Why do we do this...

Scopes are to do with splitting large programs into sections (called procedures, or functions). We imagine a 'need to know' structure. For example, the boss of a company does not need to know about some crude data scribbled on a junior's desk. That would be too much information. Such data is termed 'local' to the junior.

Section 8.0

Introduction

Problem - we want some software for a car park. There are two buttons, marked 'car has entered', and 'car has left'. When either button is clicked, the current number of cars in the park is amended and displayed.

When we drop two buttons on the form, VB creates a 'procedure' - a section of named code - for each button-click event, as in

Private Sub Button1_Click(etc...)
.... done when Button1 clicked
End Sub

Private Sub Button2_Click(etc...)
...done when Button2 is clicked
End Sub

We can try:

Private Sub Button1_Click(etc...)
dim carCount as Integer = 0
carCount=carCount+1
   ...etc
End Sub

Private Sub Button2_Click(etc...)
dim carCount as Integer = 0
carCount=carCount+1
   ...etc
End Sub

Private Sub Button1_Click()   ' start of procedure
    |
    |   Local variables declared and used here - their extent - 'scope' - is local
    |
End Sub                        ' end of procedure

   dim carCount as Integer = 0

So - carCount does not hold its value between clicks - no good for this problem!

In fact, there are 2 variables here, not one:

Analogy: a person is a procedure. Each person has some scrap paper. We ask each one to think of a number, and write it down. They may choose the same one, but probably will not. There is no connection between the numbers. Each person is free to choose.

Section 8.1

Advantages of local variables

The coder of the procedure has free choice of names. If - by coincidence - they choose the same name in use in another procedure, there is no problem.

They are sufficient for many problems. (But not for our seemingly simple carpark!)  

Section 8.2

Disadvantages

Section 8.3

Global variables

Public Class Form1
    dim carCount as integer = 0

    Private Sub Button1_Click()
        carCount=carCount+1
        MessageBox.Show(CStr(carCount))
    End Sub
    '----------------------------------------------------------
    Private Sub Button2_Click()
        carCount=carCount-1
        MessageBox.Show(CStr(carCount))
    End Sub
End Class

• The variable is declared above the two procedures, not inside them. It is inside the class/end class lines however.
• It is termed 'global' - it can be used by ANY procedure in the program.
• It can be used to communicate between the procedures.
• Incidentally, in VB we should call the variable an instance variable of the Form1 class, but GLOBAL is a term that everyone understands, and will be sufficient for your early programming.
• We will not always show the Class and End Class lines, or the full indentation, because VB does this automatically. They are always there.

Analogy - 10 people using the same whiteboard, rather than making their own local notes.

We often need a small number of globals. We can also use parameters (or arguments) to reduce globals - see later.

Note that controls we place on a form - such as a label - are global to the form. Any of our procedures can access them.

If a variable is only used inside one procedure, it should clearly be local.

Section 8.4

Name clash?

dim n as integer   ' a global one

Private Sub Button1_Click()
dim n as integer      ' local one
n=n+1      ' which n  - local or global ????

   ...etc
End Sub
 

Section 8.5

Problem

Key points

If you want a variable to be used by more than one control, make that variable global.