Tail calls and pruning

Questions 1 to 5 of 8

1. 1. True or false: the call in return n * factorial(n - 1) is a tail call.

   False, because n is multiplied by the result after the call returns.True, because the call is the last thing in the return statement.
2. 2. True or false: CPython turns this tail-recursive function into a loop, so sum_to(10000000, 0) does not raise an error. ``python def sum_to(n, acc): if n == 0: return acc return sum_to(n - 1, acc + n) ``

   False, because CPython does not turn tail calls into loops, and every call keeps its own frame, so a deep call raises RecursionError.True, because CPython rewrites tail calls as loops when it runs the file.
3. 3. True or false: pruning cannot change the set of answers, as long as each cut only skips branches that cannot end in a valid answer.

   False, because a cut drops answers that the plain search would have found.True, because a cut only skips branches that cannot end in a valid answer, so the answers stay the same.
4. 4. Why is the call factorial_acc(n - 1, n * acc) in tail position?

   It uses two parameters, so Python can reuse the frame.The call runs inside an if statement, so Python removes it.It calls a different function from the one it is inside.The multiplication happens before the call, and the value of the call is returned unchanged, so nothing is left to do when it returns.
5. 5. Why does CPython keep a frame for every call, even for a tail call?

   CPython adds a check for tail calls to every function.Frames are only reused when the arguments are int values.A traceback lists each call, so reusing frames would hide calls that the code made.The interpreter cannot store numbers in a loop.