Probably not, but you can keep asking.
Yeah, you’ll get the right amount of iterations that way. I would’t do it that way because generating the string actually involves identifying the digits (which is what you’re trying to do). Totally possible but you’re using the solution to make the solution.
When should the loop stop, what other tells are there that you’re done?
I don’t object to using strings to get the sum, but if doing that it would be by letting the string conversion do all the work. Your approach is more low-level so it ought to use simple things (just math operations)
…I totally botched that explanation, not sure how to express that in a good way.
If you implement addition, then it would be cheating to use subtraction to do it for you. (by double-negating)
But it would be clever to implement addition through subtraction, if that’s the intention.