IndexError: list index out of range for nested list (matrix)

Question

I have been trying to solve a problem on HackerRank, and here is my solution for it, And I am receiving this error IndexError: list index out of range on line 21, that is, s_sum += arr[i][n-1] , can someone help me figure this out?

def diagonalDifference(arr):
    p_sum = 0
    s_sum = 0
    for i in range(n):
        p_sum += arr[i][i]
        s_sum += arr[i][n-i]
    return abs(p_sum - s_sum)

n = int(input().strip())
arr = []
for _ in range(n):
    arr.append(list(map(int, input().rstrip().split())))
result = diagonalDifference(arr)
print(result)

Answer 1

When i = 0 , ie in the first iteration of the for loop, ni evaluates to n , but that is out of bounds, if arr is of size n .

Answer 2

When you loop -

for i in range(n):
    p_sum += arr[i][i]
    s_sum += arr[i][n-i]

Here, i starts from 0 and you have _sum += arr[i][n-1] which will be arr[0][n] in the first iteration. This will go out of bound since your arr can index only upto arr[i][n-1] . So, This is giving you list index out of range error.

You should do it correctly as follows -

for i in range(n):
    p_sum += arr[i][i]       #primary diagonal sum
    s_sum += arr[i][(n-1)-i] #secondary diagonal sum. notice the indexing arr[i][n-1-i] 
                             #which ensures that the array doesn't access out of its bounds

Hope this helps !

Answer 3

You get out of range error because you run

s_sum += arr[i][n-i]

for in the range [0:n-1] . So, when i = 0 your second index becomes n , that is out of bounds as it tries to access arr[0][n] .