What does this |_ mean in Ruby?

Question

In this code:

arr.select.each_with_index { |_, i| i.even? }

what does pipe underscore mean?

Answer 1

_ is a variable name like every other variable name (for example i).

It is a convention in Ruby to use _ as a variable name or prefix variable names with _ (like _i) as an indication that you do not plan to use that variable later on.

In your example, each_with_index yields two values in each step of the iteration: The current element and the current index.

each_with_index { |_, i| i.even? }

The author of the code needed to name both values but decided to indicate with the variable name _ that they do not care about the current value, only about the current index.