A[i, j] is the element of A at i-th row and
j-th column. Both i and j (or their elements) must be
positive integers. Note that both indices can be vectors
>> A = [8, 1, 6; 3, 5, 7; 4, 9, 2]
8 1 6
3 5 7
4 9 2
>> A[2,3]
7
>> A[1, 1 : 3] // the first row
8 1 6
>> A[1 : 3, 2] // the second column
1
5
9
>> A[[2, 3], [2, 3]] // the 2x2 submatrix on the lower right corner
5 7
9 2