1. Control Structures: Part 21

2. Outline
Introduction Essentials of Counter-Controlled Repetition For/Next Repetition Structure Examples Using the For/Next Structure Select Case Multiple-Selection Structure Do/Loop While Repetition Structure Do/Loop Until Repetition Structure Using the Exit Keyword in a Repetition Structure Logical Operators Structured Programming Summary

3. For/Next Repetition Structure
The For/Next repetition structure handles the details of counter-controlled repetition.
To illustrate the power of For/Next, we now rewrite the previous program.
The result is displayed in following figure:

4. ForCounter.vb Program Output
1 ' Fig. 5.2: ForCounter.vb
2 ' Using the For/Next structure to demonstrate counter-controlled
3 ' repetition. 4
5 Module modForCounter 6
Sub Main()
Dim counter As Integer 9
' initialization, repetition condition and
' incrementing are included in For structure
For counter = 2 To 10 Step 2
Console.Write(counter & " ")
Next 15
End Sub ' Main 17
18 End Module ' modForCounter
ForCounter.vb Program Output
Control variable initialized to 2
Step increments counter by 2 each iteration
Next marks end of loop

5. For/Next Repetition Structure
For/Next counter-controlled repetition
Structure header initializes control variable, specifies final value and increment
For keyword begins structure
Followed by control variable initialization
To keyword specifies final value
Step keyword specifies increment
Optional, Increment defaults to 1 if omitted
May be positive or negative
Next keyword marks end of structure
Executes until control variable greater (or less) than final value

6. For/Next Repetition Structure
Final value of control variable
Initial value of control variable
For keyword
Increment of control variable
For counter = 2 To 10 Step 2
Control variable name
To keyword
Step keyword
Fig. 5.3 Components of a typical For/Next header.

7. Examples Using the For/Next Structure
Vary the control variable from 1 to 100 in increments of 1
For i = 1 To 100
For i = 1 To 100 Step 1
Vary the control variable from 100 to 1 in increments of –1
For i = 100 To 1 Step –1
Vary the control variable from 7 to 77 in increments of 7
For i = 7 To 77 Step 7
Vary the control variable from 20 to 2 in increments of –2
For i = 20 To 2 Step -2
Vary the control variable over the sequence of the following values: 2, 5, 8, 11,14, 17, 20.
For i = 2 To 20 Step 3
Vary the control variable over the sequence of the following values: 99, 88, 77,66, 55, 44, 33, 22, 11, 0.
For i = 99 To 0 Step -11

8. Examples Using the For/Next Structure
The next example computes compound interest using the For/Next structure. Consider the following problem statement:
A person invests $ in a savings account that yields 5% interest. Assuming that all interest is left on deposit, calculate and print the amount of money in the account at the end of each year over a period of 10 years. To determine these amounts, use the following formula:
a = p (1 + r) n
where
p is the original amount invested (i.e., the principal)
r is the annual interest rate (e.g., .05 stands for 5%)
n is the number of years
a is the amount on deposit at the end of the nth year.

9. Select Case Multiple-Selection Structure
Tests expression separately for each value expression may assume
Select Case keywords begin structure
Followed by controlling expression
Compared sequentially with each case
Code in case executes if match is found
Program control proceeds to first statement after structure
Case keyword
Specifies each value to test for
Followed by code to execute if test is true
Case Else
Optional
Executes if no match is found
Must be last case in sequence
End Select is required and marks end of structure

10. Select Case Multiple-Selection Structure
The following program is using a Select Case to count the number of different letter grades on an exam.
Assume the exam is graded as follows: 90 and above
is an A, 80–89 is a B, 70–79 is a C, 60–69 is a D and 0–59 is an F.
This instructor gives a minimum grade of 10 for students who were present for the exam. Students not present for the exam receive a 0.

12. Optional Case Else executes if no match occurs with previous Cases
Either 0 or any value in the range
10 to 59, inclusive matches this Case.
Optional Case Else executes if no match occurs with previous Cases
Required End Select marks end of structure

13. Program Output

14. Select Case Multiple-Selection Structure
Case statements also can use relational operators to determine whether the controlling expression satisfies a condition.
For example:
Case Is < 0
uses keyword Is along with the relational operator, <, to test for values less than 0.
Best control to use with is the ComboBox.

15. Select Case Multiple-Selection Structure
Case a
Case b
Case z
. . .
Case Else action(s)
false
Case a action(s)
Case b action(s)
Case z action(s)
true
Fig Flowcharting the Select Case multiple-selection structure.

16. Do/Loop While Repetition Structure
Similar to While and Do/While
Loop-continuation condition tested after body executes
Loop body always executed at least once
Begins with keyword Do
Ends with keywords Loop While followed by condition

17. Do/Loop While Repetition Structure

18. Do/Loop While Repetition Structure
action(s)
condition
true
false
Fig Flowcharting the Do/Loop While repetition structure.

19. Do/Loop Until Repetition Structure
Similar to Do Until/Loop structure
Loop-continuation condition tested after body executes
Loop body always executed at least once

20. Print the numbers from 1–5.
1 ' Fig. 5.14: LoopUntil.vb
2 ' Using Do/Loop Until repetition structure 3
4 Module modLoopUntil 5
Sub Main() 7
Dim counter As Integer = 1 9
' print values 1 to 5 Do
Console.Write(counter & " ")
counter += 1
Loop Until counter > 5 15
End Sub ' Main 17
18 End Module ' modLoopUntil
Condition tested after body executes

21. Do/Loop Until Repetition Structure
action(s)
condition
false
true
Fig Flowcharting the Do/Loop Until repetition structure.

22. Using the Exit Keyword in a Repetition Structure
Exit Statements:
Alter the flow of control: Cause immediate exit from a repetition structure
Exit Do
Executed in Do While/Loop, Do/Loop While, Do Until/Loop or Do/Loop Until structures.
Exit For
Executed in For structures
Exit While
Executed in While structures

23. counter is 3 when loop starts, specified to execute until it is greater than 10

25. Program Output

26. Logical Operators
To handle multiple conditions more efficiently, Visual Basic provides logical operators that can be used to form complex conditions by combining simple ones.
The logical operators are AndAlso, And, OrElse, Or, Xor and Not.
We consider examples that use each of these operators.

27. Logical Operators
Logical operator with short-circuit evaluation: Execute only until truth or falsity is known:
AndAlso operator
Returns true if and only if both conditions are true
OrElse operator
Returns true if either or both of two conditions are true

28. Logical Operators
2. Logical Operators without short-circuit evaluation:
And and Or
Similar to AndAlso and OrElse respectively
Always execute both of their operands
Used when an operand has a side effect
Condition makes a modification to a variable
Should be avoided to reduce subtle errors
Xor
Returns true if and only if one operand is true and the other false

29. Logical Operators
Normally, there is no compelling reason to use the And and Or operators instead of AndAlso and OrElse. However, some programmers make use of them when the right operand of a condition produces a side effect (such as a modification of a variable’s value) or if the right operand includes a required method call, as in the following program segment:
Console.WriteLine("How old are you?")
If (gender = "F" And Console.ReadLine() >= 65) Then
Console.WriteLine("You are a female senior citizen.")
End If
Here, the And operator guarantees that the condition Console.ReadLine() >= 65 is evaluated, so ReadLine is called regardless of whether the overall expression is true or false.
It would be better to write this code as two separate statements—the first would store the result of Console.ReadLine() in a variable, then the second would use that variable with the AndAlso operator in the condition.

30. Logical Operators Logical Negation Not
Used to reverse the meaning of a condition
Unary operator
Requires one operand
Can usually be avoided by expressing a condition differently

31. Logical Operators
Fig Truth table for the AndAlso (logical AND) operator.

32. Logical Operators
Fig Truth table for the OrElse (logical OR) operator.

33. Logical Operators
Fig Truth table for the boolean logical exclusive OR (Xor) operator.
Fig Truth table for operator Not (logical NOT).

34. Logical Operators
Fig Precedence and associativity of the operators discussed so far.

35. Structured Programming Summary
Promotes simplicity
Produces programs that are easier to understand, test, debug and modify
Rules for Forming Structured Programs
If followed, an unstructured flowchart cannot be created
Only three forms of control needed
Sequence
Selection
If/Then structure sufficient to provide any form of selection
Repetition
While structure sufficient to provide any form of repetition

36. Structured Programming Summary
Sequence
Selection
If/Then structure (single selection)
Select Case structure (multiple selection) T F . . .
If/Then/Else structure (double selection) . . . F T
Fig Visual Basic’s single-entry/single-exit sequence and selection structures.

37. Structured Programming Summary
Repetition
While structure
For/Next structure T F T F
Do/Loop Until structure
Do/Loop While structure F T T F
Fig Visual Basic’s single-entry/single-exit repetition structures.

38. Structured Programming Summary
Repetition
Do While/Loop structure
Do Until/Loop structure T F F T
For Each/Next structure T F
Fig Visual Basic’s single-entry/single-exit repetition structures.