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: undefined method '+' for nil:NilClass

def score( array )
  hash = {}
  array.each{|key| hash[key] += 1}
end

Desired output is

{1 => 1, 2 => 1, 4 => 2, 5 => 1, 7 => 1 }

mr, some suggest using inject (aka reduce), which is fine, but all you have to do is change `hash = {}` to `hash = Hash.new(0)`. That tells Ruby that if it encounters `hash[key]` in a context where it must have a value (such as `hash[key] += 1` or `v = hash[key]`) and the hash does not contain the key `key`, it is to add `key=>0` to the hash before taking further action (such as `hash[key] += 1`). On the other hand, `if hash[key] == 7` will evaluate to `if nil == 7` if `key` is not in the hash; `key=>0` will not be added to the hash. — Cary Swoveland, Nov 13 '13 at 20:45
@tokland, the problem may be the same, but not the question. Here, mr. wants to know why he got a particular error, which seems to me quite ligit. — Cary Swoveland, Nov 13 '13 at 21:49
fair enough, then let's say it's related :) http://stackoverflow.com/questions/9480852/array-to-hash-words-count — tokland, Nov 13 '13 at 22:08

score 25 · Answer 1 · answered Nov 21 '17 at 00:04

25

In Ruby 2.4+:

def score(array)
  array.group_by(&:itself).transform_values!(&:size)
end

answered Nov 21 '17 at 00:04

mwp

8,217
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26

Arup Rakshit · Accepted Answer · 2013-11-13T19:58:32.687

Do as below :

def score( array )
  hash = Hash.new(0)
  array.each{|key| hash[key] += 1}
  hash
end
score([1,2,4,5,4,7]) # => {1=>1, 2=>1, 4=>2, 5=>1, 7=>1}

Or more Rubyish using Enumerable#each_with_object:

def score( array )
  array.each_with_object(Hash.new(0)){|key,hash| hash[key] += 1}
end
score([1,2,4,5,4,7]) # => {1=>1, 2=>1, 4=>2, 5=>1, 7=>1}

The reason of why NoMethodError: undefined method '+' for nil:NilClass ?

hash = {} is an empty has,with default value as nil.nil is an instance of Nilclass,and NilClass doesn't have any instance method called #+. So you got NoMethodError.

Look at the Hash::new documentation :

new → new_hash
new(obj) → new_hash

Returns a new, empty hash. If this hash is subsequently accessed by a key that doesn’t correspond to a hash entry, the value returned depends on the style of new used to create the hash. In the first form, the access returns nil. If obj is specified, this single object will be used for all default values. If a block is specified, it will be called with the hash object and the key, and should return the default value. It is the block’s responsibility to store the value in the hash if required.

score 19 · Answer 3 · answered Jun 19 '19 at 18:40

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}

score 13 · Answer 4 · answered Nov 13 '13 at 20:00

13

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 }

answered Nov 13 '13 at 20:00

Josh

5,631
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28
54

score 2 · Answer 5 · answered Nov 13 '13 at 19:52

2

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}

answered Nov 13 '13 at 19:52

CDub

13,146
4
51
68

4

That would (IMO) be cleaner with `each_with_object`: `a.each_with_object(Hash.new(0)) { |e, h| h[e] += 1 }` – mu is too short Nov 13 '13 at 20:41
1

To each their own, @muistooshort :) – CDub Nov 13 '13 at 20:43
3

I tend to use `inject` for "iterate with feedback" situations and `each_with_object` for "iterate and collect" situations. I got tired of the `; hash` stuff. – mu is too short Nov 13 '13 at 21:08

score 2 · Answer 6 · answered Nov 13 '13 at 20:15

2

Here is a short option that uses the Hash array initializer

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

answered Nov 13 '13 at 20:15

Alex.Bullard

5,533
2
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32

score 1 · Answer 7 · edited Nov 13 '13 at 20:46

1

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.

edited Nov 13 '13 at 20:46

Arup Rakshit

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answered Nov 13 '13 at 20:27

jaeheung

1,208
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7

score 0 · Answer 8 · answered Nov 13 '13 at 22:35

0

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] }]

answered Nov 13 '13 at 22:35

hirolau

13,451
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47

Create hash from array and frequency

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