用户输入方程的计算值

Question

I'm working on building a simple limit calculator in Visual Studio 2013. Right now I have my program set up in a way so that the user first enters the equation for the limit, and then the limit value. (ex. equation of x+3, limit value of 3 - the answer should then be 6)

I'm designing it so that it takes the limit from both the left and right side of the user inputted number, and if the two answers then differ by quite a bit, the program will say the limit doesn't exist. (However this code isn't in place yet - I plan to work on it after I can solve this problem)

Anyway, right now I'm stuck on one part of my code. The user can input their own custom function, as I mentioned above. I then have the program automatically replace all "x" values with the limit value that the user input as well. However, when I have the answer display, it will give me an answer of 5.0001 + 3 instead of just 8.0001. Below is my code, do you know of any way I can fix this so that it will perform the calculations needed, and just spit out a single number rather than the equation with x substituted out?

Public Class Form1

    Private Sub btnCalculate_Click(sender As Object, e As EventArgs) Handles btnCalculate.Click
        Dim Limit As Double = txtLimit.Text
        Dim Equation As String = txtEquation.Text
        Dim LeftTest As Double
        Dim RightTest As Double

        LeftTest = (Limit - 0.0001)
        RightTest = (Limit + 0.0001)

        Dim NewEquation = Replace(Equation, "x", RightTest)
        Dim FinalAnswer
        FinalAnswer = NewEquation
        MsgBox("The limit for this equation is " & NewEquation)
    End Sub
End Class

Any help would be appreciated with this.

Answer 1

You can download and use third party Parser.dll from the following link

http://simplemathparser.codeplex.com/

and code as shown below

    Dim Limit As Double = txtLimit.Text
    Dim Equation As String = txtEquation.Text
    Dim LeftTest As Double
    Dim RightTest As Double

    LeftTest = (Limit - 0.0001)
    RightTest = (Limit + 0.0001)

    Dim parser As New MathParser.Parser("x")
    Dim parsingResult = parser.Parse(Equation)
    Dim result = parsingResult.Evaluate(RightTest)
    MsgBox("The limit for this equation is " & result )