til/ruby/jump-out-of-a-nested-context-with-throw-catch.md

Jump Out Of A Nested Context With Throw/Catch

Ruby's throw/catch construct, not to be confused with its raise/rescue exception handling syntax, allows you to jump out of a nested context. This is similar to loop labels in other languages.

For example, in my recent Advent of Code solution, I was able to employ this construct. Once within a doubly-nested loop, I can throw when I find the answer I'm looking for to both break out of the loop and return an value.

answer =
  catch do |obj|
    input.each_with_index do |input1, x|
      input.each_with_index do |input2, y|
        next unless x != y

        next unless input1 + input2 == 2020

        throw(obj, input1 * input2)
      end
    end

    raise StandardError, 'No answer found'
  end

puts answer

If I were to never reach the throw before exhausting the doubly-nested loop, then the catch would product whatever value is returned within the block. In this case, I raise an error because it'd be exceptional for the throw to never be reached.

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