Why would you?

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The factorial function (FACT()), and natural logarithms (EXP() and LN()) are built into Excel. Were you looking for these specific functions or are you looking for an example of a user defined function?

function Factorial (x as long) as double
 if i<1 then
   Factorial=0 '0=error
   exit function
end if

dim i as long
Factorial=1
for i=1 to x
   Factorial=i*Factorial
next
end function

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Use can also calculate factorials using recursive algoritms.  For example,

Recursion does, however, have its limits, due to stack overflow limitations.  For Excel the above recursion routine seems to stop work at 13!, while Mathcad's numerical processor can go up to 170! and its symbolic processor can go at least to 17000!

But, short of doing a homework assignment, why would you want to duplicate a built-in function, particularly since your function will probably run 100 times slower.

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I would guess that our esteemed OP'er is a noob who still hasn't quite grasped the general concept of VB functions.

The recursive routine is quite often used for factorial calculation.  It is a classic example used to teach programing.

On my machine, the built in worksheet function took 1109 ms to run 100,000 calculations of FACT(170).  The VBA routine myFact took 2922 ms for the same computation.

The overflow is only due to the variable typing.  If you substitute double for long.  The routine will work up to 170
myFact(170)=7.25741561530799E+306

@IRStuff
It's not so bad as it looks, the function takes "only" 150-200% of the time the built-in function takes. Your point stays valid, though. Also, sticking to built-in functions will avoid "macro security" issues.

Cheers,
Joerd

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Another point to keep in mind is that FACT is not a VBA function.  To use it within a VBA routine you must reference the excel application object ie,
X=application.worksheetfunction.fact() there is an overhead cost in making the reference call.  When comparing myFact to worksheet fact I found that myFact was actually faster for arguments less than 17.  Above 17, the computational efficiency in worksheet Fact makes it a faster choice.

The slow performance of using worksheetfunction is a good point, and much more noticeable in Excel 2007 than in previous versions.

For instance, I use my own function for ATAN2, which works about 3 times faster than worksheetfunction.ATAN2() in Excel 2000, but about 8 times faster in Excel 2007.

This may be only temporary, until Microsoft fix the speed problems in Excel 2007, but for a frequently used function like ATAN2 it is worth using a UDF even for a 3 x speed increase.


Function VBAATan2(ByVal DX As Double, ByVal DY As Double) As Variant

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