Chapter 4 – Decisions

4.1 Relational and Logical Operators 4.2 If Blocks 4.3 Select Case Blocks 4.4 Input via User Selection 1

4.1 Relational and Logical Operators

• • • • • • • Conditions Relational Operators ANSI Values ASC and CHR Functions Logical Operators Using Boolean Variables Boolean Methods 2

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.” 3

Relational Operators

< <= > >= = <> less than less than or equal to greater than greater than or equal to equal to not equal to Value of a relational expression will always be True or False 4

Example

When a = 3, b = 4

(a + b) < 2 * a

3 + 4 = 7 2 * 3 = 6 7 is NOT less than 6 and so the value of the expression is False 5

Another Example

a = 4 b = 3 c = "hello"

( c.Length – b ) = ( a / 2 )

5 – 3 = 2 4 / 2 = 2

True

because 2 equals 2 6

Relational Operators and Strings • ANSI values are used to decide order for strings • • • • • “Cat” < “cat” “visual basic” > “Magic” “car” < “cat” “CAT” <> “cat” “cat” < “catalog” is True is True is True is True is True See next slide / Appendix A for ANSI values

ANSI Character Set

(Appendix A) There is a numeric representation for every key on the keyboard and for other assorted 32 (space) 48 0 33 ! 34 “ 49 1 57 9 35 # 65 A 162 ¢ 169 © 176 ° 177 ± 178 ² 179 ³ 66 B 90 Z 97 a 98 b 122 z 123 { 125 } 126 ~ 181 µ 188 ¼ 189 ½ 190 ¼ 247 ÷ 248 ø 8

Chr Function

Given

n

between 0 and 255,

Chr(n)

returns a string consisting of the character with ANSI value

n

.

Examples:

Chr(65) Chr(162) is is A ¢

9

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

10

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

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 12

Truth Tables

Truth tables help us evaluate logical operators

Example 4.3

n =

4,

answ =

“Y” Are the following expressions true or false?

Not (n < 6) (answ = "Y" ) Or (answ = "y" ) (answ = "Y" ) And (answ = "y" ) Not(answ = "y" ) 14

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) < 6)

displays True in the text box.

15

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 < 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 < n < 5 Instead use: (2 < n ) And ( n < 5 ) A complete relational expression must be on either side of the logical operators

And

and

Or.

17

• •

Common Error in Boolean Expressions

A common error is to replace the condition

Not ( 2 < 3 )

with the condition

( 2 > 3 )

.

The correct replacement is

( 2 >= 3 )

because >= is the opposite of <, just as <= is the opposite of >.

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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" )

TRUE

Note:

String literals can be used instead of string variables 19

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 20

Boolean-Valued Function IsNumeric

• 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.

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