Ruby: Collect index from Array/String Matchdata

Question

I'm new to Ruby, here's my problem : I would like to iterate through either an Array or String to obtain the index of characters that match a Regex.

Sample Array/String

 a = %q(A B A A C C B D A D)
 b = %w(A B A A C C B D A D)

What I need is something for variable a or b like ;

#index of A returns;
[0, 2, 3,8]

#index of B returns
[1,6]

#index of C returns
[5,6]
#etc

I've tried to be a little sly with

z = %w()

a =~ /\w/.each_with_index do |x, y|

 puts z < y

end

but that didn't workout so well. Any solutions ?

When you say that it "didn't work out so well", can you please describe how so? — user456814
– user456814, Commented Apr 14, 2014 at 7:48
I get a each_with_index method undefined for MacthData object error — yaboiduke
– yaboiduke, Commented Apr 14, 2014 at 7:52

xdazz · Accepted Answer · 2014-04-14 07:51:38Z

3

For array, you could use

b.each_index.select { |i| b[i] == 'A' }

For string, you could split it to an array first (a.split(/\s/)).

answered Apr 14, 2014 at 7:51

xdazz

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3 Comments

yaboiduke Over a year ago

Is there a way of iterating through the Array instead of naming each character by hand?

xdazz Over a year ago

@user3437917 What do you mean by naming each character by hand?

yaboiduke Over a year ago

e.g for this answer, the A in the block was named. I would like to do the same of the B character without having to write a new block of code for B - or maybe I just didn't understand the code - I should try it first- newbie:)

Stefan · Accepted Answer · 2014-04-14 08:21:08Z

1

If you want to get each character's index as a hash, this would work:

b = %w(A B A A C C B D A D)

h = {}
b.each_with_index { |e, i|
  h[e] ||= []
  h[e] << i
}
h
#=> {"A"=>[0, 2, 3, 8], "B"=>[1, 6], "C"=>[4, 5], "D"=>[7, 9]}

Or as a "one-liner":

b.each_with_object({}).with_index { |(e, h), i| (h[e] ||= []) << i }
#=> {"A"=>[0, 2, 3, 8], "B"=>[1, 6], "C"=>[4, 5], "D"=>[7, 9]}

answered Apr 14, 2014 at 8:21

Stefan

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Comments

LukasMac · Accepted Answer · 2014-04-14 08:31:55Z

0

If you want to count occurrences of each letter you can define helper method:

def occurrences(collection)
  collection = collection.split(/\s/) if collection.is_a? String

  collection.uniq.inject({}) do |result, letter|
    result[letter] = collection.each_index.select { |index| collection[index] == letter }
    result
  end
end

# And use it like this. This will return you a hash something like this: 
# {"A"=>[0, 2, 3, 8], "B"=>[1, 6], "C"=>[4, 5], "D"=>[7, 9]}
occurrences(a)
occurrences(b)

This should work either for String or Array.

edited Apr 14, 2014 at 8:31

answered Apr 14, 2014 at 8:25

LukasMac

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