Making Decisions in a Program Chapter Microsoft Visual Basic.NET: Reloaded 1.

Making Decisions in a Program

Chapter Microsoft Visual Basic .NET: Reloaded

1

2Microsoft Visual Basic .NET: Reloaded

Objectives

• Include the selection structure in pseudocode and in a flowchart

• Write an If…Then…Else statement

• Write code that uses comparison operators and logical operators

• Create a variable having block scope

• Use the line continuation character


Objectives (continued)

• Concatenate strings

• Use the ControlChars NewLine constant

• Change the case of a string

• Determine whether an expression can be converted to a number

• Display a message in a message box


Objectives (continued)

• Include a nested selection structure in pseudocode, flowchart, and code

• Code an If/Elseif/Else selection structure

• Include a Case selection structure in pseudocode, flowchart and code


The Selection Structure

• Also called decision structure• Make a comparison and choose between 2

paths depending upon the result of comparison (condition)


Writing Pseudocode for the If and If/Else Selection Structures

• If selection structure - 1 set of instructions which are processed if condition is true

• If/Else - 2 sets of instructions – the first when condition is true, the other when condition is false


Flowcharting the If and If/Else Selection Structures


Coding the If and If/Else Selection Structures


Coding the If and If/Else Selection Structures (continued)

• Statement Block

• Set of instructions in “true” path

• (between keywords If and Then)

• Optional set of instructions in “false” path

• (between keywords Then and End If)

• Any items in square brackets are optional


Comparison Operators• Also referred to as relational operators


Comparison Operators (continued)


Using Comparison Operators – Example 1


Using Comparison Operators – Example 1 (continued)

Note: variable intTemp has block scope and can only be used in block in which declared



• Line continuation character

• Space followed by an underscore

• Used to break up long instructions onto 2 lines

• Concatenation Operator

• The ampersand ( & )

• Used to concatenate (connect or link) strings


HOW TO…


Using Comparison Operators – Example 2


Using the ToUpper and ToLower Methods


Using the ToUpper and ToLower Methods (continued)


Logical Operators


Logical Operators (continued)


Truth Tables


Truth Tables (continued)

Note: short-circuit evaluation can occur with “And” operator. Both conditions will not be tested if first condition tests false


Using the Truth Tables• Understanding order of precedence is critical


Using the Truth Tables (continued)


Using Logical Operators in an If…Then…Else Statement


Modifying the Skate-Away Sales Application


Modifying the Skate-Away Sales Application (continued)

• Modify to eliminate unwanted error message


Modifying the Skate-Away Sales Application (continued)


The IsNumeric Function

• Function:• A predefined procedure that performs a

specific task and returns a value after completing the task

• IsNumeric function checks if an expression can be converted to a number • Returns Boolean true if able to convert to

number

• Returns Boolean false if conversion cant be made


The IsNumeric Function (continued)


The MessageBox.Show method


The MessageBox.Show method (continued)

• Arguments in the MessageBox.Show method

• Text - a string containing the text to display

• Caption - text displayed in title bar

• Buttons - buttons to display in message box

• Can be one of 6 different constants

• Icon – icon displayed in title bar

• Defaultbutton – button automatically selected if user presses the Enter key on keyboard


HOW TO…


Nested Selection Structures

• When at least one side of the selection structure’s path contains another selection structure

• Inner structure is referred to as “nested”

• “Else” is paired up with nearest preceding If regardless of indentation of code

• Use when you want to make one decision and then choose between two options depending upon the result of the first decision


Nested Selection Structures (continued)


The If/ElseIf/Else Selection Structure


The Case Selection Structure

PsuedoCode1. Get Grade

2. Grade value:A. Display “Excellent”

B. Display “Above Average”

C. Display “Average”

D,F Display “Below Average”

Other Display “Error”


The Case Selection Structure (continued)


HOW TO…


Using To and Is in an ExpressionList

• To allows range of values in Case expression• Example

• Number of items ordered Price per item

1-5$25

6-10$23

• Is allows use of comparison operatorsMore than 10

$20


Using To and Is in an ExpressionList (continued)


Programming Example – Fat Calculator

• Application allows user to enter the total number of calories and grams of fat contained in a specific food

• Calculate and display 2 values: number of fat calories, and fat percentage

• Display appropriate message depending on level of fat percentage


TOE Chart


User Interface


Objects, Properties, and Settings


Objects, Properties, and Settings (continued)


Tab Order


Pseudocode

btnExit Click event procedureclose application

btnCalc Click event procedureif the txtCalories and txtFatGrams controls contain numbers

assign the numbers to variablescalculate the fat calories by multiplying total calories by 9calculate fat percentage by dividing fat by total caloriesdisplay fat calories and fat percentage in appropriate labelsif fat percentage is over 30% display “This food is high in fat” in message boxelse display “This food is not high in fat” in message boxend if

elsedisplay “The calories and fat grams must be numbers”

end if


Code


Code (continued)


Summary

• Selection structure allows program to make a decision and then select one of 2 paths depending upon the results of that decision

• Visual Basic .NET has 4 selection structures• If If/Else

• If/ElseIf/Else Case

• Selection structures can be “nested” within other selection structures• Outer structure contains primary decision

• Inner structure contains secondary decision


Summary (continued)

• All expressions in a selection structure evaluate to either true or false

• Use line continuation character (space &) to break up long instruction into 2 physical lines

• Use concatenation operator (&) to connect (link) 2 strings together

• String comparisons are case sensitive

• Use ToUpper and ToLower functions to convert case


Summary (continued)

• ControlChars.NewLine constant advances insertion point to the next line in a control

• Use logical operators to create compound conditions (And, Or, AndAlso)

• IsNumeric function verifies expression can be converted to a number

• MessageBox.Show allows communication with user in the form of a message box

• Use If/ElseIf/Else and Case selection structures to choose between multiple alternatives

Making Decisions in a Program Chapter Microsoft Visual Basic.NET: Reloaded 1.

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