Error in linked lists function in python

Question

I have to Convert a number into linked list such that each digit is in a node and pointing to node having next digit.The function should return head of the linked list. e.g input 120 should create a list of Nodes 1 -> 2 -> 0 and return the reference.If it is a negative number say -120 it should return -1->-2->0.I tried to do it in this way:

def number_to_list(number):
    head,tail = None,None
    for x in str(number):
        if x<0:
           x = -int(x)
           node = Node(int(x))
        else:
           node = Node(int(x))
        if head:
           tail.next = node
        else:
            head = node
        tail = node
    return head
    pass

It is working fine for positive numbers but if I pass -120.It is showing an error:

ValueError: invalid literal for int() with base 10: '-'.

How can I fix it.

If you're iterating through the characters in the string representation of a number and calling int() on each, you'll need to special-case "-", because it's not an integer. — Wooble
– Wooble, Commented Feb 3, 2014 at 16:54
My function call is this:'head = number_to_list(-120)'@Wooble — Ramya
– Ramya, Commented Feb 3, 2014 at 16:54
Isn't this basically your other question asked again? stackoverflow.com/questions/21532916/… — user44484
– user44484, Commented Feb 3, 2014 at 17:06

zvisofer · Accepted Answer · 2014-02-03 16:55:16Z

1

In the first iteration with negative number you actually do:

int('-')

answered Feb 3, 2014 at 16:55

zvisofer

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Comments

Aswin Murugesh · Accepted Answer · 2014-02-03 16:59:51Z

1

The problem is you are iterating over a string

for x in string(number):

Makes x a character, which iterates over the string number. Now when you have a positive number, each digit in the number can be converted to an int. But when you pass a negative number, x takes the value of '-' which cannot be converted to an integer. There you get the error. By what I see, I think you are storing the absolute values of the digits of the numbers in the list. This can be done by checking if the character is '-', where you can just give a pass to do nothing in this iteration.

if x=='-':
    continue

edited Feb 3, 2014 at 16:59

answered Feb 3, 2014 at 16:54

Aswin Murugesh

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Comments

Kevin · Accepted Answer · 2014-02-03 17:14:20Z

1

You should add a line of code to check for '-' (and possibly '.' if you allow decimal values.)

if(x=='-'):
    # Mark the number as negative
    negative = True
    # Move on to the next character
    continue

edited Feb 3, 2014 at 17:14

answered Feb 3, 2014 at 16:56

Kevin

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Comments

Jerry_Y · Accepted Answer · 2014-02-03 16:59:50Z

0

str(-120) return '-','1','2','0'

try this:

    needMinus = number < 0
    for x in str(abs(number)):
        if needMinus:
            x = -int(x)
            needMinus = False
            node = Node(int(x))
        else:
            node = Node(int(x))
        if head:
            tail.next = node

        else:
            head = node
        tail = node

answered Feb 3, 2014 at 16:59

Jerry_Y

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Comments

zhangxaochen · Accepted Answer · 2014-02-03 17:14:05Z

Converting a string of non (base 10, by default) integer gives a ValueError:

In [1034]: int('-')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-1034-4c2a77a1869c> in <module>()
----> 1 int('-')

ValueError: invalid literal for int() with base 10: '-'

likewise, int('10L'), int('0x1a'), int('abc'), etc.

You can treat number as a positive, and then add a negative sign to each single digit if number is actually a negative:

def number_to_list(number):
    head,tail = None,None
    if number<0: number=-number
    sign=-1 if number<0 else 1

    for x in str(number):
        node = Node(sign*x)

        if head:
            tail.next = node
        else:
            head = node
        tail = node
    return head
    pass

hemanth · Accepted Answer · 2014-02-03 17:41:44Z

0

when you pass a negative number this int('-') is called in the first iteration. You can keep a boolean flag through which you can identify if a number is positive or negative.

Example: Note: I have not tested this code. This code is just to give an idea

  def number_to_list(number):
    head,tail = None,None
    positive = True
    for x in str(number):
        if x=='-':
           positive = False
           continue
        else:
          if postive:
             node = Node(int(x))
          else:
             node = Node(int("-"+x))
        if head:
           tail.next = node
        else:
            head = node
        tail = node
    return head
    pass

edited Feb 3, 2014 at 17:41

answered Feb 3, 2014 at 17:00

hemanth

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