1. Chapter 5
Decisions

2. Outline and Objectives
Relational and Logical Operators
If Blocks
Select Case Blocks
This chapter introduces the control structure, powerful programming tool that will be used in virtually all programs from this point on.’
Control structures allow the programmer to determine whether or not a specific statements are executed.
Two types of control structures:
decision structure
loop structure
Decision structures are used to make comparisons 2

3. 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

4. Relational Operators

5. Print 1<=1 True Print 1 < 1 False Print 2 < 5 True
Examples
Print 1<=1 True
Print 1 < 1 False
Print 2 < 5 True
Print 3 <> 3 False
Print 0 < True
Print 5 <= 3 False

6. Examples
Determine whether each of the following conditions is true or false
assume that:
a = 4, b = 3, c = “hello” , d = “bye”
Print (a+b) < 2 * a True
Print (Len(c) – b) = (a / 2) True
Conditions also can involve variables, numeric operators, and functions . To determine whether a condition is true or false, first compute the numeric or string values and if the resulting is true or false

7. Using Relational Operators on Strings
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 letters
A= a= sp= =48

8. Punctuation < Capitals letters < Small letters8

9. 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.

10. Example of Comparing Character Strings
Print "Chase" > "Chaz"
False
Print " Cat" < "Cat"
True
Print "Pay" < "Pay "
True
Print "Jones" < > "James"
True
Print "Hope" > "Hopeful"
False 10

11. Print c < (“good” & d) False
Example…
Determine whether the following condition is true or false
(assume that c = “hello” and d = “bye”)
Print c < (“good” & d) False

12. Logical Operators
The result of a logical operator is also True or False
The three Logical Operators are:
Not
And Or

13. Not Not: Negates a single expression Example: Suppose answ = “B”
Print Not (answ = “b”) True X
Not X F T

14. And Example: Suppose answ = “B”
Takes two expressions, returns True only if both expressions are evaluated as being true
Example: Suppose answ = “B”
Print (answ = “B”) And (answ = “b”) False X Y
X And Y F T

15. Or Example: Suppose answ = “B”
Takes two expressions, returns true if either one of the expressions is evaluated as being true.
Example: Suppose answ = “B”
Print (answ = “B”) Or (answ = “b”) True X Y
X Or Y F T

16. Chapter 5 - Visual Basic Schneider
Note:
Not ( X And Y ) = Not X OR Not Y
Not ( X Or Y ) = Not X And Not Y
Chapter 5 - Visual Basic Schneider 16

17. 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.

18. Examples…

20. 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) False
Be careful about the priorities

21. 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)

22. Are these conditions equivalent?
(wrong) correct  n>=m
Not(n<m); n>m
(a=b) and (a<b); a<>b try a=4 b=5
(wrong )
Not ((a=b) or (a=c)) ; a<>b and a<>c
(correct )
Not (a>=b); (a<b) and not (a=b)
(correct )
a<b is enough
Chapter 5 - Visual Basic Schneider 22

23. 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)
Chapter 5 - Visual Basic Schneider 23

24. Types of Decision Structures
If Block Statement
Single alternative: If...Then
Compound alternative: If Then...Else
Select Case Statement

25. Single Alternative Decision
An action is taken if the condition is true, otherwise the control goes to the next statement.
Syntax: If condition Then action End If
If condition is true, action is executed. Otherwise, action is skipped

26. Example

27. Example

28. Compound Alternative Decision
Syntax
If condition Then
action1
Else
action 2
End If

29. Example

30. Example

31. Example

32. This is easier to understand
Example
If cond1 Then
If cond2 Then
action(s)
End If
If cond1 And cond2 Then
action(s)
End If
This is easier to understand
A confusing If Block 32

33. Compound Alternative Decision
Syntax
If condition1 Then
action1
ElseIf condition2 Then
action 2
ElseIf condittion3 Then
action3
Else
action4
End 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. 33

34. Example

35. Single Line If Statement
Syntax
If 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 35

36. Example

37. Example

38. Examples If n <> 0 then If n Then action(s) action(s) End if
If 5 Then
action(s)
End If
If n <> 0 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
If 2 < n < 5 Then
action(s)
End If
Wrong (not a compiler error)
(n > 2) And (n < 5) 38

39. Similar to If statement Used instead of compound If statements
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 39

40. Select Case Block (Syntax)
Select Case selector
Case value-list-1
action1
Case value-list-2
action2
…..
Case Else
action of last resort
End Select
Select case choices are determined by the value of an expression called a SELECTOR. . Each of the possible actions is preceded by statement of the form CASE valulist., where valuelist itemizes the values of the selector for which the action should be taken 40

41. a constant a variable an expression
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. 41

42. Example

43. Example

44. Example

45. Example

46. Example

47. Example

48. Example

49. what is the output of the following code
b = 4
Select Case a = b
Case 3
Print "A"
Case 4
Print "B"
Case Not 5 > a < 2
Print "C"
Case 0
Print "D"
End Select
Answer: C

50. what is the output of the following code
a = "hello"
Select Case a
Case "Hello"
Print "B“
Case "apple" To “orange"
Print “C“
Case a < “orange”
Print “S”
Case Is <= "z"
Print “A"
Case "hello"
Print "D"
End Select
Answer: C

51. what is the output of the following code
Select Case a
Case 3.5
Print "A"
Case a > ‘Note A>5  false  0
Print "B"
Case a - 1 To a + 1
Print "C"
Case 3
Print "D"
End Select
Answer: B

52. Chapter 5 - Visual Basic Schneider
References
Chapter 5 - Visual Basic Schneider