Hi Nia,

Here is a routine originally presented by Myrna Larson that I use to test
for primes.

=IsPrime(num)

returns True or False.

Function IsPrime(TestNum As Long)
Dim PrimeCnt As Long
Dim y As Long
Dim x As Long
Dim i As Long
Dim Flag As Boolean
Dim Primes() As Long
Dim NumStop As Double
ReDim Primes(1 To 2)

NumStop = Sqr(TestNum)
If TestNum = 1 Or TestNum = 2 Or TestNum = 3 Or TestNum = 5 Then
IsPrime = True
Exit Function
End If
Primes(1) = 2
Primes(2) = 3
PrimeCnt = 2
x = 3

Do
x = x + 2
For y = 3 To Sqr(x) Step 2
If x Mod y = 0 Then GoTo NoPrime1
Next y
PrimeCnt = PrimeCnt + 1
ReDim Preserve Primes(1 To PrimeCnt)
Primes(PrimeCnt) = x
NoPrime1:
Loop Until Primes(PrimeCnt) NumStop

For i = LBound(Primes) To UBound(Primes)
If TestNum Mod Primes(i) = 0 Then
IsPrime = False
Exit Function
End If
Next
IsPrime = True
End Function


--

HTH

Bob Phillips
... looking out across Poole Harbour to the Purbecks
(remove nothere from the email address if mailing direct)

"Nia" wrote in message
...
Where can I get the function?
=IsPrime(num)

I typed it in a cell and got an error message.
Is there an add-on for more functions in Excel?

I want an IF test for a cell
=IF(IsPrime(B1),"Prime","Composite")