Chapter 4

1
Chapter 4 Decisions

• 4.1 Relational and Logical Operators
• 4.2 If Blocks
• 4.3 Select Case Blocks
• 4.4 Input via User Selection

2
4.1 Relational and Logical Operators

• Conditions
• Relational Operators
• ANSI Values
• ASC and CHR Functions
• Logical Operators
• Using Boolean Variables
• Boolean Methods

3
Condition

• A condition is an expression involving relational
  and/or logical operators
• The value of a condition is Boolean that is,
  True or False

Conditional statements we make If it is rainy
outside, then I will read a book.
4
Relational Operators

• lt less than
• lt less than or equal to
• gt greater than
• gt greater than or equal to
• equal to
• ltgt not equal to

Value of a relational expression will always be
True or False
5
Example

• When a 3, b 4
• (a b) lt 2 a

2 3 6
3 4 7
7 is NOT less than 6 and so the value of the
expression is False
6
Another Example

• a 4 b 3 c "hello"
• ( c.Length b ) ( a / 2 )

5 3 2
4 / 2 2
True because 2 equals 2
7
Relational Operators and Strings

• ANSI values are used to decide order for strings
• Cat lt cat is True
• visual basic gt Magic is True
• car lt cat is True
• CAT ltgt cat is True
• cat lt catalog is True

See next slide / Appendix A for ANSI values
8
ANSI Character Set (Appendix A)

• There is a numeric representation for every key
  on the keyboard and for other assorted
  characters.

9
Chr Function

• Given n between 0 and 255,
• Chr(n)
• returns a string consisting of the character
  with ANSI value n.
• Examples Chr(65) is A
• Chr(162) is

10
Asc Function

• For a string str,
• Asc(str)
• is the ANSI value of the first character of str.
• Examples Asc("A") is 65
• Asc("25") is 162

11
ASCI to ANSI Conversion

• Problem Create a program to convert ASCI letters
  to ANSI values and visa versa.

Public Class Converter Private Sub
btnAnsi2Ascii_Click() Handles btnAnsi2Ascii.Click
txtASCII.Text Chr(CInt(txtANSI.Text))
End Sub Private Sub btnAscii2Ansi_Click()
Handles btnAscii2Ansi.Click txtANSI.Text
CStr(Asc(txtASCII.Text)) End Sub End Class
12
Logical Operators

• Used with Boolean expressions
• Not makes a False expression True and vice
  versa
• And will yield a True if and only if both
  expressions are True
• Or will yield a True if at least one of both
  expressions are True

13
Truth Tables
Truth tables help us evaluate logical operators
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Example 4.3

• n 4, answ Y Are the following
  expressions true or false?
• Not (n lt 6)
• (answ "Y") Or (answ "y")
• (answ "Y") And (answ "y")
• Not(answ "y")

15
Boolean Expression

• An expression that evaluates to either True or
  False is said to have Boolean data type.
• Example
• The statement
• txtBox.Text CStr((2 3) lt 6)
• displays True in the text box.

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Boolean Variable

• The following statement declares var as a
  Boolean data type.
• Dim var As Boolean
• It can only be assigned one of two values
  either True and False.
• Example Dim boolVar As Boolean
• boolVar 2 lt 6
• txtBox.Text CStr(boolVar)

Displays True in the text box.
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Common Errors with Relational Operators

• The following is NOT a valid way to test whether
  n falls between 2 and 5
• 2 lt n lt 5
• Instead use
• (2 lt n ) And ( n lt 5 )

A complete relational expression must be on
either side of the logical operators And and Or.
18
Common Error in Boolean Expressions

• A common error is to replace the condition Not (
  2 lt 3 ) with the condition ( 2 gt 3 ).
• The correct replacement is ( 2 gt 3 )
• because gt is the opposite of lt, just as lt
  is the opposite of gt.

19
Boolean-Valued Method EndsWith

• The expression strVar1.EndsWith(strVar2)
• is true if the value of the first variable ends
  with the value of the second variable.
• Dim course As String Visual Basic"
• txtBox1.Text course.EndsWith(Basic")
• Note String literals can be used instead of
  string variables

TRUE
20
Boolean-Valued Method StartsWith

• The expression strVar1.StartsWith(strVar2)
• is true if the value of the first variable
  begins with the value of the second variable.
• Dim course As String Visual Basic"
• txtBox2.Text course.StartsWith(Visu")

TRUE
21
Boolean-Valued FunctionIsNumeric

• The expression IsNumeric(strVar)
• is true if the value of strVar can be converted
  to a number with CInt or CDbl.
• Examples IsNumeric("123") is true
• IsNumeric("123") is true
• IsNumeric("3 - 2") is false

Note The string literal can be replaced with a
string variable.