Decisions with Select Case and Strings

Chapter 4 Part 2

Chapter 4 2

String Storage (Section 4.8)

Stores characters (e.g., A, a, B, b) as numeric codes in Unicode format.

Displays character corresponding to actual Unicode number stored in memory.

Arranges letters in alphabetical order in Unicode sequencing. A comes before B. A has lower Unicode number than B. Uppercase before lowercase:

A before a Space before letters

Chapter 4 3

String Comparison in If Statement

John (same)JOHN (different)

Chapter 4 4

More String Comparison

John (equal) JOHN (less than John) Johnny (greater than John) Jones (greater than John)

Chapter 4 5

How Strings Are Compared

J O H N

J O N E S

Chapter 4 6

ToUpper

StringExpression.ToUpper Returns uppercase equivalent of string. Does not change original string contents.

Then store result in another variable.

Chapter 4 7

ToLower

StringExpression.ToLower Returns lowercase equivalent of string. Does not change original string contents.

Then store result in another variable.

Chapter 4 8

Tutorial 4-6 (pp. 214-216)

Illustrates how to use ToUpper within an If statement to process comparisons.

Shows that typing “prospero” equals “PROSPERO” when using: txtInput.Text.ToUpper

Chapter 4 9

StringExpression.Length Determines the length of a string. Text Box Example:

Visual Basic (typed in text box) txtProgram.Text.Length Length = 17

Variable Example: strName = txtName.Text intNumber = strName.Length

Good for getting restricted # of characters from user. (see p. 217)

Chapter 4 10

Spaces

Leading Spaces Spaces (shown here with #) before actual

characters #####Keith

Trailing Spaces Spaces after actual characters Keith#####

Chapter 4 11

Trim Methods Use Trim method to remove spaces. StringExpression.TrimStart

Removes leading spaces #####Keith becomes Keith

StringExpression.TrimEnd Removes trailing spaces Keith##### becomes Keith

StringExpression.Trim Removes leading and trailing spaces #####Keith##### becomes Keith

Trim methods do not modify actual variable; use Trim and store results in another variable.

Chapter 4 12

TrimStart Example

See p. 227 for displayingmultiple lines.

Chapter 4 13

Other String Expressions

Substring (pp. 218-219) IndexOf (pp. 219-220)

Chapter 4 14

InStr Searches for a substring within the base string

(searches are case-sensitive). Proceeds left to right. Stops at match or end of string. If successful, returns the character position at which

the match was found. If unsuccessful, returns 0. Example:

InStr("Eventful adventure","advent") Returns 10 because “a” starts in 10th position. Eventful adventure (string) advent (substring

Chapter 4 15

More InStr()

Instr(startpos, basestring, searchstring) Example:

InStr(1,”Eventful adventure”, “vent”) Starts at position 1. Looks in Eventful adventure. Finds starting position of v in vent. Returns 2.

Example: InStr(1, strWholeName, “,“) Looks within variable contents to find position of

comma.

Chapter 4 16

Trim Full Name Example

Chapter 4 17

Trim Full Name, Continued

Chapter 4 18

Select Case

Handles conditions with multiple outcomes. Tests one expression,

whereas ElseIf tests several expressions. Select Case testexpression Case expressionlist1

Statementblock1 Case expressionlist2

Statementblock2 Case Else

Statementblock End Select

Chapter 4 19

Select Case Exact Match Example

Chapter 4 20

Select Case at Run Time

1) Evaluates the test expression.2) Attempts to match the resulting value with

one of the expression lists.1) Starts searching top of expression list.2) Proceeds through subsequent expression

lists, stopping at the first match.3) Processes code at match or4) Executes the Case Else statement block if

no match is selected.

Chapter 4 21

Select Case for Ranges Select Case expression Case Is < 1 ‘requires Is before relational operator

Do something Case 1 To 5 ‘requires To keyword between values

Do something different Case 6 To 10

Do something Case Else

Do something entirely different. End Select

Chapter 4 22

Select Case Example

txtTest btnGrade

90+ Display A and “Superior” 80-89Display B and “Good” 70-79Display C and “Satisfactory” OtherDisplay failing notice

Chapter 4 23

Relational Operator >=And Range Selection

Chapter 4 24

Summary

Select Case If Statement

Test expression can be any numeric or string expression.

Must be a logical expression, such as > <.

Decision determined by a single expression at the top.

Has one condition at the top and one for each ElseIf and each of these conditions is a separate expression.

Appropriate for more “clear cut” examples.

Enables more complexity than Select Case.

Chapter 4 25

Decision Rules

Select Case If Statement

Decision has multiple outcomes, which depend only on a single expression.

Decision has only 2 outcomes & decision can be expressed as a single expression.

Outcome depends on multiple conditions that may be independent of each other.

Chapter 4 26

Radio Button Control (p. 236)

Ensures that the user selects only one option: Male/Female Age ranges

Circle with descriptive text Only 1 selected in group at

a time Mutually Exclusive:

Deselects one when youselect another radio button

Chapter 4 27

Radio Button Rules & Conventions

Use radControlName style with rad prefix. Use access keys on Text property. Set TabIndex from one radio button to

another. Must use group boxes if the form has more

than one set of radio buttons (see bulleted list and examples on p. 236).

Use no more than 7 radio buttons per set.

Chapter 4 28

Radio Button Properties

Name Control name, such as radRed

Text Descriptive text displayed by the circle

Checked Selection option value = TrueNot selected value = False

TabIndex Helps with focus (active control)

Chapter 4 29

Radio Button Results

Can trigger Click event, but typically enable user to choose and then click a button to trigger event.

Default radio button Set Checked property to True.

Chapter 4 30

Which button is clicked?

Use in IF…ElseIf statement to determine if radio button Checked property value is true (i.e., selected).

If radRed.Checked = True ThenMessageBox.Show(“You chose red.”)

ElseIf radGreen.Checked = True ThenMessageBox.Show(“You chose green.”)

ElseIf radBlue.Checked = True ThenMessageBox.Show(“You chose blue.”)

ElseIf radPurple.Checked = True ThenMessageBox.Show(“You chose purple.”)

End If

Chapter 4 31

Check Box Control (p. 238) Not Mutually Exclusive:

Enables user to select 0, 1, or more options in same category.

Name: chk prefix standard chkBold chkItalic chkUnderline

Text: caption displayed onscreen Checked: selected if True

Can set 1 or more at design time See characteristics on p. 238.

Chapter 4 32

Is a check box checked?

Although If…ElseIf statements work well for radio button groups, they do not work well for check boxes.

Use individual If statements for each check box to see if it is checked.

Use Checked to determine if check box is selected. chkBold.Checked = True

Compare radio button code (p. 239) to check box code (p. 240).

Chapter 4 33

Class-Level Variables

Review scope of a variable. Visible and accessible to statements

Local variables Declared within event procedure; local to that

procedure Class-level variables

Declared at the form level; available to all procedures on that form

Chapter 4 34

Class-Level Concerns

Wrong value can be stored; must track down code that causes problem (very troublesome in complex programs).

When 2 or more procedures modify same variable, must be careful that 1 procedure doesn’t modify it when you need original value in another procedure.

Chapter 4 35

Recommended Practice

Tutorial 4-7 (Strings) Tutorial 4-8 (Select Case) Tutorial 4-9 (Check Boxes & Radio Btns) Section 4-14 and Tutorial 10

pp. 241-250 Comprehensive review