The QA section of the current issue of our IRW Newsletter has a practical piece of knowledge.


In Excel, how do you multiply any two matrices M1 and M2 to get a third one M3?


I assume you know how to define an array in Excel (i.e., the first and last cell of a selected rectangular region defines an array).

Thus, let M1 be a matrix of r1 rows and c1 columns and M2 be a matrix of r2 rows and c2 columns. Let r1 = c2. Multiplying M1 times M2 results in a squared matrix M3 of r1 rows and c2 columns; i.e.

M1 * M2 = M3

To carry out this operation in EXCEL, do this:

1. Open a spreadsheet and enter M1 and M2 numerical data. Next, select an array region (r1 x c2) by dragging your mouse on the spreadsheet. This will be M3.

2. Type = MMULT(Array1, Array2) in the (fx) field. Next, Press F2.

3. Finally, press Ctrl + Shift + Enter and witness the magical black box of EXCEL.

An example is provided in Figure 1 of the newsletter.

From time to time, more complex math operations will be described in future issues of IRW.