Need help in understanding a vba function

[Ria_Ko]

Hello, hope everyone is doing well.
I had to create a function called "prime", which is used to return True if the number is a prime number followed by a function called "countprime" that counts the number of prime numbers between the integers n1 and n2 (inclusive).
I am new to VBA and i am not being able to understand the "countprime" function logically. If anybody could explain it to me it would be of great help. Thankyou.

Function prime(n As Integer) As Boolean
Dim i As Integer
prime = True
If n = 1 Then
prime = False
ElseIf n > 2 Then
For i = 2 To n - 1
If n Mod i = 0 Then
prime = False
Exit Function
End If
Next i
End If
End Function

Function countprime(n1 As Integer, n2 As Integer) As Integer
Dim i As Integer
For i = n1 To n2
countprime = countprime - prime(i)
Next i
End Function

 

[Fluff]

The prime function returns true or false, in VBA true equals -1 & false is 0, so -prime(i) is effectively either -0 or - -1 (ie +1)
HTH

 

[Yongle]

A few lines to help you visualise the numbers
Debug.Print writes to the Immediate Window
(In VBA editor) Display Immediate Window with {CTRL} g

I am not sure that 0 is a Prime Number but your function says it is!

This is the result of running Test with values 0 and 10

OLDcount 0
PLUS 1 0 True
NEWcount 1

OLDcount 1
PLUS 0 1 False
NEWcount 1

OLDcount 1
PLUS 1 2 True
NEWcount 2

OLDcount 2
PLUS 1 3 True
NEWcount 3

OLDcount 3
PLUS 0 4 False
NEWcount 3

OLDcount 3
PLUS 1 5 True
NEWcount 4

OLDcount 4
PLUS 0 6 False
NEWcount 4

OLDcount 4
PLUS 1 7 True
NEWcount 5

OLDcount 5
PLUS 0 8 False
NEWcount 5

OLDcount 5
PLUS 0 9 False
NEWcount 5

OLDcount 5
PLUS 0 10 False
NEWcount 5

Result 5

Sub Test()
    Debug.Print "RESULT", countprime(0, 10)
End Sub

Function prime(n As Integer) As Boolean
    Dim i As Integer
    prime = True
    If n = 1 Then
        prime = False
    ElseIf n > 2 Then
        For i = 2 To n - 1
            If n Mod i = 0 Then
                prime = False
                Exit Function
            End If
        Next i
    End If
End Function
Function countprime(n1 As Integer, n2 As Integer) As Integer
    Dim i As Integer, B As Long
    For i = n1 To n2
        If prime(i) Then B = 1 Else B = 0
Debug.Print "OLDcount", countprime
Debug.Print "PLUS", B, i, prime(i)
        countprime = countprime - prime(i)
Debug.Print "NEWcount", countprime
Debug.Print
    Next i
End Function