# Answer to Question #13766 in Visual Basic for with

Question #13766

Goldbach's conjecture says that every positive even number greater than 2 is the sum of two prime numbers. Example: 28 = 5 + 23. It is one of the most famous facts in number theory that has not been proved to be correct in the general case. It has been numerically confirmed up to very large numbers. Write a predicate to find the two prime numbers that sum up to a given even integer.

Example:

Input: 28

Output: 5,23

Example:

Input: 28

Output: 5,23

Expert's answer

Module Module1

Sub Main()

Dim lowerlimit As Integer

Dim upperlimit As Integer

Console.WriteLine("Input lower limit: ")

lowerlimit = Integer.Parse(Console.ReadLine())

Console.WriteLine("Input upper limit: ")

upperlimit = Integer.Parse(Console.ReadLine())

For i As Integer = lowerlimit To upperlimit

If i Mod 2 = 0 Then

Show(i)

End If

Next

Console.ReadLine()

End Sub

Private Sub Show(ByVal number As Integer)

Dim N As Integer = number

Dim isprime(N) As Boolean

For i As Integer = 2 To N - 1

isprime(i) = True

Next

For i As Integer = 2 To N

If (i * i < N) Then

If isprime(i) Then

For j As Integer = i To N

If i * j < N Then

isprime(i * j) = False

End If

Next

End If

End If

Next

Dim primes As Integer = 0

For i As Integer = 2 To N - 1

If isprime(i) Then

primes = primes + 1

End If

Next

Dim list(primes) As Integer

Dim nn As Integer = 0

For i As Integer = 0 To N - 1

If isprime(i) Then

nn = nn + 1

list(nn) = i

End If

Next

Dim left As Integer = 0

Dim right As Integer = primes - 1

While left <= right

If (list(left) + list(right) = N) Then

Exit While

ElseIf (list(left) + list(right) < N) Then

left = left + 1

Else

right = right - 1

End If

End While

If (list(left) + list(right) = N) Then

Console.WriteLine(N.ToString() + " = " + list(left).ToString() + " + " + list(right).ToString())

Else

Console.WriteLine(N.ToString() + " not expressible as sum of two primes")

End If

End Sub

End Module

Sub Main()

Dim lowerlimit As Integer

Dim upperlimit As Integer

Console.WriteLine("Input lower limit: ")

lowerlimit = Integer.Parse(Console.ReadLine())

Console.WriteLine("Input upper limit: ")

upperlimit = Integer.Parse(Console.ReadLine())

For i As Integer = lowerlimit To upperlimit

If i Mod 2 = 0 Then

Show(i)

End If

Next

Console.ReadLine()

End Sub

Private Sub Show(ByVal number As Integer)

Dim N As Integer = number

Dim isprime(N) As Boolean

For i As Integer = 2 To N - 1

isprime(i) = True

Next

For i As Integer = 2 To N

If (i * i < N) Then

If isprime(i) Then

For j As Integer = i To N

If i * j < N Then

isprime(i * j) = False

End If

Next

End If

End If

Next

Dim primes As Integer = 0

For i As Integer = 2 To N - 1

If isprime(i) Then

primes = primes + 1

End If

Next

Dim list(primes) As Integer

Dim nn As Integer = 0

For i As Integer = 0 To N - 1

If isprime(i) Then

nn = nn + 1

list(nn) = i

End If

Next

Dim left As Integer = 0

Dim right As Integer = primes - 1

While left <= right

If (list(left) + list(right) = N) Then

Exit While

ElseIf (list(left) + list(right) < N) Then

left = left + 1

Else

right = right - 1

End If

End While

If (list(left) + list(right) = N) Then

Console.WriteLine(N.ToString() + " = " + list(left).ToString() + " + " + list(right).ToString())

Else

Console.WriteLine(N.ToString() + " not expressible as sum of two primes")

End If

End Sub

End Module

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