Know Ruby: with_index

Have you ever used with_index? Not each_with_index which is similar but slightly different. Did you know that you can do map.with_index?
Adding with_index to an enumeration lets you enumerate that enumeration. Say that ten times fast. A quick example will clarify that a bit. Let’s say I have a list of three, I don’t know, famous Martians.

martians = ["Marvin", "J'onn J'onzz", "Mark Watney"]

I’ll list them along with their current position in the array.

> martians
*   .each
*   .with_index(1) do |martian, i|
*     puts "#{i}) #{martian}"
>   end
1) Marvin
2) J'onn J'onzz
3) Mark Watney
=> ["Marvin", "J'onn J'onzz", "Mark Watney"]

As I mentioned earlier, with_index isn’t limited to each. I could replace each with map in the example above.

> martians
*   .map
*   .with_index(1) do |martian, i|
*     "#{i}) #{martian}"
>   end
=> ["1) Marvin", "2) J'onn J'onzz", "3) Mark Watney"]

You probably noticed that I’m passing 1 to with_index. It accepts an integer offset defaulted to 0. I’ve found this to be handy when generating user-facing information. They usually don’t want their lists to be zero-indexed. No more having to do i + 1 inside the block. It’s also useful when you have a dynamic list that starts after some hard-coded entries.

Alexander M almost 6 years ago
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