Sadly I can't answer this question, because I don't have an edition of "How to Guess Programs", however libraries near you have a book called "How to Design Programs".
Emphasis Design, nobody wants to play "fix my program bingo" forever.
If you don't follow the advice and strategies you are given, then you don't learn to solve easy problems like this in a strategic way.
This is still "what does the function you call do?" like here:
Don't just assume you understand parts of the program, actually proof to yourself that everything works as you think it does. Because only if you test your assumptions, you will find the mistakes.