1. 3.2 Working with Data
Scope of variables
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2. Learning Objectives Explain the concept of lifetime.
State the lifetime of variables declared with the Dim statement.
Explain how to extend this lifetime.
Explain what the scope of a variable refers to.
Name the two main scope levels of variables, state their ‘scopes’ and explain how to declare variables with each type of scope.
Explain the concept of “narrow scope”.
Explain how to produce Message Boxes.
State the general form of code which increments variables.
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3. Lifetime
The lifetime of a variable represents the period of time during which it can hold a value.
Its value can change over its lifetime, but it always holds the last defined value.
A variable declared with the Dim statement has a lifetime equal to its scope.
i.e. The variable is reset every time the Dim statement is encountered.
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4. Scope of a variable
Refers to the part of the program code that can use a variable.
There are two main levels of scope:
Local
Global
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5. Scope of a variable Local scope Global scope Level Description
Available to all code within the procedure in which it is declared
Global scope
Available to all code within the module, class, or structure in which it is declared i.e. defined outside a procedure.
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6. MsgBox
You inform a user using labels but an MsgBox is another method of informing the user.
It is useful because it less easy to ignore a MsgBox.
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7. Incrementing Variables
… = … + increment value
e.g. Number = Number + 1
Add on 1 to the variable Number each time.
Same Variable Name / Identifier
Note that this line makes no sense mathematically.
Imagine: x = x + 1
However, remember that in programming = is an arithmetical operator meaning to assign.
It does not mean “it is equal” it means “make it equal”.
Left Side -> Right Side
You can alternatively use … += increment value
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8. Incrementing Variables
For example:
Number = Number + 1 OR
Number += 1
Means add on another 1 to the variable Number each time from the previous Number (so if Number starts from 0, it will become 1, then 2, then 3, etc….).
Total = Total + NewNumber
Total += NewNumber
Means add on a new number to the variable Total each time from the previous Total.
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9. Program 3.2 Global Variables
Specification:
Demonstrate the difference between local and global variables.
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10. Program 3.2 Global Variables
Create a new project named ‘Global Variables’.
Change the form’s Text property to ‘Global Variables’.
Place two labels in the upper left and right areas of the form and one button control in the lower centre of the form.
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11. Properties to set: Upper left label: Upper right label:
Text: Number of clicks
Size: 14
Bold: True
Upper right label:
Name: lblClicks
BorderStyle: FixedSingle
Text: (Blank)
Note that the Border allows the control to remain visible. If you make the text property of a label blank without creating a border first you will find the label becomes invisible and almost impossible to move again even though VB still knows it exists.
Lower centre button:
Name: butOK
Text: OK
Size: 18
Bold: True

12. Program 3.2 Global Variables
Double click the OK button and enter the following code inside its procedure template:
Dim Number As Integer
Number = Number + 1 ‘Increments the count of the number of clicks by one.
lblClicks.Text = Number ‘Display the number of clicks.
Run the program and click the button a few times.
The program keeps saying you have only clicked once. Why?
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13. Local Variables
Between each click the variable: ‘Number’ ceases to exist (i.e. its lifetime is only as long as the procedure). It is reset every time the procedure is called with the Dim statement.
Local variables are said to have a scope limited to within the module/block/subroutine/procedure (button) it is declared in.
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14. Program 3.2 Global Variables
Move up out of the procedure to the beginning, before (or outside of) any procedures or “modules”, and create a blank line.
Select and move the Dim statement here.
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15. Program 3.2 Global Variables
Run the program again.
It now works.
Add the comment to the following line:
Dim Number As Integer ‘Declare the variable Number globally, so that it continues to exist while the program is running, this means its value is “remembered” from one button click to another.
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16. Global Variables
Global variables are said to have a scope limited only to entire lifetime of the program.
Meaning that it is valid and “remembered” from the time the program is run to the time the program is stopped (not just within a procedure (button) in a program – like local variables are).

17. Narrow scope
You may be thinking: “To avoid this issue why don’t we declare all variables globally?”
However, it is good programming practice and efficient to declare variables with as narrow a scope as possible.
Certain errors/problems can occur if you don’t:
Name Conflict Avoidance
Several different procedures can use the same variable name/identifier. As long as each instance is declared as a (local) procedure variable, each procedure recognizes only its own version of the variable.
Debugging Problems:
Difficult to debug as difficult to find where variable value was changed as different procedures can modify the value.

18. Extension “Running Total” Program
Write a program to allow a user to enter a series of numbers in a text box and see their running total.
One button:
Declare a variable to store the number entered and one to hold the total so far.
Store number from the text box in a variable.
Calculate total so far.
Display total.
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19. Plenary Explain the concept of lifetime.
The lifetime of a variable represents the period of time during which it can hold a value.
State the lifetime of variables declared with the Dim statement.
A variable declared with the Dim statement has a lifetime equal to its scope.
Explain how to extend this lifetime.
Declare variables globally.
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20. Plenary State the length of this extension.
Variables declared with Static statement hold their values as long as the application it was declared in is running i.e. have lifetimes as long as the lifetime of the application.
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21. Plenary Explain what the scope of a variable refers to.
Refers to the part of the program code that can use a variable.
Should we declare all variables globally.
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22. Narrow scope
No, we should not declare all variables globally because it is good programming practice and efficient to declare variables with as narrow a scope as possible.
Certain errors/problems can occur if you don’t:
Name Conflict Avoidance
Several different procedures can use the same variable name/identifier. As long as each instance is declared as a (local) procedure variable, each procedure recognizes only its own version of the variable.
Memory Consumption
(Local) Procedure variables consume memory only while their procedure is running. Their memory is released when the procedure finishes. (Global) Module variables obviously use memory until the module finishes i.e. longer.
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23. Plenary
Name the two main scope levels of variables, state their ‘scopes’ and explain how to declare variables with each type of scope.
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24. Scope of a variable Level Description (Local) Procedure scope
Available to all code within the procedure in which it is declared
(Global)
Module scope
Available to all code within the module, class, or structure in which it is declared i.e. defined outside procedure.
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25. Plenary
How do we increment a variable
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26. Incrementing Variables
… = … + increment value ‘Add a value to the number
Same Variable Name / Identifier
Note that this line makes no sense mathematically
Imagine: x = x + 1
However, remember that in programming the assignment statement “=“ does not mean “it is equal” it means “make it equal”.
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