Math expression parsing: incorrect output when a + followed by a -

问题

I am trying to create a program that parses an expression such as 3x^3 + 7x^2 + 6x - 9. I am using a recursive system to do this. At the moment, I'm just testing to see if I get the correct output for the expression I input by only using constants in the expression. The code works fine with most expressions, but when I have a minus followed by a positive, both terms are subtracted (first term should be subtracted, second term should be added). An example of this is entering 4*3^2-7*3+5*3. The answer to this is 30 but the program does 4*3^2-7*3-5*3 instead and outputs 0. I am unsure how to solve this.

Code:

    Private Function ParseExpr(ByRef expression As String)

        Dim op, op1 As Integer
        op = ParseFactor(expression)
        If expression.Length <> 0 Then

            If (expression(0) = "+") Then
                expression = expression.Substring(1, expression.Length - 1)
                op1 = ParseExpr(expression)
                op += op1
            ElseIf (expression(0) = "-") Then
                expression = expression.Substring(1, expression.Length - 1)
                op1 = ParseExpr(expression)
                op -= op1
            End If
        End If
        Return op
    End Function

回答1:

All is well with raising to a power and multiplication. At that point we are simplified to 36-21+15. Here we must remember that we are adding -21. So we need to go back to -7 . When I build my list of numbers (it handles numbers of more than one digit) I add the minus sign to the number it precedes.

This code does not handle decimal numbers or parenthesis. I think you will be able to add the division operator, if you wish.

Private NumList As New List(Of Double)
Private OperatorList As New List(Of String)

Private Sub OpCode()
    Dim Input = "14*3^2-7*3+15*3"
    PopulateLists(Input)
    Dim OpIndex As Integer
    Dim NewNum As Double
    Dim operators = {"^", "*", "+"} 'Note: no minus sign, the minus goes with the number
    For Each op In operators
        Do
            OpIndex = OperatorList.IndexOf(op)
            If OpIndex = -1 Then
                Exit Do
            End If
            Select Case op
                Case "^"
                    NewNum = NumList(OpIndex) ^ NumList(OpIndex + 1)
                Case "*"
                    NewNum = NumList(OpIndex) * NumList(OpIndex + 1)
                Case "+"
                    NewNum = NumList(OpIndex) + NumList(OpIndex + 1)
            End Select
            NumList.RemoveAt(OpIndex + 1)
            NumList(OpIndex) = NewNum
            OperatorList.RemoveAt(OpIndex)
        Loop
    Next
    MessageBox.Show(NumList(0).ToString) 'Displays 150
End Sub

Private Sub PopulateLists(Input As String)
    Dim strNum As String = ""
    For Each c As Char In Input 'Be careful here, the IDE wants to add () at the end of this line - it doesn't belong
        If Char.IsDigit(c) Then
            strNum &= c
        ElseIf c = "-" Then
            OperatorList.Add("+") 'We are adding a negative number
            NumList.Add(CDbl(strNum)) 'Add the last number we accumulated so we can start a new one with the minus sign
            strNum = "-" 'Start a new number with the minus sign
        Else 'The other operators are added to the list
            OperatorList.Add(c)
            NumList.Add(CDbl(strNum))
            strNum = ""
        End If
    Next
    NumList.Add(CInt(strNum)) 'The last number which will not be followed by an operator
End Sub

来源：https://stackoverflow.com/questions/65798782/math-expression-parsing-incorrect-output-when-a-followed-by-a