Finding duplicate keys across hashes?

Question

To simplify things let's say I have the hashes below.

I would like to find the keys that are in multiple hashes and the name of the hashes. So ideally I would like to end up with

A is also in  a
A is also in  b
B is also in  a
B is also in  b
D is also in  b
D is also in  c
E is also in  b
E is also in  c

The only way I can think of is: putting all keys in an array, sort it, remove unique elements, search each hash that contain the remaing array elements.

It is somewhat complicated and hacky I guess.

Question

Is there an easier way to find duplicate keys across hashes?

!/usr/bin/ruby                                                                                          

require 'ap'

a = {}
b = {}
c = {}

a["A"] = 1
a["B"] = 1
a["C"] = 1

b["A"] = 1
b["B"] = 1
b["D"] = 1
b["E"] = 1

c["D"] = 1
c["E"] = 1
c["F"] = 1

What is the "name of the hashes"? Hashes don't have names in Ruby. — Jörg W Mittag, Sep 12 '16 at 10:01

score 1 · Answer 1 · answered Sep 12 '16 at 09:38

To find the hashes with a certain key you can do.

def find_key_in list_of_hashes, key
  list_of_hashes.select { |one_hash| one_hash.detect { |k,v| k == key }}
end

Call it like this:

irb(main):016:0> find_key_in [{a: 3, b: 2}, {b: 3, x: 4} ], :x
=> [{:b=>3, :x=>4}]

Printing the names of the hashes is tricker. See this question.

Bartłomiej Gładys · Answer 2 · 2016-09-12T09:48:29.997

1

something like this?

arr = { 'a' => a, 'b' => b, 'c' =>c}
#=> {"a"=>{"A"=>1, "B"=>1, "C"=>1}, "b"=>{"A"=>1, "B"=>1, "D"=>1, "E"=>1}, "c"=>{"D"=>1, "E"=>1, "F"=>1}}

def my_method(letter, arr)
 arr.map { |el|  "#{letter} is in #{el[0]}" if !el[1]["#{letter}"].nil? }.compact
end

example:

my_method("A", arr)
#=> ["A is in a", "A is in b"]

edited Sep 12 '16 at 09:48

answered Sep 12 '16 at 09:41

Bartłomiej Gładys

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score 1 · Accepted Answer · answered Sep 12 '16 at 09:47

You could build another hash to store each key and its hashes:

keys = Hash.new { |hash, key| hash[key] = [] }
a.each_key { |k| keys[k] << :a }
b.each_key { |k| keys[k] << :b }
c.each_key { |k| keys[k] << :c }

More precisely, keys stores an array of symbols. It looks like this after running the above code:

keys
#=> {"A"=>[:a, :b],
#    "B"=>[:a, :b],
#    "C"=>[:a],
#    "D"=>[:b, :c],
#    "E"=>[:b, :c],
#    "F"=>[:c]}

To get your expected output:

keys.each do |key, hashes|
  next if hashes.size < 2
  hashes.each { |hash| puts "#{key} is also in #{hash}" }
end

Prints:

A is also in a
A is also in b
B is also in a
B is also in b
D is also in b
D is also in c
E is also in b
E is also in c

Finding duplicate keys across hashes?

3 Answers3

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