IndexError: list index out of range in"if... else..."

Question

i practice a test of "Remove Duplicates from Sorted Array" in leetcode.Everything is fine but when i test input[1,1] ,it failed with:

IndexError: list index out of range

May i know why?(i already got a correct solution but i still wonder this error.)

here is my code:

class Solution(object):
    def removeDuplicates(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        index_o=0
        i=1
        while i <len(nums):   
              if nums[index_o]==nums[i]:
                  nums.remove(nums[i])
                  index_o-=1
                  i-=1
                  print("nums[index_o]:",nums[index_o])
                  print("index_o:",index_o)
                  print("nums[i]:",nums[i])
                  print("i:",i)
                  print("nums:",nums)
              else:
                  index_o+=1
                  i+=1
        return len(nums)

n=[1,1]
a=Solution()
print(a.removeDuplicates(n))
print(n)

here is the result:

f:leetcode>python 190626.py
nums[index_o]: 1
index_o: -1
nums[i]: 1
i: 0
nums: [1]
Traceback (most recent call last):
  File "190626.py", line 228, in <module>
    print(a.removeDuplicates(n))
  File "190626.py", line 217, in removeDuplicates
    print("nums[index_o]:",nums[index_o])
IndexError: list index out of range

here is a correct solution:

class Solution(object):
    def removeDuplicates(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        index_o=0
        i=1
        while i <len(nums): 
              if nums[index_o]==nums[i]:
                  nums.remove(nums[index_o])
              else:
                  index_o+=1
                  i+=1
        return len(nums)

Answer 1

When the first time arrives to execute the while loop and the statement of if is true, then the variable index_o will take the value -1, the variable i will be 0 and the while loop will be run again as i=0 < len(nums)=1. So the next time, if statement will check if nums[-1] == nums[0] which is out of index. I suggest you to add an if statement just to make sure your array pointers will not get negative values.

Answer 2

In the third iteration of the loop, you have the variables i=-1 and n=[]. Your code enters to first if as -1 < 0 equals True.*

At this point you try to access your array with the variable index_o, which is -2. Because your array is empty, n[-2] returns IndexError: list index out of range error.

Some comments on your code:
- The algorithm does not do what it supposed to do but I can understand your thinking.
- Why would remove duplicates function would return the length of the output array, and not the array itself ?
- To test that algorithm, you can use the set() built-in, that will remove the duplicates from an array. Then you can compare them.
- When developing algorithms, always use debugging by printing the values, as advised by @Mark-meyer in the comments.

*I didn't run your algorithm locally and debugged, I just tracked it from head. So some values might be off by one.