Create hash from array and frequency

Question

I have an array [1,2,4,5,4,7] and I want to find the frequency of each number and store it in a hash. I have this code, but it returns NoMethodError: undefine

Answer 1

Ruby 2.7 onwards will have the Enumerable#tally method that will solve this.

From the trunk documentation:

Tallys the collection. Returns a hash where the keys are the elements and the values are numbers of elements in the collection that correspond to the key.

["a", "b", "c", "b"].tally #=> {"a"=>1, "b"=>2, "c"=>1}

Answer 2

Just use inject. This type of application is exactly what it is meant for. Something like:

a.inject(Hash.new(0)) {|hash,word| hash[word] += 1; hash }

Answer 3

The point here is that hash[1] doesn't exist (nil) when it first sees 1 in the array.

You need to initialize it somehow, and hash = Hash.new(0) is the easiest way. 0 is the initial value you want in this case.

Answer 4

Here is a short option that uses the Hash array initializer

Hash[arr.uniq.map {|v| [v, arr.count(v)] }]

Answer 5

Or use the group by method:

arr = [1,2,4,5,4,7]

Hash[arr.group_by{|x|x}.map{|num,arr| [num, arr.size] }]

Answer 6

Love me some inject:

results = array.inject(Hash.new(0)) {|hash, arr_element| hash[arr_element] += 1; hash }

1.9.3p448 :082 > array = [1,2,4,5,4,7]
 => [1, 2, 4, 5, 4, 7] 
1.9.3p448 :083 > results = array.inject(Hash.new(0)) {|hash, arr_element| hash[arr_element] += 1; hash }
 => {1=>1, 2=>1, 4=>2, 5=>1, 7=>1}