Ruby - Count the number of strings in an array that contain either of two characters

Question

I have this array:

@a = ["P1 - D", "P3 - M", "P1 - D", "P1 - M", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - M", "P2 - D", "P2 - D", "P2 - D", "P2 - M", "P2 - M", "P3 - D", "P3 - D", "P - D", "P1 - M", "P - D", "P - D", "Post - D", "S1 - D", "P1 - M"]

Every string is based on Page# - Device. So P1 - D is: Page1 - Desktop & P3 - M is: Page3 - Mobile

How can find how many of the strings inside array has D or M?

Might be worth putting what the answer is, do you want a total, i.e. 27 or something like `{"D"=>20, "M"=>7}`. — Kris, Feb 27 '17 at 14:40
You need to edit the title to make it consistent with the question. Perhaps, "Count the number of strings in an array that contain either of two characters." — Cary Swoveland, Feb 27 '17 at 22:26

score 4 · Accepted Answer · edited May 23 '17 at 11:53

4

a.group_by { |e| e[-1] }.each_with_object({}) { |(k, v), hash| hash[k] = v.count }
#=> {"D"=>20, "M"=>7}

Steps:

groups = a.group_by { |e| e[-1] }
# {
#   "D"=> ["P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P2 - D", "P2 - D", "P2 - D", "P3 - D", "P3 - D", "P - D", "P - D", "P - D", "Post - D", "S1 - D"],
#   "M"=> ["P3 - M", "P1 - M", "P1 - M", "P2 - M", "P2 - M", "P1 - M", "P1 - M"]
# }

group_counts = groups.each_with_object({}) { |(k, v), hash| hash[k] = v.count }
#=> {"D"=>20, "M"=>7}

group_counts['M']
#=> 20

edit

With Ruby 2.4+ you can make use of Hash#transform_values (credits go to Alex Golubenko :)):

a.group_by { |e| e[-1] }.transform_values(&:size)
#=> {"D"=>20, "M"=>7}

edited May 23 '17 at 11:53

Community

1
1

answered Feb 27 '17 at 14:28

Andrey Deineko

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Thanks @Andrey, When I run this, I get error: `undefined method last` Please take a look at this link: [function on repl.it](https://repl.it/FzZ2/1) – Rubioli Feb 27 '17 at 14:32
@user6589814 that is strange... you could use `group_by{ |el| el[-1] }` then – Andrey Deineko Feb 27 '17 at 14:36
`a.group_by { |e| e[-1] }.transform_values(&:size)` ;) – Oleksandr Holubenko Feb 27 '17 at 14:46
@AlexGolubenko `transform_values`!! Cool, did not use it before – Andrey Deineko Feb 27 '17 at 14:47
Isn't `String#last` a Rails method? – Eric Duminil Feb 27 '17 at 14:55
@EricDuminil that's why it is not working for OP, I tested in rails console, thx! Editing – Andrey Deineko Feb 27 '17 at 14:55
BTW, it's not very efficient to use `group_by` just to get the sizes of the arrays. – Eric Duminil Feb 27 '17 at 15:06
@EricDuminil I had the version identical to what you showed (using hash `default_proc`) but just did not paste it (it should have been "Now, more efficient way.." part). Well, busy working day.. – Andrey Deineko Feb 27 '17 at 15:08
No need for `default_proc` AFAICT, since `0` isn't mutable. – Eric Duminil Feb 27 '17 at 15:10
Thanks a bunch @AndreyDeineko Works like a charm :) – Rubioli Feb 28 '17 at 09:58

score 3 · Answer 2 · answered Feb 27 '17 at 14:32

3

Possible solution:

@a.group_by { |e| e[-1] }.map {|e, a| [e, a.size]}.to_h
=> {"D"=>20, "M"=>7}

answered Feb 27 '17 at 14:32

Ilya

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Oleksandr Holubenko · Answer 3 · 2017-02-27T14:42:01.200

2

@a = ["P1 - D", "P3 - M", "P1 - D", "P1 - M", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - D", "P1 - M", "P2 - D", "P2 - D", "P2 - D", "P2 - M", "P2 - M", "P3 - D", "P3 - D", "P - D", "P1 - M", "P - D", "P - D", "Post - D", "S1 - D", "P1 - M"]
@a.count { |string| string.match(/D|M/) }
#=> 27

edited Feb 27 '17 at 14:42

answered Feb 27 '17 at 14:32

Oleksandr Holubenko

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I like this best because it's efficient and reads best. Having `count` up-front tells the reader what you are doing. In Ruby v2.4+ you can use `string.match? /D\M/`, which arguably reads even better. You could of course use `/[DM]/` instead of `/D}M/`. I suggest you delete the first line. Since the OP defined `@a` it's redundant and it makes your answer less visually attractive. – Cary Swoveland Feb 27 '17 at 22:21

score 1 · Answer 4 · answered Feb 27 '17 at 14:39

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@a.select { |word| word.include?('D') || word.include?('M') }.size
# => 27

answered Feb 27 '17 at 14:39

Kris

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Eric Duminil · Answer 5 · 2017-02-27T15:01:44.790

1

If you just want the total, you could use :

@a.grep(/D|M/).count
#=> 27

If you want subtotals, this should be the most efficient method, since it doesn't create any temporary array :

@a.each_with_object(Hash.new(0)) { |string, count| count[string[-1]] += 1 }
#=> {"D"=>20, "M"=>7}

edited Feb 27 '17 at 15:01

answered Feb 27 '17 at 14:53

Eric Duminil

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Ruby - Count the number of strings in an array that contain either of two characters

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