Variables and constants
- 1. Bug Hunt Starter • Use your developing knowledge of VB to find and correct the errors in the example program.
- 2. Objectives • Identify the scope of variables and distinguish between local and global identifiers • Identify and explain the use of constants and variables in algorithms and programs.
- 3. Variables (general) • A variable is a name used to refer to a particular memory location that is used to store data • Value can change while the program is running. • Purpose if to allow the programmer to write code that manipulates the data without knowing, yet, what the data will be.
- 4. What data types that can be applied to variables? String Integer Byte Long Double Decimal Boolean Date/Time
- 5. Types of variables • Declared at the beginning of the code and is available throughout Global the code, including all its subroutines. • Declared and used inside a module or subroutine and is only Local available in the module. Created when subroutine is called and destroyed when subroutine exits.
- 6. Program Global Form 1 Local Sub Procedure Local Sub Procedure Local Sub Procedure Local Sub Procedure
- 7. Program Global (to form) Form 1 Global (to form) Form 2 Local Sub Procedure Local Sub Procedure Local Sub Procedure Local Sub Procedure Local Sub Procedure Local Sub Procedure Local Sub Procedure Local Sub Procedure
- 8. Program Global (to form) Form 1 Global (to form) Form 2 Global (all) Module Local Sub Procedure Local Sub Procedure Local Function Local Sub Procedure Local Sub Procedure Local Function Local Sub Procedure Local Sub Procedure Local Sub Procedure Local Sub Procedure
- 9. In action…
- 10. Variables top tips! Use of spaces is not allowed. Use underscore or capitalisation to make variables easier to spot. E.g. TimeOfArrival or Time_of_Arrival Give them sensible names, avoiding reserved words (IF, END, OR, PRINT) Use prefixes to help you remember data type e.g. strFirstname, intAge Select an appropriate data type – why?
- 11. Test yourself 1. What statement do you use to declare a local variable in VB? 2. What statement do you use to declare a global variable in VB? 3. What operator do you use to assign a value to a variable? 4. In terms of security, why is a local variable better than a global variable? 5. How should I create a variable that can we used program wide and available to all forms?
- 12. Constants • Used where values will not change • Examples? Declared as: Const Public VAT as Decimal = 17.5 • Doesn’t require program to lookup value, always known – time efficient.
- 13. In summary.. Global Variables: a variable declared at the beginning of a program and accessible from anywhere in the program Local Variable: a variable declared in a program block & accessible only within the program block Constant: a value that does not change throughout the program.
Editor's Notes
- #7: Global variables across forms = variable in module