Udf (or actual function) to tell if a number is prime.

[Gene Klein]

I am sure I asked this years ago, but I am looking for a function that determines if a number is prime. It can be a UDF or an actual built in function. (I tried =isprime of course)

It would work like this =isprime(5) would return 1
=isprime(6) would return 0

Gene Klein

 

[Kyle123]

I wasn't answering your question, I was answering Rick's/Mark's

 

[Rick Rothstein]

I don't think that was the cause of my original issue. I never had decimals as input.

Gene

I wasn't answering your question, I was answering Rick's/Mark's

And that is why one should use "Reply With Quote" (removing any non-relevant parts of the quoted message) when there are multiple respondents in a thread so everyone knows who a response is directed at.

At a guess because Prime numbers have to be whole numbers, if declared as long decimals will be coerced into their whole number value - 5.3 would become 5 for example which would return as Prime when it is clearly not.

The input as Single prevents this

shg's function addresses with:

  ElseIf Int(d) <> d Then
    IsPrime = CVErr(xlErrValue)
    Exit Function  
  End If

Okay, I see your point, but theoretically no one should be asking if a calculated floating point number is prime given the vagaries of significant digits out around the 14th/15th decimal position when floating point values are converted to binary values (what a computer does internally to perform floating point math). However, given that, shg's use of Double is much better than the original function's use of Single (try any IsPrime function, where the argument is declared as Single, with an assumed calculated value like 11.0000000999).

 

[GTO]

Thank you so much Kyle and sorry I missed seeing your response earlier.

Mark