检查元素是否在链表中

Question

I'm trying to implement a method that returns the index of a node at a particular index. I can successfully retrieve the index when the element exists; however when it doesn't I get an error 'NoneType' object has no attribute 'data' I can't quite get why that happens. here are my linked list and node classes

class Node:
    def __init__(self,data = None,next = None):
        self.data = data
        self.next = next
class Listy:
    def __init__(self):
        self.head = None

and this is my get_index function

    def get_index(self,key):
        temp = self.head
        count = 0
        while temp.data != key:
            temp = temp.next
            count += 1
        print(count)

Thank you for your help

Answer 1

You get an error because you are trying to get property 'data' of 'temp' in the while loop but your 'temp' is 'None' because you assigned it to 'self.head' which is actually 'None' in case of empty list. I hope this makes sense

Answer 2

The root caused is because the last node of LinkedList returns None, let's change a bit Your Listy class code to avoid None value.

class Listy:
    def __init__(self, nodes=None):
        self.head = None
        if nodes is not None:
            node = Node(nodes.pop(0))
            self.head = node
            for elem in nodes:
                node.next = Node(elem)
                node = node.next
    
    def get_index(self, key):
        temp = self.head
        count = 0
        currentNode = temp.data
        while temp is not None:
            if count == key:
                currentNode = temp.data
                break
            temp = temp.next
            count += 1
        return currentNode

Let's try to call:

if __name__ == "__main__":
    nodes = ["a", "b", "c"]
    ll = Listy(nodes)
    idx_1 = ll.get_index(0)
    idx_2 = ll.get_index(1)

    print(idx_1)
    print(idx_2)

Return will be as follow:

a
b

Answer 3

Your code is too optimistic about finding the key: it does not check whether you have reached the end of the list before finding the value, and so temp will become None resulting in an exception when temp.data is evaluated.

So change your loop to make sure that temp is still not None , and return the index when the key is found (instead of printing it):

def get_index(self, key):
    temp = self.head
    count = 0
    while temp:
        if temp.data == key:
            return count
        temp = temp.next
        count += 1

Note how this will return None when the key is not found.