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Is there a way to modify the implementation of map() in the Array class such that it only affects certain indices of the array?

Example:

a = [1, 2, 3, 4, 5]
a.map(2..4) { |x| x*2 }

Now, a = [1, 2, 6, 8, 10] since the map function was only used on indices 2 and 3.

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  • What happened to index 4? Commented Oct 16, 2020 at 0:15
  • Let r = 2..4. Then you need to write a.map.with_index { |n,i| r.cover?(i) ? 2*n : n } #=> [1, 4, 6, 8, 10]. That returns a new array and leaves a unchanged. If you wish to modify a use map! rather than map. See Range#cover?. Commented Oct 16, 2020 at 1:45
  • Sorry about that. I'm still very new to Ruby and forgot that ".." is inclusive. I have fixed it in my question. Commented Oct 17, 2020 at 1:41

1 Answer 1

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You could (but really shouldn't) do this:

Array.class_eval do
  def map(range = nil)
    return super() if range.nil?
    return self[range].map unless block_given?

    self[range].map { |x| yield x }
  end
end

[1, 2, 3, 4, 5].map(2..4) { |x| x * 2 }
# => [6, 8, 10]

Ruby already has a MUCH more normal/nice/better way of "select only certain indices", with arr[2..4]:

arr = [1, 2, 3, 4, 5]
arr[2..4].map { |x| x * 2 }
# => [6, 8, 10]

I've avoided mutation above, but if you must have that as well, you can do something similar to the above, just with map! instead.

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1 Comment

Thank you so much. I would have just used the Ruby implementation of what I was talking about, but my professor ONLY wants us to modify the Array class. I have been trying to solve this problem for a week but it seems my Ruby fundamentals are still not that great. Thank you for providing your insight, I really appreciate it.

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