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Chapter 4 Introduction to Control Statements (Branching Statements)

Chapter 4 Introduction to Control Statements (Branching Statements). Section 1 - Additional Java Operators Section 2 - If Statements Section 3 - If-Else Statements Section 4 - Extended If Statements. Go. Go. Go. Go. Chapter 4 Section 1 Additional Java Operators.

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Chapter 4 Introduction to Control Statements (Branching Statements)

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  1. Chapter 4Introduction to Control Statements(Branching Statements) Section 1 - Additional Java Operators Section 2 - If Statements Section 3 - If-Else Statements Section 4 - Extended If Statements Go Go Go Go

  2. Chapter 4 Section 1Additional Java Operators • Extended Assignment Operators • Increment & Decrement Operators • Relational Operators • Logical Operators 2

  3. 4.1 Extended Assignment Operators These are special assignment operators for arithmetic operations and concatenation. They have the same low precedence as the standard assignment operator =. Here are the extended assignment operators: += Addition and Concatenation -= Subtraction *= Multiplication /= Division %= Modulus 3

  4. 4.1 Extended Assignment Operators Look closely at the lines of code that use the extended assignment operators below and see the equivalent lines of code. Look closely at the values stored in the variables x and str after the lines are executed. int x = 10; String str = “Java”; x += 5; // equivalent to x = x + 5;x now holds 15 x -= 3; // equivalent to x = x - 3;x now holds 12 x *= 2; // equivalent to x = x * 2; x now holds 24 x /= 4; // equivalent to x = x / 4;x now holds 6 x %= 5; // equivalent to x = x % 5;x now holds 1 str += “ Rules!”; // equivalent to str = str + “ Rules”;str holds “Java Rules” 4

  5. 4.1 The Increment & Decrement Operators The Increment operator is++ • It increases the value of an int or double variable by 1. The Decrement operator is-- • It decreases the value of an int or double variable by 1. The following code shows you how to use ++ and -- int x = 10; int y = 20; double w= 7.8; double z = 14.3; x++; // equivalent to x = x + 1; or x += 1; x now holds 11 y--; // equivalent to y = y - 1; or y -= 1;y now holds 19 w++; // equivalent to w = w + 1; or w += 1; w now holds 8.8 z--; // equivalent to z = z - 1; or z -= 1;z now holds 13.3 5

  6. Relational operators are used to form Boolean Expressions for if and loop statements. Here are the six relational operators: 4.1 Relational Operators You’ll see how to use these soon. Do not place a space between the operators that use two symbols … >= <= == or != 6

  7. Logical operators are used to modify or form compound Boolean Expressions for if and loop statements. There are three logical operators: 4.1 Logical Operators Do not place a space between the operators that use two symbols … && and | | You’ll see how to use these soon. 7

  8. 4.1 Updated Precedence Chart for Operators 8

  9. Chapter 4 Section 2If Statements 9

  10. 4.2 Introduction to If Statements In Java you can execute a segment of code if a Boolean condition is true. So if the Boolean condition is false, then nothing is executed. You do thisby using an if statement. Here is the general form of an if statement: if (some Boolean expression is true) { // execute this code only between the curly braces } The code inside the if statement is NOT executed if the boolean condition is false. Note that curly braces { } encompass all of the statements inside the body of the if statement. There can be one or many. 10

  11. int a = 3, b = 7, c = 10, d = -20; if (a < b) is true if (a <= b) is true if (a == b) is false if (a != b) is true if (a - b > c + d) is true if ( a < b < c) invalid Java statement but valid in algebra if (a == b == c) invalid Java statement You will be shown how to accomplish the last two statements later. 4.2 Evaluating Boolean Expressions 11

  12. 4.2 If Statement Example Consider the following code: System.out.print (“Enter an integer: ”); int num = reader.nextInt(); if (num % 5==0) { System.out.print (num + “is evenly divisible by 5”); } Note the use of the equals operator == to compare the two values of num % 5 and 0 to see if they are equivalent. If they are equivalent the entire expression evaluates to true, otherwise it evalutes to false and nothing is printed. 12

  13. 4.2 if Statement Code Example • Here is code that will calculate a salesperson’s pay. It will double a salesperson’s commission from 10% to 20% if sales are over $2,000.00: • double commission = 0.10; // regular commission is 10% • System.out.print(“Enter the person’s sales for the month: ”); • double sales = reader.nextDouble(); • if (sales > 2000.00) • commission *= 2; • double pay = sales * commission; • System.out.println(“Your pay is: ” + pay); Note: if there is only one line of code inside the if statement, then you don’t have to put curly braces around the body of the if, but we indent to show for readability to sho that it is inside the if statement. If we didn’t indent it would still be inside the if statement. 13

  14. 4.2 if Statement Code Example • If a worker is paid $8.50 per hour for all hours up to and including 40 hours per week, but is paid a time and a half wage of $12.75 per hour for any hours over 40 hours per week, then the person’s pay could be calculated using the following: • System.out.print(“Enter the number of hours worked this week: ”); • double hoursWorked = reader.nextDouble(); • double pay = hoursWorked * 8.50; • if (hoursWorked > 40) • { • double overTimeHours = hoursWorked - 40; • pay += overTimeHours * 4.25; • } • System.out.println(“The employee’s pay is: ” + pay); 14

  15. 4.2 The Form of an if Statement • Either use of curly braces below works, but the second is preferred for readabiltiy and troubleshooting! • if (condition) { // saves lines by putting { on same line of condition • statement; • statement; • } No semicolon goes here! • if (condition) No semicolon goes here! • { // adds readability by putting curly brace { here • statement; // because braces are indented at • statement; // the same level (preferred) • } // Easier to find curly brace problems!!! • Note: the statements are executed only if the condition is true,otherwise this control statement is skipped and nothing is executed. 15

  16. No curly braces needed when only one statement is in the branch. • if (condition) • statement; • If curly braces aren’t used, then only the first statement following the if (condition) is in the branch,even if the second statement is accidently indented.Remember: compilers ignore spacing and indentation. Programmers use them for readability. • if (condition) • statement1; • statement2; • statement2 is indented but it is not in the if statement since there are no curly braces. Another example is on the next slide. 4.2 The Form of an if Statement 16

  17. 4.2 if Statement Indentation Error • if (grade > 100) • System.out.println(“You cannot have a grade over 100.”); • System.out println(“Show your parents your grade before continuing.”); • Here the first println statement is executed only when grade is greater than 100. The second println statement is incorrectly indented and will be executed even if the first one is not. • Sometimes programmers get in a hurry and don’t write branching statements correctly. • How might you rewrite the one above? 17

  18. 4.2 if Statement Indentation Error • How might you rewrite the one on the previous slide so it is readable and shows what is intended? • if (grade > 100) • { • System.out.println(“You cannot have a grade over 100.”); • } • System.out println(“Show your parents your grade before continuing.”); • or • if (grade > 100) • System.out.println(“You cannot have a grade over 100.”); • System.out println(“Show your parents your grade before continuing.”); • Note the blank line after the first System.out.println in the second version. first version second version 18

  19. Chapter 4 Section 3If-Else Statements 19

  20. 4.3 Introduction to If-Else Statements In Java you can execute one segment of code if a boolean condition is trueor you can execute another segment of code if the condition is false by using an if-else statement. An if-else statement looks like this: if (some Boolean expression is true) { // execute this code only if the Boolean expression is true } else { // execute this code only if the Boolean expression is false } Note the { } that encompass all of the statements inside the if branch and another set of { } encompass all of the statements inside the else branch. Curly braces always occur in pairs! 20

  21. 4.3 if-else Statement Code Example • System.out.print(“Enter a celsius temperature: ”); • double celsius = reader.nextDouble(); • double fahr; • if ( celsius > -273.16) • { • fahr = 9.0 * 5.0 * celsius + 32.0; • } • else • { • System.out.print(“The fahrenheit temperature ”); • System.out.println(“cannot be calculated.”); • } 21

  22. Here is code that will calculate a salesperson’s pay. It will double a salesperson’s commission from 10% to 20% if sales are over $2,000.00, but it is written differently from what we saw before: • double commission; • System.out.print(“Enter the person’s sales for the month: ”); • double sales = reader.nextDouble(); • if (sales > 2000.00) • { • commission = 0.20; // bonus commission is 20% • } • else • { • commission = 0.10; // regular commission is 10% • } • double pay = sales * commission; • System.out.println(“Your pay is: ” + pay); 4.3 if-else Statement Code Example 22

  23. 4.3 if-else Statement Code Example • If a worker is paid $8.50 per hour for all hours up to and including 40 hours per week, but is paid a time and a half wage of $12.75 per hour for any hours over 40 hours per week, then the person’s pay could be calculated using the following: • System.out.print(“Enter the number of hours worked this week: ”); • double hoursWorked = reader.nextDouble(); • double pay; • if (hoursWorked <= 40) • pay = hoursWorked * 8.50; • else • { • pay = 40 * 8.5 + (hoursWorked - 40) * 12.75; • } • System.out.println(“The employee’s pay is: ” + pay); 23

  24. 4.3 Checking Input with if-else Statements • if-else statements are commonly used to check user inputs before processing them. • System.out.print("Enter the radius: "); • double radius = reader.nextDouble(); • if (radius < 0) • System.out.println("Error: Radius must be >= 0"); • else • { • double area = Math.PI * Math.pow(radius, 2); • System.out.println("The area is " + area); • } 24

  25. 4.3 Comparison of if-else Statement Formats • if (condition) • { • statement; • statement; • } • else • { • statement; • statement; • } • greater readability • if (condition) { • statement; • statement; • } else { • statement; • statement; • } • less lines Here you see the two ways of formatting an if-else statement. Either works. One places the curly braces on the same lines as the if condition and the else keyword. The other one doesn’t so that the curly braces can more easily seen as matched pairs. Either use of curly braces works: Note: the statements in the if branch are executed only if the condition is true, otherwise the statements in the else branch are executed. One of the two branches will always be executed when you have an if-else statement. 25

  26. 4.3 Forms of if and if-else Statements • if (condition) • { • statement; • ….. • statement; • } • else • statement; • if (condition) • statement; if and if-else statements can have one or more statements in any branch as seen in the variety below: • if (condition) • { • statement; • statement; • } • if (condition) • statement; • else { • statement; • ……….. • statement; • } • if (condition) • statement; • else • statement; 26

  27. 4.3 Important Things about if Statements • It is better to over-use braces than to under-use them. This can help to eliminate logic errors by adding readability. • The condition of an if statement must be a Boolean expression that evaluates to either true or false. • A flowchart can be used to illustrate the behavior of if and if-else statements and develop an over all flow-of-control for a programming segment. 27

  28. 4.3 if Statement Flow Chart true false boolean condition Statements 28

  29. 4.3 if-else Statement Flow Chart true false boolean condition Statements Statements 29

  30. 4.3 Simple Boolean Conditions You have learned that Java allows you to use an if statement of the form: if (condition) { …. } The condition can be a simple boolean expression or a compound boolean expression. Here is a simple boolean expression: if (grade > 90) System.out.println(“Your grade is A”); 30

  31. 4.3 Compound Boolean Conditions • Java allows you to use an if statement that has a compoundboolean expression in the condition. Compound boolean expressions use the logical operators: • && (referred to as “AND”) • | | (referred to as “OR”) • ! (referred to as “NOT”) • Here are examples using each one: • if (grade > 90 && grade <= 100) • System.out.println(“Your grade is A”); • if (grade < 0 | | grade > 100) • System.out.println(“Grade not possible.”); • if ( !(grade > 59) ) // this one is a simple not a compound • System.out.println(“Your grade is an F”); 31

  32. Chapter 4 Section 4Nested Ifand Extended IfStatements 32

  33. 4.4 Extended if Statement Form Extended if statements have two or more branches. They have the following general form: if(condition) { … code } else if(condition) { … code } else { … code } Only one of the three branches of code will be executed. The else if branch has a condition. The else branch is optional and doesn’t have a condition, but if none of the conditions in the other branches are true then the else branch will be executed. 33

  34. 4.4 Extended if Statement without Else An extended if statement need not have an else branch. If it doesn’t, then code will only be executed if one of the two conditions is true, otherwise nothing is executed. if(condition) { … code } else if(condition) { … code } 34

  35. 4.4 Multiple Branches of an Extended if if(condition) { … code } else if(condition) { … code } else if(condition) { … code } ….. possible other else if branches else { … code } An extended if statement may have many branches. No matter how many branches it has only one branch of code will be executed. If it does not have an ending else branch,thenagainit is possible no code will be executed. However, if it does have an ending else branch then if none of the conditions in the other branches are true then the else branch will be executed. 35

  36. 4.4 Modifying Three if Statements Consider the pseudocode for the following simple if statements. Even though they have compound boolean expressions, the intention of the code is clear when they follow one another in this order. Here all three if conditions are tested but it is possible that some of them may not be executed: if (the time is after 7pm && you have a book) read the book if (the time is after 7pm && ! you have a book) watch TV if ( ! the time is after 7pm) go for a walk We will convert the above code to an extended if statement. When if statements are written so that the intent is to have only one thing executed, like read a book, watch TV, or go for a walk, then there is a better way to write the code. 36

  37. 4.4 Extended if Statements Here the if statements have been rewritten as an extended if statement and the code overall is more readable and it is more efficient because once Java finds a true condition, then that branch is executed and all other branches are skipped! if (the time is after 7pm && you have a book) read the book else if (the time is after 7pm && ! you have a book) watch TV else if ( ! the time is after 7pm) go for a walk Remember ... you don’t have to have a final else to end an extended if statement, but you can if you want something to be executed if none of the boolean conditions of the extended if are true. 37

  38. 4.4 Extended if Statements if (the time is after 7pm && you have a book) read the book else if (the time is after 7pm && ! you have a book) watch TV else if ( ! the time is after 7pm) go for a walk In an extended if statement like this one, it is easier to read and trace the logic and make sure that every possible situation is covered. Therefore, we are able to avoid logic errors. It is more difficult in the next nestedif statement. 38

  39. 4.4 Nested if Statement Logic Errors The nested if statement below is equivalent to the previous extended if, but it is not as easy to read and is more difficult to trace the logic to make sure every possibility is covered. It takes more time to think about it and verify its correctness. if (the time is after 7pm) { if (you have a book) read the book else watch TV } else go for a walk The inner if-else statement is nested inside the first if branch, therefore we call the first if a nested if. 39

  40. 4.4 Flow Chart for a Nested If Statement This flowchart of a nested if statement helps us verify the code in a visual manner. Most of the time when we can visualize code we can verify its correctness. The flowchart for an extended-if statement would look exactly the same. 40

  41. 4.4 Evolution of an Extended If Statement The next three slides will show you how extended if statements are actually nested if statements. 41

  42. 4.4 Nestedif Statement Example with the { } braces in the elses. if (testAverage >= 90) System.out.print(“Grade is A. ”); else { if (testAverage >= 80) System.out.print(“Grade is B. ”); else { if (testAverage >= 70) System.out.print(“Grade is C. ”); else { if (testAverage >= 60) System.out.print(“Grade is D. ”); else { System.out.print(“Grade is F. ”); } } } } Note: there is only one java statement inside each if branch so there are no curly braces for those branches. There is also just one statement in each else branch even though they have { }, but we will remove the curly braces. (See next slide) 42

  43. 4.4 Nestedif Statement Example without the { } braces. if (testAverage >= 90) System.out.print(“Grade is A. ”); else if (testAverage >= 80) System.out.print(“Grade is B. ”); else if (testAverage >= 70) System.out.print(“Grade is C. ”); else if (testAverage >= 60) System.out.print(“Grade is D. ”); else System.out.print(“Grade is F. ”); Note: java doesn’t care about spacing or indentation so on the next slidewe can move the if statements up to the same line as the word else once the curly braces are removed. (See next slide) 43

  44. 4.4 Nestedif Statement Example without the { } braces and reformatted to be an extended if statement. if (testAverage >= 90) System.out.print(“Grade is A. ”); else if (testAverage >= 80) System.out.print(“Grade is B. ”); else if (testAverage >= 70) System.out.print(“Grade is C. ”); else if (testAverage >= 60) System.out.print(“Grade is D. ”); else System.out.print(“Grade is F. ”); Now we have a nice readable statement that makes sense because it is easy to read. 44

  45. 4.4 Extended if Statement Order if (testAverage >= 60) System.out.print(“Grade is D. ”); else if (testAverage >= 70) System.out.print(“Grade is C. ”); else if (testAverage >= 80) System.out.print(“Grade is B. ”); else if (testAverage >= 90) System.out.print(“Grade is A. ”); else System.out.print(“Grade is F. ”); If we coded the previous extended if with the conditions in reverse order and entered 93 for the testAverage, which branch would be executed? (see next slide) 45

  46. 4.4 Extended if Statement Order if (testAverage >= 60) System.out.print(“Grade is D. ”); else if (testAverage >= 70) System.out.print(“Grade is C. ”); else if (testAverage >= 80) System.out.print(“Grade is B. ”); else if (testAverage >= 90) System.out.print(“Grade is A. ”); else System.out.print(“Grade is F. ”); If we coded the previous extended if with the conditions in reverse order and entered 93 for the testAverage, what would happen? The first branch would be executed and the program would display a “D”. 46

  47. 4.4 Extended if Statementswith compound boolean expressions However if we change the simple boolean expression listed first to a compound boolean expression then we ensure the code’s accuracy. if (testAverage >= 60) System.out.print(“Grade is D. ”); becomes if (testAverage >= 60 && testAverage < 70) System.out.print(“Grade is D. ”); (see next slide for the expanded example) 47

  48. 4.4 Extended if Statementswith compound boolean expressions Here is a full extended if with compound boolean expressions, where the order of the branches doesn’t matter because of the compound boolean expressions. if (testAverage >= 90 && testAverage <= 100) System.out.print(“Grade is A. ”); else if (testAverage >= 80 && testAverage < 90) System.out.print(“Grade is B. ”); else if (testAverage >= 70 && testAverage < 80) System.out.print(“Grade is C. ”); else if (testAverage >= 60 && testAverage < 70) System.out.print(“Grade is D. ”); else System.out.print(“Grade is F. ”); 48

  49. Chapter 4 Review • The Math class provides several useful methods, such as pow, sqrt,random and abs. • Java’s extended assignment operators are +=, -=, *=, /=, %= • Java’s has increment++ and decrement-- operators that modify a double or int variable by 1. • Java has the logical operators&&, ||, and ! that can be used in forming boolean expressions. • Java has the relational operators<, <=, >, >=, ==, and != They compare two values and return a Boolean value. They are used to form Boolean expressions in the conditions of control statements. 49

  50. Chapter 4 Review • Use an if-else statement rather than two if statements when the alternative courses of action are mutually exclusive. This makes your code more efficient when the segment needs to executed many times. • The presence or absence of the {} symbols can seriously affect the logic of a control statement. • When testing programs that use if or if-else statements, use test data that forces the program to exercise all logical branches. 50

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