1. CIS16 Application Development Programming with Visual Basic
Chapter Three
Variables and Constants

2. How a computer works

3. Question?
Can you add 3 numbers and calculate the sum before you get them?

4. LOGIC

5. Pseudocode and Flowcharts
Many programmers use development planning tools such as pseudocode and flowcharts.
Pseudocode uses short phrases to describe the steps
a procedure must take to accomplish its goal.
A flowchart, on the other hand, uses standardized
symbols to illustrate a procedure’s steps.
Programmers use the planning tool of their choice as a guide when coding the procedure.

6. Pseudocode and Flowcharts

7. Pseudocode and Flowcharts
The flowcharts contain three different symbols: an oval, a rectangle, and a parallelogram
The oval symbol is called the start/stop symbol. The start and stop ovals indicate the beginning and end, respectively, of the flowchart.
The rectangles are called process symbols. You use the process symbol to represent tasks such as ending the application and making calculations.
The parallelogram in a flowchart is called the input/output symbol, and it is used to represent input tasks

8. Internal Memory
Figure 3-1: Illustration of shoe boxes and memory locations
Memory locations are reserved (declared as a variable) by a programmer for use in a program

9. Variables
A variable is a computer memory location that a programmer uses to temporarily store data while an application is running

10. Variables
Assigning the variable a data type indicates the type of data (for example a numeric or string) the memory location will store
Reasons to use variables
To hold information that is not stored in a control on the form
To allow for more precise treatment of numeric data
To enable code to run more efficiently

11. Byte Whole#’s from 0-255

12. Variables (cont’d.)
Variables assigned the Integer, Long, or Short data type can store integers, which are positive or negative numbers that do not have any decimal places
The differences among these three data types are in the range of integers each type can store and the amount of memory each type needs to store the integer
Variables assigned the Decimal, Double, or Single data type, on the other hand, can store real numbers, which are numbers that contain a decimal place
The Char data type can store one Unicode character, while the String data type can store from zero to approximately two billion Unicode characters

13. Using Variables to Store Information (cont.)
The Integer data type is used for all integers
Either the Decimal or Double data type are used for numbers containing decimal places or numbers used in calculations
The String data type for text or numbers not used in calculations
The Boolean data type for Boolean values

14. Using Variables to Store Information (cont.)
Selecting a Name for a Variable
Names must begin with a letter or underscore
Names can contain only letters, numbers, or underscores
No punctuation, special characters, or spaces are allowed
The recommended length for a name variable is 32 characters
Variable names cannot be reserved words (such as Sub or Double)
Three-character IDs and examples

15. Using Variables to Store Information (cont.)
Valid names:
intFeb_Income
decSales2014
dblEastRegion
strName
Invalid Names:
4thQuarter
The name must begin with a letter or underscore
dblWest Region
The name cannot contain a space
strFirst.Name
The name cannot contain punctuation
decSales$East
The name cannot contain a special character

16. Variables (cont’d.) Declaring a Variable in Code
The declaration statement tells the computer to set aside a small section of its internal memory, and it allows the programmer to refer to the section by the variable’s name
The {Dim | Private | Static} portion of the syntax indicates that you can select only one of the keywords appearing within the braces

17. Using Variables to Store Information (cont.)
If no initial value is given to a variable when declaring it, the computer stores a default value
Numeric variables are set to 0
Boolean variables are set to False
Object and String variables are set to Nothing
Date variables are set to 1/1/ :00:00AM

18. LITERALS AND CONSTANTS

19. Literals
A value is called a literal when it literally is the value required by the assignment statement

20. Forced Literal Types
Sometimes you might want a literal to be a specific data type than the Visual Basic default type

21. Constants
A constant variable will contain one permanent value throughout the execution of the program
The declaration of a constant begins with the letters Const, not the letters Dim
You must assign the value to be contained in the constant on the same line as its definition
You cannot attempt to change the value of the constant anywhere in the program. If you do, you will produce a compiler error
The letter c often is placed before the constant name to identify it throughout the program as a variable that cannot be changed
Other than the letter c, constant names are formed using the same rules and techniques as other variable names

22. Named Constants (cont.)
Figure 3-20 Syntax and examples of the Const statement

23. Assigning data to variables

24. Assigning Data to an Existing Variable
Assignment statement
Assigns a value to a variable at run time
Syntax: variablename = expression
An expression may include literal constants, object properties, variables, keywords, and arithmetic operators
Literal constant
A data item whose value does not change while the application is running
Example: The string “Mary”
Literal type character
Forces a literal constant to change its data type

25. Referencing a Variable
When a variable is declared, it will be underlined with a green squiggly line until it is referenced in a statement
When using a variable in a program, it is mandatory that you define the variable before using the variable name in a statement

26. Assigning Data to an Existing Variable

27. Parsing, converting and formatting

28. Val( ) function
Val temporarily converts a string to a number and returns the number
Doesn’t change the string
Syntax: Val(string)
Microsoft Visual Basic 2012: Reloaded, Fifth Edition

29. TryParse() Using the TryParse Method
Each data type in Visual Basic is a class
Most classes have one or more methods that perform a specific task for the class
All of the Visual Basic numeric data types (such as Double, Decimal, or Integer) have a TryParse method whose task is to convert a string to that particular data type (i.e. convert a string to double, decimal, or integer)

30. TryParse( ) Converts a string to a number Arguments
You specify the data type to parse to
Will not produce an error if the conversion fails
Arguments
dataType: A numeric data type, such as Integer
String: A string to be converted
Variable: A variable that receives the numeric value
SYNTAX:
datatype.TryParse(from, into)
TryParse Is preferred over Val
Allows the programmer to specify the data type
Val only returns a Double number

31. TryParse( )

32. TryParse results for double, decimal and integer data types

33. Convert( )
At times, you may need to convert a number (rather than a string) from one data type to another
Visual Basic includes several procedures that allow you to convert one data type to another data type
Convert Methods include ToDecimal, ToDouble, ToInt32, and ToString

34. Convert( )

36. Conversion method review
Ex.
Dim W as Double
Dim X as String =“10”
Dim Y as integer
Dim Z as decimal
Val temporarily converts a string to a number and returns the number (returns the numbers in a string)
Doesn’t change the string (useful for getting numeric info from a textbox)
No control over the resulting data type
Y = Val(x) ‘ Y is now equal to 10 and can be used in a math equation
Y = Val(" ") ' Y is now 2457
TryParse() converts a string to a number
Returns 0 if it fails
Useful for getting numeric info from a textbox, because it will always return a numeric value
ex:
Double.TryParse(X, Y)
Convert() changes a number from one data type to another
W = Convert.ToDouble(Z);
Chapter 4: Variables and Arithmetic Operations

37. ToString ( ) Used for Formatting numbers into strings
Specifying decimal places and special characters to display
Syntax: variablename.ToString(FormatSpecifier)
variablename: The name of a numeric variable
FormatSpecifier: The format you want to use

38. Format Specifiers for the ToString Function
Use the format specifier to identify the format for the numeric data to be returned by the ToString function

39. Precision Specifier
The precision specifier is a number included within the quotation marks that identifies the number of positions that should be returned to the right of the decimal point

40. Arithmetic expressions

41. Arithmetic Operators

42. Arithmetic Expressions

43. Arithmetic Expressions (cont’d.)
The integer division operator is used to divide two integers and then return the result as an integer
The modulus operator (sometimes referred to as the remainder operator) is also used to divide two numbers, but the numbers do not have to be integers
After dividing the numbers, the modulus operator returns the remainder of the division

44. Arithmetic Expressions (cont’d.)
Arithmetic Assignment Operators
Used to abbreviate an assignment statement that contains an arithmetic operator
variableName = variableName arithmeticOperator value
age = age
is the same as
age += 1
Both statements tell the computer to add the number 1 to the contents of the age variable and then store the result in the variable

46. Scope

47. The Scope and Lifetime of a Variable
Indicates where a variable can be used
Lifetime
How long a variable remains in memory
Scope and lifetime are determined by where a variable is declared:
either the General Declarations section
or the form’s Declaration section
Three types of scope:
Class: The variable can be used by all procedures in a form
Procedure: The variable can be used within a procedure
Block: The variable can be used within a specific code block

48. Scope and Lifetime (cont’d.)
Variables with Class Scope
A class-level variable has have class scope and is declared in the form’s Declarations section, which begins with the Public Class clause and ends with the End Class clause
Class-level variables retain their values and remain in the computer’s internal memory until the application ends
Declare a class-level variable using the Private keyword
When a variable is referenced in two different event handling procedures, it must be defined at the class level instead of the procedure (event handler) level

49. Using a Procedure Two types of Procedures
A procedure that performs its task but does not return a value is called a Sub procedure
A procedure that returns a value is called a Function procedure, or a function
An argument identifies a value required by a procedure
Every procedure is part of a Class
Variables with Procedure Scope
Can be used only by that procedure
Declared at the beginning of the procedure using the Dim keyword
Removed from memory when the procedure ends

50. Scope and Lifetime (Cont’d.)

51. Scope and Lifetime (Cont’d.)
Variables are declared using the Dim, Private, or Static keywords
A static variable is a procedure-level variable that remains in memory, and also retains its value, even when the procedure in which it is declared ends
Like a class-level variable, a static variable is not removed from the computer’s internal memory until the application ends
Unlike a class-level variable, which can be used by all the procedures in a form, a static variable can be used only by the procedure in which it is declared

52. Scope and Lifetime (cont’d.)

53. Option statements

54. Option Statements Every variable must be declared Option Explicit
flags any undeclared variables
Option Infer
Prevents you from using undeclared variables
Ensures that every variable is declared with a data type
Option Strict - If the value’s data type does not match the memory location’s data type, implicit type conversion occurs to convert the value to fit the memory location
Implicit type conversion - Converts the right-side value to the data type on the left side
Promotion - Data is converted to a greater precision number (e.g., Integer to Decimal)
Demotion = Data is truncated (e.g., Decimal to Integer)
Data loss can occur when demotion occurs

55. Option Statements
When a value is converted from one data type to another data type that can store either larger numbers or numbers with greater precision, the value is said to be promoted
Changing a short (2 bytes) to a Long (8 bytes)
Dim X as Short = 2
Dim Y as Long
Y =X
When a value is converted from one data type to another data type that can store only smaller numbers or numbers with less precision, the value is said to be demoted
Changing a long to a short
X=Y

56. TESTing and debugging

57. Testing and Debugging the Application
Applications must be tested to verify that the code works correctly
A logic error can occur for a variety of reasons
A run time error causes the application to end abruptly
Your sample input data should include both valid and invalid data
Valid data is data that the application is expecting the user to enter
Invalid data is data that the application is not expecting the user to enter

58. Debugging Your Program
A Format Exception occurs when the user enters data that a statement within the program cannot process properly

59. Debugging Your Program
An Overflow Exception occurs when the user enters a value greater than the maximum value that the statement can process
It is not possible to divide by zero, so if your program contains a division operation and the divisor is equal to zero, the Divide By Zero Exception will occur