How can I solve my "sub main was not found" problem? - vb.net

Question

Module PrimePairs
Public Function IsPrime(n As Long) As Boolean
Console.WriteLine("Please enter the value: ")
n = Console.ReadLine()
n = Math.Abs(n) ' Allows to consider negative prime numbers
If n < 2 Then ' Disallows -1, 0, 1
Return False
End If
Dim i As Long
i = 2
While i < n ' Note that for n = 2 we don't enter the loop and thus return True.
If n Mod i = 0 Then
Return False
End If
i += 1
End While
Return True
End Function
Public Function PrimePairs(ByVal n As Long, ByVal n2 As Long) As Integer
Dim count As Integer = 0
Console.ReadLine()
If n Mod 2 = 0 Then
For i = 1 To (n / 2) + 1
n2 = n - i
If IsPrime(i) And IsPrime(n2) = True Then
count += 1
End If
Next
Else
n2 = n - 2
If IsPrime(n2) = True Then
count = +1
End If
End If
Console.WriteLine("The result is:", count)
Return n
End Function
End Module
I want my code to calculate how many prime number twins can write the input I gave.

Answer 1

The problem is that your project actually does not contain a Sub Main(), as the error states.
You defined two functions in the Module Program, but console applications need a predefined entry point, which usually is the Main method.
The compiler is telling you that your project is not valid because it didn't find any entry point.
Just add a Sub Main() to get your project working, then call your functions from that method.
Example:
Option Strict On
Module Program
Sub Main(args As String())
Console.WriteLine("Please enter the value: ")
Dim input As String = Console.ReadLine()
Dim number As Long
If Long.TryParse(input, number) Then // More about this function in the answer below
Dim prime As Boolean = IsPrime(number)
If prime Then
Console.WriteLine(number & " is prime.")
Else
Console.WriteLine(number & " is not prime.")
End If
End If
Console.ReadLine()
End Sub
Function IsPrime(n As Long) As Boolean
n = Math.Abs(n) ' Allows to consider negative prime numbers
If n < 2 Then ' Disallows -1, 0, 1
Return False
End If
Dim i As Long = 2
i = 2
While i < n ' Note that for n = 2 we don't enter the loop and thus return True.
If n Mod i = 0 Then
Return False
End If
i += 1
End While
Return True
End Function
End Module
Also, I suggest you to enable Option Strict On as I added at the beginning of file. This prevents the compiler from doing implicit casts and forces you to explicitly declare your variables.
E.g., your line of code
n = Console.ReadLine()
is not valid with Option Strict On, because n is supposed to be a long, but Console.ReadLine() returns a string.
If you are a beginner, this will allow you to better understand how programming works and will help you to avoid errors - take good habits from the beginning, you can thank me later ;)
That's why I changed your code withLong.TryParse(input, number): this function returns true if provided input can be cast (=converted) to a long, and assigns the casted value to number variable.
There's a lot more I'd like to suggest you, but I would go off-topic.