Help prime co prime

help im trying to output prime and coprime AND NEITHER DEPENDING ON THE NUMBERand its not working
i have been trying for hours and its not working
swift help would be appreciated

Function IsPrime(TestPrime As Long)
    Dim TestNum As Long
    Dim checkPrime As Boolean
    Dim TestLimit As Long
    If TestPrime Mod 2 = 0 Then Exit Function
    TestNum = 3
    TestLimit = TestPrime
    Do While TestLimit > TestNum
    If TestPrime Mod TestNum = 0 Then
    Exit Function
    End If
    TestLimit = TestPrime \ TestNum
    TestNum = TestNum + 2
    Loop
    checkPrime = True
    
    
     If checkPrime = True Then
     IsPrime = "prime"
     Else: IsPrime = "coprime"
     End If
    
     If TestPrime = 1 Then
     IsPrime = "neither"
     End If
     
     
  
    
End Function

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One way:

	A	B	C
1	264,627,858,537,807	3	B1: =isprime(A1, TRUE)
2	250,522,486,845,971	6719759
3	336,502,419,415,881	3
4	213,751,729,707,615	5
5	215,771,119,714,057	164513
6	384,515,184,772,903	17
7	383,451,182,203,411	7
8	234,532,507,205,987	TRUE
9	328,100,503,187,517	3
10	200,768,597,700,379	17

Function IsPrime(dNum As Double, Optional bFirstFactor As Boolean = False) As Variant

  '   Returns                 if dNum is
  ' -----------  -----------------------------------
  '  #VALUE!      <> Int(dNum) or dNum < 2
  '  "Too big!"   > 1E+15
  '  FALSE        composite and bFirstFactor = False
  '  1st factor   composite and bFirstFactor = True
  '  TRUE         prime

  Dim dDiv          As Double ' divisor
  Dim dQuo          As Double ' quotient

  If Int(dNum) <> dNum Then
    IsPrime = CVErr(xlErrValue)

  Else
    If dNum < 2# Then
      IsPrime = CVErr(xlErrValue)

    Else
      If dNum = 2# Or dNum = 5# Then
        IsPrime = True

      Else
        If dNum > 1E+15 Then
          IsPrime = "Too big!"
        
        Else
          Select Case Right(Format(dNum, "0"), 1)
            Case "0", "2", "4", "6", "8"
              IsPrime = IIf(bFirstFactor, 2#, False)
            Case "5"
              IsPrime = IIf(bFirstFactor, 5#, False)
            Case Else
              ' can't use Mod with numbers bigger than Longs, so ...
              For dDiv = 3# To Sqr(dNum) Step 2#
                dQuo = dNum / dDiv
                If Int(dQuo) = dQuo Then
                  IsPrime = IIf(bFirstFactor, dDiv, False)
                  Exit Function
                End If
              Next dDiv
              
              IsPrime = True
          End Select
        
        End If
      End If
    End If
  End If
End Function

I don't know how it's supposed to work. "co-prime" is a property of two numbers, not 1.

"Doesn't work" can be a rather unhelpful way to describe a problem. Can you explain what you mean by "doesn't work"? When I copy your function into a blank worksheet and use it, it seems to "work" (meaning that it runs without syntax or run-time errors) just fine. It returns 0 (the value written to Excel for empy variants) for non-prime numbers and "prime" for prime numbers -- as it has been instructed to do. I would guess that these are not your expected results. What are your expected results?

As a debugging step, I added a few Stop statements to your UDF.

Function IsPrime(TestPrime As Long)
    Stop
    Dim TestNum As Long
    Dim checkPrime As Boolean
    Dim TestLimit As Long
    If TestPrime Mod 2 = 0 Then Stop: Exit Function
    TestNum = 3
    TestLimit = TestPrime
    Do While TestLimit > TestNum
    If TestPrime Mod TestNum = 0 Then
    Stop
    Exit Function
    End If
    TestLimit = TestPrime \ TestNum
    TestNum = TestNum + 2
    Loop
    Stop
    checkPrime = True
    
    
     If checkPrime = True Then
     IsPrime = "prime"
     Else: IsPrime = "coprime"
     End If
    
     If TestPrime = 1 Then
     IsPrime = "neither"
     End If
     
     
  
    Stop
End Function

I recommend that you setup your spreadsheet and see what is happening inside of the code. Identify where the code is doing something you don't expect. Then you will be in a better position to change the code to do what you want. If you need help making the edits, tell us where it is doing something unexpected or unwanted, explain what you want it to do instead, and we should be able to help you edit the code.

I also note, like shg, that there is nothing in the code for computing coprimes. The code as written will only identify prime or not-prime numbers. I note, from the Wikipedia page for coprimes (https://en.wikipedia.org/wiki/Coprime ) that there may be simpler algorithms based on some of the properties of coprime numbers.
One algorithm might use the built in GCD() function https://support.office.com/en-us/art...c-ddfc21b5073a
Another might use the built in LCM() function. https://support.office.com/en-us/art...c-06ede5563c94