How to find the same item in an array with Ruby?

Question

I have an array like this.

@arr  =  ["Ac", "Ba", "Ca", "Dd", "aC", "bD", "cA", "dD"]

I want to swap the letter if the first one is a capital. In this case, Ac becomes cA, Ba -> aB etc. This becomes like this.

@arr = ["cA", "aB", "aC", "dD", "aC", "bD", "cA", "dD"]

Then I want to find the same item. In this case there are two, cA and dD.

@newarr = ["cA", "dD"]

This is what I have got so far:

@firstarr = @arr.map{|item| 
      if item[0] =~ /[A-Z]/
        item = item[1]+item[0]
      else
        itme = item
      end
    }

This gives

@firstarr = ["cA", "aB", "aC", "dD", "aC", "bD", "cA", "dD"]

possible duplicate of [Ruby: How to find and return a duplicate value in array?](http://stackoverflow.com/questions/8921999/ruby-how-to-find-and-return-a-duplicate-value-in-array) — morten.c, Apr 06 '14 at 22:17

score 1 · Answer 1 · answered Apr 06 '14 at 22:21

For your reversing criteria:

foo = @arr.map do |x|
  (x[0].downcase!).nil? ? x : x.reverse
end
# => ["cA", "aB", "aC", "dD", "aC", "bD", "cA", "dD"]

downcase! returns nil if the receiver is already in downcase, using this, above code checks if x[0] can be downcase-d; if yes, that means the first character is in uppercase and it reverse the word, else it returns the same word.

For duplication criteria:

foo.select { |x| foo.count(x) > 1 }.uniq
# => ["cA", "aC", "dD"]

bjhaid · Answer 2 · 2014-04-07T00:06:33.820

1

arr = ["Ac", "Ba", "Ca", "Dd", "aC", "bD", "cA", "dD"]
arr.each_with_object(Hash.new(0)) { |x,h| 
  (x !~ /[a-z][A-Z]/) ? h[x.reverse] += 1 : h[x] += 1 }.select { |k,v| v > 1 }.keys
#=> ["cA", "aC", "dD"]

edited Apr 07 '14 at 00:06

answered Apr 06 '14 at 23:33

bjhaid

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score 1 · Answer 3 · answered Apr 06 '14 at 23:39

An approach intended to read well:

Code

arr = ["Ac", "Ba", "Ca", "Dd", "aC", "bD", "cA", "dD"] 

new_arr = arr.map { |str| str[0] == str[0].upcase ? str.reverse : str }
  #=> ["cA", "aB", "aC", "dD", "aC", "bD", "cA", "dD"]

new_arr.group_by { |e| e }
       .select { |_,v| v.size > 1 }
       .keys
  #=> ["cA", "aC", "dD"]

Explanation

The calculation of new_arr is straightforward.

new_arr = ["cA", "aB", "aC", "dD", "aC", "bD", "cA", "dD"]

a = new_arr.group_by { |e| e }
  #=> { "cA"=>["cA", "cA"], "aB"=>["aB"], "aC"=>["aC", "aC"],
  #     "dD"=>["dD", "dD"], "bD"=>["bD"] }

b = a.select { |_,v| v.size > 1 }
  #=> { "cA"=>["cA", "cA"], "aC"=>["aC", "aC"], "dD"=>["dD", "dD"] }

b.keys
  #=> ["cA", "aC", "dD"]

How to find the same item in an array with Ruby?

3 Answers3