using inject to count elements in array

Question

I'm trying to count the occurrences of elements in an array and save it in a hash. I'd like to use the inject function. I have this code:

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

I don't understand why I get the following error:

TypeError: can't convert String into Integer
    from (irb):47:in `[]'
    from (irb):47:in `block in irb_binding'
    from (irb):47:in `each'
    from (irb):47:in `inject'

Also, I don't know how to fix it.

When you don't know how to fix something, do debug printing. Print everything that happens. You'll likely have insights. — Sergio Tulentsev, May 31 '13 at 15:29
Related: http://stackoverflow.com/a/4314292/2988 and http://stackoverflow.com/a/10576460/2988 — Jörg W Mittag, Jun 01 '13 at 04:51

score 10 · Accepted Answer · edited May 31 '13 at 16:38

10

inject calls your block with two parameters, memo and current element. It then takes return value of the block and replaces memo with it. Your block returns integers. So, after first iteratation, your memo is not a hash anymore, it's an integer. And integers don't accept strings in their indexers.

Fix is easy, just return hash from the block.

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

You may prefer each_with_object because it doesn't replace memo. Note that each_with_object accepts parameters in reverse (element first, memo second).

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

edited May 31 '13 at 16:38

steenslag

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answered May 31 '13 at 15:24

Sergio Tulentsev

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probably good answer, but for self learning I would ask you - *each_with_object accepts parameters in reverse (element first, memo second).* So how this reverse orders helps, which `inject` doesn't. Can I have more flavor on this? – Arup Rakshit May 31 '13 at 15:30
2

It's not the order that helps. It's that `each_with_object` doesn't replace memo. – Sergio Tulentsev May 31 '13 at 15:31
@RubyLovely the order has nothing to do with it, it's just how `each_with_object` works. – Patrick Oscity May 31 '13 at 15:33
make sense now from the line *each_with_object doesn't replace memo*..forgot for a while..Thanks Buddy :) – Arup Rakshit May 31 '13 at 15:35
@steenslag Intersting.. to know `inject` returns 1. But any doc for the same to see? – Arup Rakshit May 31 '13 at 17:29
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@RubyLovely `inject` does not return 1; `hash[word] += 1` does. – steenslag May 31 '13 at 18:13

using inject to count elements in array

1 Answers1