Working with Variables, Constants, Data Types, and Expressions

1
Chapter 4

• Working with Variables, Constants, Data Types,
  and Expressions

2
Objectives

• Learn to use the RadioButton and GroupBox
  controls, and Layout toolbar
• Learn to use the Pmt function, and the Format
  function
• Lock controls on a form
• Visual Basic .NET data types
• Declare and use variables and constants within
  code
• Convert between data types
• Use the Option Strict statement
• Use arithmetic expressions
• Describe the order of operator precedence in code

3
Problem Car Loan Calculator

• Develop a window application for Automobile Loan
  Calculator. It will calculate a customers
  monthly automobile loan payment based on a loan
  amount, interest rate, and the term, or length,
  of the loan.
• MonthlyPayment (InterestRate
  (1InterestRate) NumberofPayments) / ((1
  InterestRate) NumberofPayments-1) LoanAmount

4
Program Development
5
Problem Analysis
Question What are the inputs, outputs?
6
Interface Design

• The increment of Loan amount is 1000. The
  maximum value allowed is 25000

• The Current interest rate is incremented or
  decremented by 0.05 with minimum 5.0 and
  maximum 15.00.
• The format of Monthly payment is currency

7
Program Design

• Two tasks
• Calculate monthly payment
• Reset the input and output values

8
Validate Design

• Validate the design by stepping through the
  requirements and make the design addresses each
  requirement
• For example,
• The formula in the Algorithms section of the
  requirements document is handled in the design by
  the Computer Payment Click event
• The items in the first note in the Notes section
  are shown as requested on the storyboard

9
Starting a Project

• Start VB .NET and click the New Project button on
  the Start Page
• Click Create directory for Solution, type
  Automobile Loan Calculator in the Name box
• Browse data disk for Chapter4, and then click
  Open
• Click the OK button

10
Setting Form Properties

• Set the properties of Form1 using the table as a
  guide

11
Adding Controls

• Add three Label, two NumericUpDown, one TextBox,
  and two Button controls
• Add a GroupBox control

12
Adding RadioButton Controlsto a Group Box

• Drag the RadioButton button to GroupBox1 control.
  Be sure to release the mouse button while the
  mouse pointer is within the borders of the
  GroupBox1
• Repeat this step for two more RadioButton controls

13
Change Properties of Controls
14
Change GroupBox Properties

• The GroupBox is used to group functionally
  similar controls together

15
Change GroupBox Properties
Checked property value is true
16
Using Layout Toolbar

• The layout toolbar contains tools that allow you
  to adjust the alignment, spacing, and size of
  controls
• Select the Label2 control, the Current interest
  rate ()
• Resize the control so that the text displays on a
  single line. Press and hold down the CTRL key
  and then click the other two Label controls on
  the form
• Click the Align Rights button on the Layout
  toolbar and then click the Make Vertical Spacing
  Equal button on the Layout toolbar.

• When use Layout toolbar to size and align
  multiple controls, the last control you select is
  used as the basis for aligning the controls

17
Sizing and Aligning Controls

• Select the nudLoanAmount, nudRate, and
  txtMonthlyPayment controls. Click the Make Same
  Size button and then Align Lefts button on the
  toolbar.

18
Setting a Default Button

• The default button is the Compute Payment button.
  Pressing the ENTER key is equivalent to clicking
  the button
• Select the AcceptButton property in the
  properties window to btnComputePayment

19
Locking Controls Positions

• Disallows the ability to move controls or modify
  control sizes on a form during design time
• Click Format on the menu bar, and select Lock
  Controls

20
Constants and Variables

• A value is a number or string that programmers
  use in the code
• A variable represents a location in computer
  memory that can change values as the code
  executes
• A constant represents a location in computer
  memory that cannot be changed during execution
• By the name of a constant, we can easily tell the
  use or the purpose of the constant

21
Data Types

• Computer uses binary numbers to represent
  everything
• 1310000011012 c 9910 011000112
• The data type of a variable or a constant
  determines what kind of data the variable or
  constants can store
• Try to use the data type that takes up the
  smallest amount of memory
• Try to use integral data type if decimal number
  is not necessary

22
Data Types
23
Declaring Constants
24
Declaring Constants

• Double-click the Form1 form in an area that does
  not contain a control.
• When the code window displays, enter the seven
  lines of code below

25
Coding Form Load Event Procedure

• The form Load Event Procedure executes when the
  form first displays on screen. Execution happens
  before any other events.
• Click the Form1_Load event procedure and enter
  the two lines of code below.

26
Coding the btnReset_Click Event Procedure

• Click the Form1.vbDesign tab and then
  double-click the btnReset control. Enter the 7
  lines of code below.

27
Option Strict Statement

• To force you to ensure that all assignment
  statements use the same data type on both sides
  of the assignment
• In the code window on line 1, type Option Strict
  On

28
Declaring Variables
29
Scope of a Variable

• Scope refers to a variables accessibility. It is
  defined by the placement of the variable in the
  code.
• MaximumLoanAmount declared in the general area of
  the form is a global constant
• Variable declared inside a procedure is a local
  variable
• Local variable is not accessible outside that
  procedure

30
Global Variable

• Append a letter g in front of a variable name to
  indicate a global variable
• Place global variable after the comment header

31
Event Procedures of RadioButton

• The CheckChanged event procedure executes when
  user clicks a RadioButton
• Double-click the radTwoYears control on the
  Form1.vbDesign, enter the code
• Repeat for radFiveYears and radSixYears buttons

32
Declaring Local Variables

• Click the Form1.vbDesign tab and then
  double-click the btnComputePayment control.
  Enter the code below in the code window.

33
Converting Data Types

• To ensure that all data being used in the
  calculation has the same data type, we may use
  Convert.ToDouble() method.

34
Arithmetic Operators

• Arithmetic operator is used to manipulate two or
  more numeric values
• The Mod operator divides two numbers and return
  the remainder of division operation
• The \ operator truncates any decimal numbers to
  integers, divides, and returns an integral
  quotient

34 mod 6 is 4 6.6 \ 2.5 is 3 6.6 / 2.5 is 2.64
35
Precedence of Operators

• Precedence of operators determine the sequence an
  expression is evaluated by a computer
• An expression is evaluated from left to right
• (), , , /, \, Mod, , -

(2 3 4 / 5) 2 5 / (4 3 2 3) (2
3 4 / 5) 2 5 / (4 3 8) (2 2.4) 2
5 / 4 (-0.4) 2 5 / 4 0.16 5 / 4 0.16
1.25 1.41
36
In-Class Exercise

• Evaluate each of the following expressions
• 4 5 3 / 6 6 2 / 12
• 7 5 / 2 9 Mod 3 3
• If necessary, insert parentheses so that each
  numeric expression results in the value indicated
  on the right side of the arrow
• 8 / 2 2 12 ? 14
• 7 3 4 2 3 / 13 ? 22

37
Construction of Error-Free Numeric Expressions

• Do not attempt to divide by 0
• Do not attempt to take square root of a negative
  number
• Do not attempt to compute a value larger than the
  maximum a data type may represent

38
Calculating a Monthly Interest Rate

• Enter the lines of code below and do not press
  the ENTER key

39
Intrinsic Functions

• Intrinsic functions are VB .NET built-in
  functions
• The pmt() function is in the financial category
  of the run-time library functions Pmt( rate,
  nper, pv, fv, due)
• The function Pmt() requires a negative value for
  the present value (pv) argument
• The function returns a value of certain data type

40
The Format Function

• The Format function is another intrinsic
  function
• strPayment Format(dblPayment, Currency)
• Currency is one of many predefined format name

41
Finish btnComputePayment_Click
42
Summary

• Use the RadioButton and GroupBox controls, and
  Layout toolbar
• Set a default button on a form, and lock controls
  on a form
• Declare and use variables and constants
• Describe data types and Convert between data
  types
• Use arithmetic expressions
• Code a form Load event procedure
• Describe the order of operator precedence in code
• Use the intrinsic function like Pmt, and Format
  functions