist.

unique_array = original_array.uniq

get a unique version of your array

if original_array == unique_array then return true else return false

compare it to your original array.

Simple!

edited Sep 11, 2017 at 15:23

answered Sep 11, 2017 at 15:16

Callan A. Carr

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Comments

ingrid · Accepted Answer · 2015-02-04 15:26:12Z

0

If you just want to get rid of duplicates, the easiest thing to do is take the array and do array&array. Use the & operator.

If you want to know what those repeats are, just compare array to array&array.

answered Feb 4, 2015 at 15:26

ingrid

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Comments

astgtciv · Accepted Answer · 2017-09-01 19:32:55Z

0

If array is sortable, then something like below will return only the duplicates.

array.sort.each_cons(2).select {|p| p[0] == p[1] }.map &:first

Sorts the array, then maps it to consecutive pairs of elements, selects pairs which are same, maps to elements.

answered Sep 1, 2017 at 19:32

astgtciv

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