problem with controlling program flow vb tutorial

fuze · April 3rd, 2009, 11:29 AM

Hi everyone, I am really confused, I am trying to teach myself the basics of visual basic and i am following tutorials and am stuck on lesson 9, link here http://www.vbtutor.net/vb2008/vb2008_lesson9.html

In the example 9.4 it is a programme designed to show pupils grades based on their mark, but when i copy the code into the button i get the error message 'mynumber is not declared'

this is the code

Code:

 						 							 							Private Sub  							Button1_Click(ByVal sender As System.Object, ByVal e  							As System.EventArgs) Handles Button1.Click
							Dim Mark As Integer
 							 							Dim Grade as String
 							 							
							Mark = TextBox1.Text
							If myNumber >=80 Then
							Grade="A"
 							 							ElseIf  Mark>=60  							and Mark<80 then
 		  							 							Grade="B"
 							 							ElseIf  Mark>=40  							and Mark<60 then
  							 							 							Grade="C"
 							 							Else
 							 							Grade="D"
 							 							
							End If
							End Sub

I am really confused, because I tried to change mynumber to mark, because that seemed to make sense but when I do that i get the error message Conversion from string "" to type 'Integer' is not valid. and Mark = TextBox1.Text is highlighted.

Is there something silly I am doing wrong?

The tutorial doesnât say what labels and textboxes you need to make it work, so can someone explain what I am doing wrong?
Thankyou in advance

gbianchi · April 3rd, 2009, 11:41 AM

Hello.
MyNumber should be Mark.
The other error is because you maybe has option extrict turned ON. (which is good :) )
to avoid the error, you need to cast the string contained in textbox1.text to an integer. You can do it using Cint, int.parse or any other function that convert a string into a number.

fuze · April 3rd, 2009, 02:13 PM

Quote:

Originally Posted by gbianchi

Hello.
MyNumber should be Mark.
The other error is because you maybe has option extrict turned ON. (which is good :) )
to avoid the error, you need to cast the string contained in textbox1.text to an integer. You can do it using Cint, int.parse or any other function that convert a string into a number.

Thank you very much for this, but how do i do this? I am very new to visual basic and I dont even know what the objects you mentioned are, let alone how to use them!

Th==Cheers in advance, your help is much appreciated!

dparsons · April 3rd, 2009, 02:28 PM

What Gonzalo means is do something like this:

Mark = CInt(TextBox1.Text)
Mark = Convert.ToInt32(TextBox1.Text)
Mark = (Integer)TextBox1.Text

They all pretty much do the same thing.

hth.
-Doug

fuze · April 4th, 2009, 07:59 AM

Quote:

Originally Posted by dparsons

What Gonzalo means is do something like this:

Mark = CInt(TextBox1.Text)
Mark = Convert.ToInt32(TextBox1.Text)
Mark = (Integer)TextBox1.Text

They all pretty much do the same thing.

hth.
-Doug

right, i tried replacing mark = textbox1.text with each of the statements above but I get this in the immediate window -
A first chance exception of type 'System.InvalidCastException' occurred in Microsoft.VisualBasic.dll

am i just doing something silly. Should the grade just appear because it is a string or would it appear in a label or textbox because at the moment when i debug it, it runs but nothing happens when i put a mark in the textbox and click the button, i just get that message I mentioned

when i use Mark = Convert.ToInt32(TextBox1.Text i get the error messageA first chance exception of type 'System.FormatException' occurred in mscorlib.dll

and if i use Mark = '(Integer)TextBox1.Text' i get the error saying expression expected.

I really dont know why I am going wrong and why the tutorial is published with errors in it.

In my programme I have only got the button with the code and the text box for the mark, should I have somehting else? I dont suppose someone can look at the tutorial and see what is wrong with it please/

Thanks for all this help by the way, i am just such a novice!

gbianchi · April 4th, 2009, 09:15 PM

Are you inserting a number into the textbox???

The tutorial is not bad. It's just lack the error control part, maybe because it will be teach to you later.

fuze · April 6th, 2009, 10:28 AM

Yes I have been inserting a number into the textbox.

Why, does it work when you try it? Am i just being stupid?

Cheers!

dparsons · April 6th, 2009, 10:37 AM

Hmm. If you are entering a number into TextBox1 and it is throwing an exception there is something else at play.

You can try this:

visualbasic Code:

Dim outInteger as Integer
If Integer.tryParse(TextBox1.Text, outInteger) Then
'Process data using hte outNumber variable
Else
'Textbox1 does not contain a number
End If

You could, of course, use the IsNumeric() function to test if a string is actually a number but I prefer tryParse since there isn't a built in IsNumeric function in C# ;]

Anyway, Integer.tryParse takes two parameters: a string which you want to try and parse into an integer, and an output parameter (in this case outInteger). The tryParse function returns a Boolean; if the first parameter (the string) is able to be parsed into an Integer it returns True otherwise it returns False. Finally, if tryParse succeeds the second parameter (the output parameter which is outInteger) will then contain the integer value of whatever was contained within in the first parameter.

You would then use this output parameter to do any processing related to the integer value.

hth.
-Doug

fuze · April 6th, 2009, 01:27 PM

None of the errors appear any more in the immediate window but nothing happens when i run the programme. this is really confusing me, maybe im not cut out for visual basic! thanks for being so patient with me and trying to help though!

I have a label with mark in, the textbos, and the button and this is the code I am using

Code:

Public Class Form1




    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim Mark As Integer

        Dim Grade As String
        Dim outInteger As Integer
        If Integer.TryParse(TextBox1.Text, outInteger) Then
            'Process data using the outNumber variable
        Else
            'Textbox1 does not contain a number
        End If

        Mark = TextBox1.Text
        If Mark >= 80 Then
            Grade = "A"

        ElseIf Mark >= 60 And Mark < 80 Then

            Grade = "B"

        ElseIf Mark >= 40 And Mark < 60 Then

            Grade = "C"

        Else

            Grade = "D"


        End If
    End Sub



End Class

gbianchi · April 6th, 2009, 01:51 PM

mmm.. you are at lesson 9. I didn't read the tutorial, but at least you learned how to use if..then..else..endif and you already has to know that ' means that the line is a comment, so will not execute.

Maybe the tutorial is to hard for you??? Do you at least has a background in programming or this is even your first time programming??

The code that Doug gave you lacks something to do if the number is parse correctly or not, he just gave you something to start to work it out the problem.
Do you have any background programming??? saying "I don't know VB" is not the same that saying "I don't know even how to do a hello word in any language".