Question

In most other languages, the catch and throw statements do what the begin, rescue, and raise statements do in Ruby. I know the you can do this with these two statements:

catch :done do
  puts "I'm done."
end

and

if some_condition
  throw :done
end

But what is this useful for? Can somebody please give me an example of what catch and throw statements are used for in Ruby?

Answer 1

You can use this to break out of nested loops.

INFINITY = 1.0 / 0.0
catch (:done) do
  1.upto(INFINITY) do |i|
    1.upto(INFINITY) do |j|
      if some_condition
        throw :done
      end
    end
  end
end

If you had used a break statement above, it would have broken out of the inner loop. But if you want to break out of the nested loop, then this catch/throw would be really helpful. I have used it here to solve one of the Euler problems.