Neat!

@snake2

Assume that there is no match for the midpoint, so you'd return -1.

arr.each_index do |i| will iterate n times (where n is the length of arr).
Inside each iteration, the reduce call is itself iterating m times (0...i) and l times (i+1..-1) [l + m = n - 1, though, which is effectively n].
So you are doing n times n (or n²) iterations, so you have a function that costs O(n²) to run.

This can be solved with a cost of O(n). Someone smarter than me might have one even better. Maybe.

what makes it not eff? also what of my code? its a little similar, but im just a beginner.

So cool!

Well, I mean, the instructions say to return a variable 'l', and it's part of the code that's automatically put into the solution. I know my comment seems pedantic, but that wasn't my intention. I really do think this is the best answer, just that it doesn't follow that specific instruction.

Neither efficient or readable