Python IndexError : list assignment index out of range

Question

class Solution:
    def trap(self, height: List[int]) -> int:
        n = len(height)
        mxl = []
        mxr = []
        mxl.append(height[0])
        for i in range(1,n):
            mxl[i] = max(mxl[i-1],height[i])

Line 8 gives Index error. Is it wrong way to assign value in a list at particular index?

`mxl[i]` is trying to assign to an index that doesn't exists. Use `mxl.append` instead. — Guy, Mar 09 '22 at 11:20
Yes. You are assigning `mxl[i]` when `i==1`. At this point `mxl` only has one element. — quamrana, Mar 09 '22 at 11:20

score 0 · Answer 1 · answered Mar 09 '22 at 11:26

0

Fix:

class Solution:
    def trap(self, height: list[int]) -> int:
        n = len(height)
        mxl = []
        mxr = []
        for item in height:
            mxl.append(item)
        for i in range(1, n):
            mxl[i] = max(mxl[i - 1], height[i])

answered Mar 09 '22 at 11:26

TheCoder1001

22
4

1

Why defining `mxr` when it's not being put to any use? – Learning is a mess Mar 09 '22 at 11:26
Why to add items to `mxl` just to override them later? Why not just `mxl.append(max(mxl[i - 1], height[i]))`? – Tomerikoo Mar 09 '22 at 11:28
I just wanted to fix the code without making any major changes. And I think the person who asked the question was trying to solve a leetcode problem – TheCoder1001 Mar 09 '22 at 13:48

score 0 · Answer 2 · answered Mar 09 '22 at 11:26

0

By the way, np.maximum.accumulate is the fastest (and reliable) way of doing what you are trying to achieve here

mxl = np.maximum.accumulate(height)

answered Mar 09 '22 at 11:26

Learning is a mess

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Python IndexError : list assignment index out of range

2 Answers2