CODEWITHSUNDEEP
pythonif-statementternary-operator
jayko03
jayko03

Reputation: 2481

python ternary if statement does not catch None

I am implementing the algorithm that adds two numbers from two linked lists in python. (From cracking the coding interview 2-5)

For example,

first: 7 -> 1 -> 6      617
second: 5 -> 9 -> 2    +295
                       -----
                        912
output: 2 -> 1 -> 9 ( which indicates 912 )

This is my code,

class Node:
    def __init__(self, val=None):
        self.data = val
        self.Next = None

class LinkedList:
    def __init__(self):
        self.head = None
        self.size = 0

    def __repr__(self):
        temp = self.head
        alist = []
        while temp:
            alist.append(temp.data)
            temp = temp.Next
        return str(alist)

    def add(self, val):
        cur = self.head
        prev = None
        if cur is None:
            self.head = Node(val)
        else:
            while cur:
                prev = cur
                cur = cur.Next
            prev.Next = Node(val)
        self.size += 1

def adding(p1,p2):
    pointer1 = p1.head
    pointer2 = p2.head
    remainder = 0
    sum_list = LinkedList()

    while pointer1 is not None or pointer2 is not None:
        first = 0 if pointer1.data is None else pointer1.data
        second = 0 if pointer2.data is None else pointer2.data
        sum_ = first + second + remainder

        remainder = 1 if sum_ >= 10 else 0

        sum_ %= 10

        sum_list.add(sum_)

        if pointer1 is not None:
            pointer1 = pointer1.Next
        if pointer2 is not None:
            pointer2 = pointer2.Next
    if remainder > 0:
        sum_list.add(remainder)
    return sum_list

My problem is first = 0 if pointer1.data is None else pointer1.data. It is working when the size of both linked lists are same, however, if one is shorter than others, the shorter one becomes None. So I expect my if statement catches this and makes variable(first) as 0. However it throws AttributeError: NoneType object has no attribute 'data'.

It is working if I write normally, not ternary operator

if pointer1 is None:
    first = 0
else:
    first = pointer1.data
if pointer2 is None:
    second = 0
else:
    second = pointer2.data

Did I miss anything when I use ternary operator? Thanks!

Upvotes: 8

Views: 12098

Answers (2)

John Szakmeister
John Szakmeister

Reputation: 47012

Yes, you aren't actually doing what the if/else statement is doing with the ternary operator.

This:

if pointer1 is None:
    first = 0
else:
    first = pointer1.data
if pointer2 is None:
    second = 0
else:
    second = pointer2.data

Would be the following:

first = 0 if pointer1 is None else pointer1.data
second = 0 if pointer2 is None else pointer2.data

In your version:

first = 0 if pointer1.data is None else pointer1.data

It's possible that pointer1 is None, and therefore doesn't have a data attribute, which is why you're getting the exception. So you need to check that pointer1 is not None before accessing data.

Upvotes: 13

Izaak Weiss
Izaak Weiss

Reputation: 1310

The problem is that you are checking if pointer.data is None. But it is actually pointer which will be None, so when your code gets to the ternary if, it will first try and get the attribute data from a None object.

You need to change your code so that it checks to see if pointer is None: first = 0 if pointer1 is None else pointer1.data

Upvotes: 2

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