CODEWITHSUNDEEP
pythonnodessingly-linked-list
mesunmoon
mesunmoon

Reputation: 35

Function not returning anything when it should be returning int

So I've got this function that is supposed to take an Optional[Node] as a parameter. The Node class is recursive, and takes in data: int and next: Optional[Node] as parameters. The idea of my function is to return the last data int value associated with the linked list, and to use recursion to do so.

My function does print "Returns {head.data}" upon completion, but isn't accompanied by the return statement. It's almost as if it never reaches the return statement and just stops at the print function. I'm completely stumped.

Here's the Node class:

class Node:
    """An item in a singly-linked list."""
    data: int
    next: Optional[Node]

    def __init__(self, data: int, next: Optional[Node]):
        """Construct a singly linked list. Use None for 2nd argument if tail."""
        self.data = data
        self.next = next

    def __repr__(self) -> str:
        """Produce a string representation of a linked list."""
        if self.next is None:
            return f"{self.data} -> None"
        else:
            return f"{self.data} -> {self.next}"

And the problematic function:

def last(head: Optional[Node]) -> Optional[int]:
    """Returns the last value of a Linked List, or None if the list is empty."""
    if head is None:
        print("Returning None...")
        return None
    else:
        if head.next is None:
            print(f"Returns {head.data}")
            return head.data
        else:
            print("Calling last...")
            last(head.next)

My terminal output:

>>> last(Node(110, Node(210, Node(310, None))))
Calling last...
Calling last...
Returns 310

So, the function is supposed to return 310, but it doesn't. It just prints out the print statement and stops there. What could be wrong? Is it the line "if head is None:"? Though, it never prints out "Returning None...", so that doesn't seem to be the problem.

Upvotes: 1

Views: 229

Answers (2)

Red
Red

Reputation: 27577

I believe you neglected to return last(head.next). Note that the else statements aren't necessary, as the return statements does the work for you:

def last(head: Optional[Node]) -> Optional[int]:
    """Returns the last value of a Linked List, or None if the list is empty."""
    if head is None:
        print("Returning None...")
        return None
    if head.next is None:
        print(f"Returns {head.data}")
        return head.data
    print("Calling last...")
    return last(head.next)

Upvotes: 3

buckley-w-david
buckley-w-david

Reputation: 301

You've forgotten to actually return the result of the recursive call to last

def last(head: Optional[Node]) -> Optional[int]:
    """Returns the last value of a Linked List, or None if the list is empty."""
    if head is None:
        print("Returning None...")
        return None
    else:
        if head.next is None:
            print(f"Returns {head.data}")
            return head.data
        else:
            print("Calling last...")
            return last(head.next) # Added return here

Upvotes: 0

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