Control Structures

1
Chapter 4

• Control Structures
• Decisions
• Loops

2
Chapter 4 selection structures

• This chapter begins a new path in our programming
  ability
• Basically we can cause our programs to follow
  different paths, react differently to different
  situations.
• Its an exciting time to be learning Java
  programming

3
Chapter Overview

• Control structures
• Boolean expressions
• If statements
• Nested if
• Switch
• Looping

4
Section 4.1 Control structures

• Sequence
• Selection
• Alter normally sequential flow of a program
• Repetition
• Alter normally sequential flow of a program

5
Boolean Expressions

• Boolean has two possible values
• True
• False
• Simplest Boolean expression is variable
• boolean leapYear true
• Can only be assigned true or false

6
Relational operators

• lt less than
• lt less than or equal
• equal
• gt greater than
• gt greater than or equal
• ! not equal

7
Reading Boolean data

• Use JOptionPane.showConfirmDialog(null, Is this
  fun?)
• This is the result of the statement
• Returns 0 for yes, 1 for no, 2 for cancel

8
Using the result

• To convert this value to boolean simply use the
  result like this
• boolean myBool (returnNum 0)
• See methods top of page 186 for method to handle
  this

9
Operands

• Operands can be
• Literals
• Variables

10
Boolean Operators

• and
• or
• ! not

11
And
And 1 0
1 True False
0 False False
12
Or
Or 1 0
1 True True
0 True False
13
not
not Result
1 False
0 True
14
Boolean Operators

• (salary lt minimumSalary) (dependents gt 5)
• (temperature gt 90.0)
• (humidity gt 0.90)

15
Boolean variables in expressions

• winningRecord (!onProbation)

16
Boolean Assignment

• variable expression
• same true
• same (x y)

17
Short-circuit

• Short circuit evaluation
• Stops evaluating as soon as knows the outcome.
• Can cause problems depending on what is in
  statement

18
Writing Conditions

• (min lt x) (x lt max)

x
min
max
19
Comparing Characters

• c lt d true
• a gt A true
• 3 gt 4 false

20
Comparing Strings

• Must use String methods
• string1.equals(string2)

21
Lexicographic Comparison

• string1.compareTo(string2)
• Negative value if string 1 lt string 2
• Value 0 if string 1 string 2
• Positive value if string 1 gt string 2
• See table pg 195

22
Section 4.3 if statement

• if (gross gt 100.00)
• net gross tax //if expression is true
• else
• net gross //if expression is false

true
false
23
One selection

• if (x ! 0.0)
• product product x

24
Syntax single selection

• if (condition)
• statement

25
Syntax 2 alternatives

• if (condition)
• statement
• else
• statement

26
Look at web example

• http//faculty.juniata.edu/thomas/cs110/ifelse/Pag
  e1.htm

27
If and compound statements

• Use the braces to create a block of code in the
  if statement.
• if (x gt y)
• temp x
• x y
• y temp
• //end if

28
If else and compound

• See example top page 200

29
Returning booleans from methods

• Look at example 4.8 page 201
• good way
• need not do as shown on the bottom of the page

30
5.5 Decision steps in Algorithms

• Decision step selects one of several actions.
• Review example 4.13 Pages 204-205

31
Case Study page 205

• Payroll problem
• Analysis
• Design
• Implementation
• Page 213 variable scope
• Local
• Data fields

32
4.5 nested ifs

• Besides using the Boolean operators and !
  We can also create nested if statements.
• Nested if statements are often more efficient
  than a sequence of if statements

33
Sequence versus Nested

• if (x gt 0)
• y y 1
• if (x lt 0)
• y y 1
• if (x 0)
• y y 2

• if (xgt 0)
• y y 1
• else
• if (x lt 0)
• y y 1
• else //btw x is 0
• y y 2

34
Matching else with If

• You must use indentiation to make it clear how
  your if/else match.
• BUT!! The compiler ignores the white space
• Java matches each else with its closest
  preceding if that is not already matched with an
  else

35
Good or Bad

• if (x gt 0)
• y y 4
• if (x lt 0)
• y y - 4
• else
• y y 4

36
Multiple alternative format

• if (score gt 90)
• displayResult(A)
• else if (score gt 80)
• displayResult(B)
• else if (score gt 70)
• displayResult(C)
• .
• .

37
Example

• Order matters big time see page 220
• Review tax example page 221

38
Tips

• Code nested ifs one statement at a time.
• Code outer if then the internal ifs
• TEST TEST TEST

39
Switch statement

• Switch allows you to select from several
  alternatives.
• Works especially well when based on the value of
  one variable

40
This is what it looks like

• switch (editOp)
• case 0
• search()
• break
• case 1
• insert()
• break
• case 2
• delete()
• break
• case 3
• replace()
• break
• case 4
• displayResult("All done")
• break
• default
• displayResult("Invalid operation.")

41
Rules

• The switch selector must be an ordinal data type.
• Primitive
• All values maybe listed
• int, boolean, char
• Not double
• Break causes control to pass to statement after
  the switch.
• Break statements are not always necessary

42
Another example

• switch (month)
• case 12 julian julian day
• case 11 if (month 11)
• julian julian day
• else
• julian julian 30
• case 10 if (month 10)
• julian julian day
• else
• julian julian 31
• case 9 if (month 9)
• julian julian day
• else
• julian julian 30
• case 1 if (month 1)
• julian julian day
• else
• julian julian 31
• //end switch

Case 8 2 go in here, slides just arent big
enough
43
Returning a value

• Use of a return in a switch case statement also
  stops execution of the statement.
• I am a little fussy with entry level programmers
  having multiple exit points in a method, although
  will allow it in a switch statement.

44
Repetition Structures

• In the programs we have written each line only
  executes at most 1 time.
• There are times that we want statements to
  execute multiple times.
• When would you want this?
• Repetition is 3rd type of control structure
• Sequence, selection, repetition

45
Overview

• Loops
• Counting loops
• Sentinel controlled loops
• Flag controlled loops
• Menu driven loops
• Loop types
• while
• for
• do-while

46
Counting loops

• Loop, repetition of steps in a program.
• Counting loop repeats a predetermined number of
  times.
• Name some real life examples of count controlled
  loops
• Counter-controlled loops are controlled by a
  variable that keeps track of the number of
  repetitions performed.

47
While statement

• Syntax
• while (condition)
• statement
• Statement is the body of the loop
• Can be compound
• Condition, continues looping while condition
  remains true

48
Example

• int countTenSum 1
• int tenSum 0
• while (countTenSum lt 11)
• tenSum
• tenSum countTenSum
• countTenSum
• countTenSum 1

49
Example

• int numberEmp readInt(number of employees)
• int countEmp 0
• while (countEmp lt numberEmp)
• //read pay data computer gross and net
• //add one to counter
• countEmp countEmp 1

50
Syntax and formatting

• while (repetitionCondition)
• loopBody
• Must use indentation for clarity
• As in other cases white space is totally ignored
  by compiler

51
Flow chart
countEmp lt numberEmp
Get Data Perform Calculations Increment counter
true
false
52
Loop-control variable

• You must perform the following steps to your
  loop-control variable
• Initialize
• Test
• Update
• What is a loop that never ends called?
• Can you sing the song that never ends?

53
For statement

• This is used just for count controlled loops.
• It is a special loop format that compresses count
  controlled into a special format.
• Count control loops are used quite often.

54
While versus for

• counter initial
• While (counter lt final)
• //loop processing
• counter

• for (counter initial
• counter ltfinal
• counter)
• //loop processing

55
Syntax for statement

• For (initialization statement
• repetition condition
• update statement)
• loop body
• Initialization statement occurs when for loop
  begins execution
• Prior to each loop repetition (include 1st)
  condition tested
• Update statement executes after each repetition

56
For examples

• See code page 230 Example 4.18
• (int i 10 i gt 0 i--)
• System.out.println(i)
• OK to create variable local in the loop

57
Accumulating a Sum

• Loops can be used to accumulate a sum
• Must have an accumulator
• Loop causes accumulator to increase with each
  time loop is processed
• This is a common use of a loop.
• See page 231

58
New operators!!!

• count count 1
• count
• accum accum count
• accum count
• countDown countDown 1
• countDown--

59
Code

• Review code on pages 234-236

60
State-controlled loops

• Rather than loop for a specific number of
  repetitions (count control)
• We can loop while a condition is true
• Repetition stops when a particular state is
  reached. This state causes the loop-repetition
  condition to become false.
• Can we always be sure these will end??

61
Code

• See exams pages 240 - 241

62
4.8 Case Study

• Problem
• Write program to generate and solve arithmetic
  problems.
• Has feature of varying difficulty
• Checks answers and corrects when wrong

63
Analysis

• Need class that generates math problems.
• Problem represented by an operator and its left
  and right operands.
• Operands generated randomly
• Gets larger as difficulty increases
• Called MathProblem

64
Analysis

• Another class will use MathProblem to generate
  individual problems
• Will also allow for looping and user interaction
• Will additionally store statistics
• Called MathDrill

65
Design

• Table page 244
• Table page 245

66
Implementation

• See code pages 248 - 251

67
Section 4.9 Debugging and testing

• Jbuilder includes a Debugger that allows user to
• single-step through program
• trace
• set breakpoints
• watch variables

68
Without debugger

• Use diagnostic output statements
• Put in println and showMessageDialog in
  appropriate places in program to monitor code and
  variables

69
4.10 Common Programming Errors

• Fully parenthesis your boolean expressions also
• boolean flag
• flag x y //works but should be
• flag (x y)

70
Other common errors

• Watch use of versus
• Watch use of and indentation
• if (xy)
• x 2
• else
• y 3
• x 1
• //continue code here

71
Other loop errors

• Off by one
• One to few
• One to many
• Check you loop boundaries
• initial and final values of loop control
  variables are they right?