Замена первого и последнего элемента матрицы на максимальное и минимальное значение

import random
import sys

arr = [
    [random.randint(10, 99) for i in range(20)] for j in range(20)
]

maxVal = ~sys.maxsize
minVal = sys.maxsize
indexMax = 0
indexMin = 0

def printMatrix(arr):
    for i in range(len(arr)):
        for j in range(len(arr[i])):
            print(arr[i][j], end=" ")
        print()

print("Начальная матрица:")
printMatrix(arr)

for i in range(len(arr)):
    for j in range(len(arr[i])):
        if maxVal < arr[i][j]:
            maxVal = arr[i][j]
        if minVal > arr[i][j]:
            minVal = arr[i][j]

        if j == (len(arr[i]) - 1):
            for a in range(len(arr[i])):
                if arr[i][a] == maxVal:
                    indexMax = a
                if arr[i][a] == minVal:
                    indexMin = a

            arr[i][0], arr[i][indexMax] = arr[i][indexMax], arr[i][0]
            arr[i][-1], arr[i][indexMin] = arr[i][indexMin], arr[i][-1]
    maxVal = ~sys.maxsize
    minVal = sys.maxsize


print("Конечная матрица: ")
printMatrix(arr)