Chapter 4 Control Structures –Decisions –Loops

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. It’s an exciting time to be learning Java programming

Chapter Overview Control structures Boolean expressions If statements Nested if Switch Looping

Section 4.1 Control structures Sequence Selection –Alter normally sequential flow of a program Repetition –Alter normally sequential flow of a program

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

Relational operators < less than <= less than or equal == equal > greater than >= greater than or equal != not equal

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

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

Operands Operands can be: –Literals –Variables

Boolean Operators && and || or ! not

And True False 0

Or True 0 False

not Result 1 False 0 True

Boolean Operators (salary 5) (temperature > 90.0) && (humidity > 0.90)

Boolean variables in expressions winningRecord && (!onProbation)

Boolean Assignment variable = expression same = true; same = (x == y);

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

Writing Conditions (min <= x) && (x <= max) x min max

Comparing Characters ‘c’ < ‘d’ true ‘a’ > ‘A’ true ‘3’ > ‘4’ false

Comparing Strings Must use String methods string1.equals(string2)

Lexicographic Comparison string1.compareTo(string2) –Negative value if string 1 < string 2 –Value 0 if string 1 = string 2 –Positive value if string 1 > string 2 See table pg 195

Section 4.3 if statement if (gross > ) net = gross – tax; //if expression is true else net = gross; //if expression is false gross > net = gross - tax net = gross true false

One selection if (x != 0.0) product = product * x;

Syntax single selection if (condition) statement;

Syntax 2 alternatives if (condition) statement; else statement;

Look at web example else/Page1.htm else/Page1.htm

If and compound statements Use the braces to create a block of code in the if statement. if (x > y) { temp = x; x = y; y = temp; } //end if

If else and compound See example top page 200

Returning booleans from methods Look at example 4.8 page 201 –good way –need not do as shown on the bottom of the page

5.5 Decision steps in Algorithms Decision step: selects one of several actions. Review example 4.13 Pages

Case Study page 205 Payroll problem –Analysis –Design –Implementation Page 213 variable scope –Local –Data fields

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

Sequence versus Nested if (x > 0) y = y + 1; if (x < 0) y = y – 1; if (x ==0) y = y + 2; if (x> 0) y = y + 1; else if (x < 0) y = y – 1; else //btw x is 0 y = y + 2;

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 it’s closest preceding if that is not already matched with an else

Good or Bad if (x > 0) y = y * 4; if (x < 0) y = y * - 4; else y = y + 4;

Multiple alternative format if (score >= 90) displayResult(“A”); else if (score >= 80) displayResult(“B”); else if (score >= 70) displayResult(“C”);.

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

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

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

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."); }

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

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 aren’t big enough

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.

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 3 rd type of control structure –Sequence, selection, repetition

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

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.

While statement Syntax: while (condition) statement; Statement is the body of the loop –Can be compound { } Condition, continues looping while condition remains true

Example int countTenSum = 1; int tenSum = 0 while (countTenSum < 11) { tenSum = tenSum + countTenSum; countTenSum = countTenSum + 1; }

Example int numberEmp = readInt(“number of employees”); int countEmp = 0; while (countEmp < numberEmp) { //read pay data computer gross and net //add one to counter countEmp = countEmp + 1; }

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

Flow chart countEmp < numberEmp false Get Data Perform Calculations Increment counter true

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?

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.

While versus for counter = initial; While (counter <= final) { //loop processing counter++; } for (counter = initial; counter <=final; counter++){ //loop processing }

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 1 st ) condition tested Update statement executes after each repetition

For examples See code page 230 Example 4.18 (int i = 10; i >= 0; i--) System.out.println(i); OK to create variable local in the loop

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

New operators!!! count = count + 1; –count++; accum = accum + count; –accum += count; countDown = countDown – 1; –countDown--;

Code Review code on pages

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??

Code See exams pages

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

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

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

Design Table page 244 Table page 245

Implementation See code pages

Section 4.9 Debugging and testing Jbuilder includes a Debugger that allows user to –single-step through program –trace –set breakpoints –watch variables

Without debugger Use diagnostic output statements Put in println and showMessageDialog in appropriate places in program to monitor code and variables

4.10 Common Programming Errors Fully parenthesis your boolean expressions also boolean flag; flag && x == y; //works but should be flag && (x == y);

Other common errors Watch use of == versus = Watch use of {} and indentation if (x==y) x = 2; else y = 3; x = 1; //continue code here

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?