Multiplying Matrices (Subscript out of Range) - vba

Question

I tried putting the program into VBA, but I keep getting an error. This line was highlighted:
B(i, j) = C(i, j) + A(i + 1, j) * A(i, j + 1)
I am trying to multiply two of the same matrices and then add it to itself.
Suppose I wanted to multiply a 15 x 15 matrix, would I use the same codes except just change the dimensions and ranges?
Sub testing()
Dim A As Variant
Dim B(1 To 2, 1 To 2)
Dim C(1 To 2, 1 To 2)
A = Range("A1:B2").Value
i = 1
C(1, 1) = 0
Do Until i = 3
j = 1
Do Until j = 3
C(i, j) = C(i, j) + A(i, j) * A(j, i)
**B(i, j) = C(i, j) + A(i + 1, j) * A(i, j + 1)**
j = j + 1
Loop
i = i + 1
Loop
Range("E1:F2").Value = B
End Sub
Thanks!

Answer 1

Do...Loop is not the right tool for this job. Use For...Next instead. Also, to help make sure you don't overrun the array bounds and get a subscript out of range error, you can use LBound and UBound instead of hard-coding the index bounds. That way you don't have to change the code if you change the size of your input matrix.
This will get you part of the way. For simplicity I assumed A is a square matrix.
Dim i As Long, j As Long
Dim A As Variant
Dim B() As Variant
Dim C() As Variant
A = Range("A1:B2").Value
ReDim B(LBound(A, 1) To UBound(A, 1), LBound(A, 2) To UBound(A, 2))
ReDim C(LBound(A, 1) To UBound(A, 1), LBound(A, 2) To UBound(A, 2))
For i = LBound(A, 1) To UBound(A, 1)
For j = LBound(A, 2) To UBound(A, 2)
C(i, j) = C(i, j) + A(i, j) * A(j, i)
B(i, j) = C(i, j) + A(i + 1, j) * A(i, j + 1) ' still bad
Next j
Next i
Now you'll still get an error on that same line, due to the i+1 and j+1 indices. Those will have a value of 3 at some point, which is beyond your array bounds of 2. I can't help you fix that line, because I don't understand what you are trying to do with it.
Which brings me to my next point: if you think you're doing a matrix multiplication with this line
C(i, j) = C(i, j) + A(i, j) * A(j, i)
then you are wrong. The correct definition is:
and this is not what you have implemented.
A perhaps more convenient way to do it is to use the built-in MMult function:
C = WorksheetFunction.MMult(A, A)