I implemented a recursive Sieve of Erasthotenes, which, from the debug statements used, appears to work, but returns None.
The code with the debug statements looks like this:
def sieb2 (iterable, container):
print("Just called:", iterable, container, '\n')
if len(iterable) != 1:
container.append(iterable [0])
iterable = [item for item in iterable if item % iterable [0] != 0]
print("New call:", iterable, container, '\n')
sieb2(iterable, container)
else:
container.append(iterable[0])
print("Return:", iterable, container, '\n')
print("Container:", container)
return container
The function is (for instance) called with:
lst = list(range(2, 10) # I might add statements for removing everything <2 and sorting
primes = []
print(sieb2(lst, primes))
The output for this input looks like this:
# Debug-Statements from the function:
Just called: [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] []
New call: [3, 5, 7, 9, 11, 13, 15, 17, 19] [2]
Just called: [3, 5, 7, 9, 11, 13, 15, 17, 19] [2]
New call: [5, 7, 11, 13, 17, 19] [2, 3]
Just called: [5, 7, 11, 13, 17, 19] [2, 3]
New call: [7, 11, 13, 17, 19] [2, 3, 5]
Just called: [7, 11, 13, 17, 19] [2, 3, 5]
New call: [11, 13, 17, 19] [2, 3, 5, 7]
Just called: [11, 13, 17, 19] [2, 3, 5, 7]
New call: [13, 17, 19] [2, 3, 5, 7, 11]
Just called: [13, 17, 19] [2, 3, 5, 7, 11]
New call: [17, 19] [2, 3, 5, 7, 11, 13]
Just called: [17, 19] [2, 3, 5, 7, 11, 13]
Mew call: [19] [2, 3, 5, 7, 11, 13, 17]
Just called: [19] [2, 3, 5, 7, 11, 13, 17]
Return: [19] [2, 3, 5, 7, 11, 13, 17, 19]
Container: [2, 3, 5, 7, 11, 13, 17, 19]
# Printed return:
None
From what I can see the function works as it should and goes into the else-statement with the return, the print-statement is executed properly too and outputs the final container.
Why does the return-statement output a NoneType and not the container list?
You're just missing one return statement from your recursive function:
def sieb2(iterable, container):
print("Just called:", iterable, container, '\n')
if len(iterable) != 1:
container.append(iterable[0])
iterable = [item for item in iterable if item % iterable[0] != 0]
print("New call:", iterable, container, '\n')
return sieb2(iterable, container) # FIXED
else:
container.append(iterable[0])
print("Return:", iterable, container, '\n')
print("Container:", container)
return container
Always remember to return the functions which you're calling recursively so that their values get correctly returned to the stack.