[tinco]

Sure, simply do:

    module Enumerable
      def dollar(c)
        map.with_index{|a,i| a == c ? i : nil }.reject{|i| i.nil? }
      end
    end

And then you could do:

    c = a.dollar("\n")

There is of course a reason this dollar method is not a part of the standard library. Its name makes no sense and it's oddly specific, how often would you want the indexes of matches to a character? Most modern languages don't like to work with indexes a lot, and in my day to day work I don't need indexes very frequently either. This I guess is just a thing that these finance/apl people do more often, so they have a specialized standard library.

(So when I said 'a neat way to do $' I meant it has no find_all_with_index method, which would make my implementation much cleaner)

[Widdershin]

Regarding find_all_with_index, you can do:

    .find_all.with_index { |item, index| ... }

[dragonwriter]

except .find_all.with_index passes the index to the predicate block, but still only returns a view of the matching elements from the list.

What I think is being sought is more like:

  module Enumerable
    def find_indices
      each.with_index
          .find_all {|x,_| yield x}
          .map {|_,y| y}
    end
  end