How do I create a multiple if statement in VB.NET?

Question

So I'm trying to make an infix to reverse Polish notation program by pushing the expression into a stack. The program iterates the expression looking for an operator and once it finds it, it pops the 2 values and performs the calculations. However if I try to add multiple if statements a conversion error pops. The line “if express(i) <> “+” works for addition however if I were to extend it by adding multiple conditions “ if express(i) <> “+” or “-“ or “*”” then it says “Conversion from string “-“ to type Boolean is not valid. Could anyone help me with this? Thanks.

Module Module1

Sub Main()
    Dim expres As String
    Console.WriteLine("Enter infix expression")
    expres = Console.ReadLine()
    Dim S As New Stack
    Dim current(1) As Integer
    Dim temp_val As Integer
    For i = 0 To expres.Length - 1
        If expres(i) <> "+" Then
            S.Push(expres(i))
        End If

        If expres(i) = "+" Then
            current(0) = S.Pop().ToString
            current(1) = S.Pop().ToString
            temp_val = current(0) + current(1)
            Console.WriteLine(temp_val)
            S.Push(temp_val)

        End If
        If expres(i) = "-" Then
            current(0) = S.Pop().ToString
            current(1) = S.Pop().ToString
            temp_val = current(0) - current(1)
            Console.WriteLine(temp_val)
            S.Push(temp_val)

        End If
        If expres(i) = "*" Then
            current(0) = S.Pop().ToString
            current(1) = S.Pop().ToString
            temp_val = current(0) * current(1)
            Console.WriteLine(temp_val)
            S.Push(temp_val)

        End If
        If expres(i) = "/" Then
            current(0) = S.Pop().ToString
            current(1) = S.Pop().ToString
            temp_val = current(0) / current(1)
            Console.WriteLine(temp_val)
            S.Push(temp_val)

        End If
        If expres(i) = "^" Then
            current(0) = S.Pop().ToString
            current(1) = S.Pop().ToString
            temp_val = current(0) ^ current(1)
            Console.WriteLine(temp_val)
            S.Push(temp_val)

        End If
        If expres(i) = "~" Then
            current(0) = S.Pop().ToString
            current(1) = S.Pop().ToString
            temp_val = current(0) + current(1)
            Console.WriteLine(temp_val)
            S.Push(temp_val)

        End If
    Next
    Console.ReadLine()
End Sub

End Module

Answer 1

you should state the left expression each time you compare, something like

if express(i) <> "+" or express(i) <> "-" or express(i) <> "*"

Answer 2

Try this one

Module Module1

    Sub Main()
        Dim expres As String
        Console.WriteLine("Enter infix expression")
        expres = Console.ReadLine()
        Dim s As New Stack
        Dim current(1) As Integer
        Dim tempVal As Integer
        For i = 0 To expres.Length - 1

            If Array.IndexOf({"+", "-", "*", "/", "^", "~"}, expres(i)) = -1 Then
                s.Push(expres(i))
            End If

            current(0) = s.Pop().ToString
            current(1) = s.Pop().ToString

            Select Case expres(i)
                Case "+"
                    tempVal = current(0) + current(1)

                Case "-"
                    tempVal = current(0) - current(1)

                Case "*"
                    tempVal = current(0) * current(1)


                Case "/"
                    tempVal = current(0) / current(1)

                Case "^"
                    tempVal = current(0) ^ current(1)


                Case "~"
                    tempVal = current(0) + current(1)

            End Select
            Console.WriteLine(tempVal)
            s.Push(tempVal)

        Next
        Console.ReadLine()
    End Sub
End Module