Hi,
I have a problem with visual basic. Details:
-I have a TextBox
-I have a Button
-When I press the button it replaces any "x" found on the textbox with Replace(TextBox1.Text, Chr(120), "1")
-I want to set an integer from the TextBox, but i get an ERROR when the program convert the TextBox content to an Integer variable
For example I type in the TextBox "2*x+1", and I get an error "Conversion from string "2*x+1" to type 'Integer' is not valid.
How can I resolve this problem?VB can easily convert a string to an integer. HOWEVER, what you wan is an expression evaluator. The following are valid symbols for a number: 0123456789
And the two following may come at the left of the number
- +
All other symbols are not numbers, but  tokens that can modify rubbers. Use of these makes you string into an expression.

Want you want is the thing that evaluates an repression.
http://msdn.microsoft.com/en-us/library/ms164903.aspx
Converting from a string to a number does not involve parsing embedded arithmetic expressions. In order to do that, you will either have to write your own code to do that, Or, use one that is available for download.

You don't mention whether you are using VB6 or a later version. I've written Expression Evaluators for both.

the VB6 version is part of BCScript

After installing that you could add it in the references (Project->References) dialog and use code like this, once the appropriate controls are added:

Code: [Select]Option Explicit
Private ParserObject As CParser
Private Sub cmdEval_Click()
    If ParserObject Is NOTHING Then
        Set ParserObject = new CParser
        ParserObject.Create()
    End If
    Dim XVar as CVariable 'Reference to the Variable we create.
    Set XVar = ParserObject.Variables.Add("X",1)
    ParserObject.Variables.Add "x",XVar  'make lowercase refer to other variable. (optionally, just ucase the string being evaluated...)
    Dim Result As Variant
    ParserObject.Expression = txtexpression.Text
    ParserObject.ExecuteByRef Result
    txtexpression.Text=Result

End Sub


For Visual Basic.NET, you could use my very similar library that is essentially a C# rewrite of my earlier VB6 version, BASeParser.NET. This one is packaged as a zip file. Extract it somewhere, and then add a reference using Project->Add Reference, and add a reference to BASeParser.dll (from the zip, not the earlier version). Then code like this will work: (again, once the appropriate controls are there)


Code: [Select]Imports BASeParser
Public Class Form1

    Private ParserObject As CParser
    Private XVariable As BASeParser.Variable


    Private Sub cmdEval_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdEval.Click
        If ParserObject Is Nothing Then
            ParserObject = New CParser()
            XVariable = ParserObject.Variables.Add("X", 1.0) '1.0 is needed- (double) (rather than 1 (Integer)) otherwise the Parser will break :(
            ParserObject.Variables.Add("X",XVariable) 'make the lower case also refer to the same variable.
        End If

        Dim Result = ParserObject.Execute(txtEvaluate.Text)
        txtEvaluate.Text = Result.ToString()

    End Sub

If this is homework, however, using either of those (or any library, for that matter) will probably get you an instant fail... Otherwise you should be set.

EDIT: Note also that Geek-9pm's suggestion has to do with the immediate window, and doesn't actually provide a programmatic method for doing this.I solved the problem with a much simpler solution, using CodeDOM expression evaluator.
Thank you for helping me. Quote from: alecsillidan on January 28, 2012, 05:00:18 AM

I solved the problem with a much simpler solution, using CodeDOM expression evaluator.
Thank you for helping me.

I had no idea it could be so hard.