Excel's Formula - sisman

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Part VI: Developing Custom Worksheet Functions
The Excel SUM function considers a text string to have a value of 0 unless it appears as a
literal argument (that is, as an actual value, not a variable). Therefore, MYSUM adds the
cell’s value only if it can be evaluated as a number (VBA’s IsNumeric function is used
for this).
Dealing with Boolean arguments is tricky. For MYSUM to emulate SUM exactly, it needs to
test for a literal TRUE in the argument list and compensate for the difference (that is, add
2 to –1 to get 1).
For range arguments, the function uses the Intersect method to create a temporary
range that consists of the intersection of the range and the sheet’s used range. This handles
cases in which a range argument consists of a complete row or column, which would
take forever to evaluate.
You may be curious about the relative speeds of SUM and MYSUM. MYSUM, of course, is much
slower, but just how much slower depends on the speed of your system and the formulas themselves.
On my system, a worksheet with 5,000 SUM formulas recalculated instantly. After I
replaced the SUM functions with MYSUM functions, it took about 8 seconds. MYSUM may be
improved a bit, but it can never come close to SUM’s speed.
By the way, I hope you understand that the point of this example is not to create a new SUM
function. Rather, it demonstrates how to create custom worksheet functions that look and work
like those built into Excel.