Throws an IndexError: list index out of range error. What's the matter?

Question

Arrays

matrix = []
import random
m,n = int(input()),int(input())
for i in range(m):
for j in range(n):
print(i,j)
matrix[i][ j] = random.randint(1,9)
print()
print(matrix)

Answer 1

matrix - you have an empty list, and you are trying to write something into the [i, j]-th cell. The error about that also says that there is an address on an invalid index.
You, after entering m, n, do this: And only then do your 2 cycles with generation. Well, or in cycles, add new elements to matrix:
matrix = [[0 for x in range(n)] for y in range(m)]

matrix = []
for i in range(m):
  matrix.append([])
  for j in range(n):
    matrix[i].append(random.randint(1,9))