Chapter 5

Decisions

Outline and Objectives

Relational and Logical Operators If Blocks Select Case Blocks

Condition

A condition is a Boolean expression that can be either true or false

Conditions can be formed by using the six Relational operators and the three Logical operators

Relational Operators

Mathematical Notation

Visual Basic Notation

Numeric Meaning

= = Equal to

≠ < > Not equal to

< < Less than

> > Greater than

<= <= Less than or equal to

>= >= Greater than or equal to

Examples

1<=1 True1 < 1 False2 < 5 True3 <> 3 False0 < 3.5 True5 <= 3 False

Examples

Determine whether each of the following conditions is true or false

assume that: •a = 4, b = 3, c = “hello” , d = “bye”

(a+b) < 2 * a True

(Len(c) – b) = (a / 2) True

Computers use a special coding system to compare character strings called ANSI

With ANSI each character has a number associated with it, this number is called ANSI value of the character.

The ANSI values of characters are given in following table. (Appendix A)

You do not need to memorize the table

Just know the numbers, small and capital lettersA= 65 a=97 sp=32 0=48

Using Relational Operators on Strings

8

Punctuation < Capitals letters < Small letters

Using Relational Operators on Strings

The string str1 is said to be less than the string str2 if str1 precedes str2 alphabetically when using the ANSI (or ASCII) table

Two strings are compared working from left to right, character by character, to determine which one should precede the other.

Example of Comparing Character Strings

“Hope” < “Hopeful”

“Chase” < “Chaz” True

“ Cat” < “Cat” True

“Pay” < “Pay “ True

“Jones” <> “James” True

True

Example…

Determine whether the following condition is true or false

(assume that c = “hello” and d = “bye”)

c < (“good” & d) False

Logical Operators

The result of a logical operator is also True or False

The three Logical Operators are: Not And Or

Not Not: Negates a single expression Example: Suppose answ = “Y”

Not (answ = “y”) is True

X Not X

F T

T F

And Takes two expressions, returns True only if

both expressions are evaluated as being true

Example: Suppose answ = “Y”

•(answ = “Y”) And (answ = “y”) is False

X Y X And Y

F F F

F T F

T F F

T T T

Or

Takes two expressions, returns true if either one of the expressions is evaluated as being true.

Example: Suppose answ = “Y”

•(answ = “Y”) Or (answ = “y”) is True

X Y X Or Y

F F F

F T T

T F T

T T T

Truth Tables

X Y X And Y X Or Y Not X

F F F F T

F T F T T

T F F T F

T T T T F

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Not ( X And Y ) = Not X OR Not Y Not ( X Or Y ) = Not X And Not Y

Precedence

First, all Arithmetic operations are carried out

Then all expressions involving >, <, and = are evaluated to true or false

The logical operations are next applied in the order Not, then And, and Finally Or.

Examples…

Evaluate the following to True or False

Assume a=4, b=3 Print a*3-2=5-b False Print a=b+3<=a+b True Print not (a<2) or b<4 = a+b True Print Not(a<b)=(a>=b+2)

FalseBe careful about the priorities

Notes A Condition such as 2<n<5 (not a

syntax error) but should never be used, because visual basic will not evaluate it as intended.

The correct condition is (2<n) And (n<5)

The following conditions are equivalent:a<>b Not (a=b)a>b Not (a<=b)Not (n<m) (n>=m)

Are these conditions equivalent?

Not(n<m); n>m n>=m

(a=b) and (a<b); a<>b try a=4 b=5

Not ((a=b) or (a=c)) ; a<>b and a<>c

Not (a>=b); (a<b) and not (a=b)

a<b is enough

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Write an Exp equal to the negation of

a>b a<b Or a=b a<=b (a<b) and (c<>d) (a>=b) Or (c=d) (a=b) or (a=d) (a<>b) and (a<>d) Not ( (a=b) or (a>b) ) (a=b) or (a>b)

(a<> “”) and (a<b) and (len(a) < 5)(a =“”) Or (a>=b) Or (len(a)>=5)

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Types of Decision Structures

If Block Statement•Single alternative: If...Then

•Compound alternative: If Then...Else

Select Case Statement

Single Alternative Decision An action is taken if the condition is

true, otherwise the control goes to the next statement.

Syntax:If condition Then actionEnd If

If condition is true, action is executed. Otherwise, action is skipped

Example

Example

Compound Alternative Decision

SyntaxIf condition Then action1 Else action 2End If

Example

Example

Example

Example

If cond1 Then If cond2 Then action(s) End IfEnd If

If cond1 And cond2 Then action(s)End If

This is easier to understandA confusing If Block

Compound Alternative Decision

SyntaxIf condition1 Then action1 ElseIf condition2 Then action 2 ElseIf condittion3 Then action3 Else action4End If

This block statement searches for the first True condition, carries out its action, and then skips to the statement following end if.

If none of the conditions is true, then else’s action is carried out.

In general, an IF block can contain any number of ElseIf clauses.

Example

Single Line If Statement

SyntaxIf condition Then action

There is no End If If the condition is true, the action will

be executed If the condition is false, the (else)

action will be taken

Example

Example

Examples

If 2 < n < 5 Then

action(s)

End If

Wrong (not a compiler error)

(n > 2) And (n < 5)

If 5 Then

action(s)

End If

If n Then

action(s)

End If

If Not(n < m) Then

action(s)

End If

Equivalent to

If (n >= m) Then

action

End If

If n <> 0 then

action(s)

End if

The Select Case Block

Similar to If statement Used instead of compound If

statements Action is selected from a list of

alternatives Avoids confusion of deeply nested If

blocks

Select Case Block (Syntax)

Select Case selector Case value-list-1 action1 Case value-list-2 action2 ….. Case Else action of last resortEnd Select

Select Case Block

Each value-list contains one or more of the following types of items separated by a comma

a constant a variable an expression an inequality sign preceded by Is and

followed by a constant, variable, or expression

a range expressed in the form a To b, where a and b are constants, variables, or expressions.

Example

Example

Example

Example

Example

Example

Example

what is the output of the following code

a = 3b = 4Select Case a = b

Case 3Print "A"Case 4Print "B"Case 1Print "C"Case 0Print "D"

End Select

Answer: D

what is the output of the following code

a = "hello"Select Case a

Case "apple" To "orange"Print "A"Case "Hello"Print "B"Case Is <= "z"Print "C"Case "hello"Print "D"

End SelectAnswer: A

what is the output of the following code

a = 0Select Case a

Case 3.5Print "A"Case a > 5Print "B"Case a - 1 To a + 1Print "C"Case 3Print "D"

End SelectAnswer: B

References

Chapter 5 - Visual Basic Schneider