Test for specific intersection on a matrix when there is no data

In a matrix, I wish to test for a specific  combination of row value and column value, and then return "n/a" for that intersection - key is that there will be no data for that combination. I don't however want to return "n/a" for other intersections where there is no data. I thought the following might work:

IF( VALUES(rowColName)="xxx" && VALUES(colColName)="yyy", "n/a", do something else) - however, it only works correctly if there is some data - presumably because it can find something to test...?

On another thread someone suggested a left join approach but I would like to avoid complex or intensive operations due to performance issues. However if that's the only solution, then I'll try it out.

Thanks in advance.