40 Absolute difference 42 Trying it 43 Trying it 44 Concepts covered in this chapter 0-2

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1 List of Slides 1 Title 2 Chapter 4: Conditional execution 3 Chapter aims 4 Section 2: Example:Oldest spouse 1 5 Aim 6 Execution: conditional execution 7 Expression: boolean 8 Expression: boolean: relational operators 11 Oldest spouse 1 12 Statement: if else statement 14 Oldest spouse 1 15 Trying it 16 Coursework: MaxTwoDoubles 17 Section 3: Example:Oldest spouse 2 18 Aim 19 Statement: if else statement: nested 0-0

2 21 Oldest spouse 2 22 Oldest spouse 2 23 Trying it 24 Trying it 25 Trying it 26 Trying it 27 Coursework: DegreeCategory 28 Section 4: Example:Film certificate age checking 29 Aim 30 Statement: if statement 31 Film certificate age checking 32 Trying it 33 Trying it 34 Trying it 35 Coursework: PassFailDistinction 36 Section 5: Example:Absolute difference 37 Aim 38 Statement: compound statement 0-1

3 40 Absolute difference 42 Trying it 43 Trying it 44 Concepts covered in this chapter 0-2

4 Title Java Just in Time John Latham October 25, 2017 October 25, 2017 Java Just in Time - John Latham Page 1(0/0)

5 Chapter 4 Conditional execution October 25, 2017 Java Just in Time - John Latham Page 2(0/0)

6 Chapter aims Not every part of a program need be executed every time. Some parts only under certain conditions. We meet if statements and if else statements. October 25, 2017 Java Just in Time - John Latham Page 3(0/0)

7 Section 2 Example: Oldest spouse 1 October 25, 2017 Java Just in Time - John Latham Page 4(0/0)

8 Aim AIM: To introduce the idea of conditional execution, implemented by the if else statement, and controlled by boolean expressions based on the use of relational operators. October 25, 2017 Java Just in Time - John Latham Page 5(0/0)

9 Execution: conditional execution Not sufficient to always obey all instructions every run some problems need conditional execution execute some bits conditionally: depending on values of variables. October 25, 2017 Java Just in Time - John Latham Page 6(0/0)

10 Expression: boolean An expression which evaluates to true or false a condition also called a boolean expression. Used for controlling conditional execution. October 25, 2017 Java Just in Time - John Latham Page 7(0/0)

11 Expression: boolean: relational operators Six relational operators for comparing values to make up conditions show us relations between items in pairs of values binary infix operators... yield true or false. October 25, 2017 Java Just in Time - John Latham Page 8(0/0)

12 Expression: boolean: relational operators Operator Title Description == Equal This is the equal operator, which provides the notion of equality. a == b yields true if and only if the value of a is the same as the value of b.!= Not equal This is the not equal operator, providing the the notion of not equality. a!= b yields true if and only if the value of a is not the same as the value of b. < Less than This is the less than operator. a < b yields true if and only if the value of a is less than the value of b. October 25, 2017 Java Just in Time - John Latham Page 9(0/0)

13 Expression: boolean: relational operators Operator Title Description > Greater than This is the greater than operator. a > b yields true if and only if the value of a is greater than the value of b. <= Less than or equal This is the less than or equal operator. a <= b yields true if and only if the value of a is less than value of b, or is equal to it. >= Greater than or equal This is the greater than or equal operator. a >= b yields true if and only if the value of a is greater than value of b, or is equal to it. October 25, 2017 Java Just in Time - John Latham Page 10(0/0)

14 Oldest spouse 1 Coffee time: Which of the following conditions are true? a) 5 < 4 b) 5 < 5 c) 5 < 6 d) 5 > 4 e) 5 > 5 f) 5 > 6 g) 5 <= 4 h) 5 <= 5 i) 5 <= 6 j) 5 >= 4 k) 5 >= 5 l) 5 >= 6 m) 5 == 4 n) 5 == 5 o) 5 == 6 p) 5!= 4 q) 5!= 5 r) 5!= 6 s) 5 > 4 == 9 < 15 t) 5 < 4!= 9 > 15 Do the last two examples above suggest that == and!= have a different operator precedence from the other relational operators? If so, are they higher or lower? October 25, 2017 Java Just in Time - John Latham Page 11(0/0)

15 Statement: if else statement The if else statement allows conditional execution. Three parts condition / boolean expression true part statement for when condition is true false part statement for when condition is false Syntax: reserved word if condition in brackets true part reserved word else false part October 25, 2017 Java Just in Time - John Latham Page 12(0/0)

16 Statement: if else statement E.g. assuming variable noofpeopletoinvitetothestreetparty if (noofpeopletoinvitetothestreetparty > 100) else System.out.println("We will need a big sound system!"); System.out.println("We should be okay with a normal HiFi."); Get one message or the other depending on noofpeopletoinvitetothestreetparty never both. Notice brackets, semi-colons and indentation. October 25, 2017 Java Just in Time - John Latham Page 13(0/0)

17 Oldest spouse 1 001: public class OldestSpouse 002: { 003: public static void main(string[] args) 004: { 005: int husbandsage = Integer.parseInt(args[0]); 006: int wifesage = Integer.parseInt(args[1]); 007: 008: if (husbandsage > wifesage) 009: System.out.println("The husband is older than the wife"); 010: else 011: System.out.println("The husband is not older than the wife"); 012: } 013: } October 25, 2017 Java Just in Time - John Latham Page 14(0/0)

18 Trying it Console Input / Output $ java OldestSpouse The husband is older than the wife $ java OldestSpouse The husband is older than the wife $ java OldestSpouse The husband is not older than the wife $ java OldestSpouse The husband is not older than the wife $ java OldestSpouse The husband is not older than the wife $ _ Run October 25, 2017 Java Just in Time - John Latham Page 15(0/0)

19 Coursework: MaxTwoDoubles (Summary only) Write a program to find the maximum of two given numbers, using an if else statement. October 25, 2017 Java Just in Time - John Latham Page 16(0/0)

20 Section 3 Example: Oldest spouse 2 October 25, 2017 Java Just in Time - John Latham Page 17(0/0)

21 Aim AIM: To introduce the idea of nesting if else statements. October 25, 2017 Java Just in Time - John Latham Page 18(0/0)

22 Statement: if else statement: nested Can have if else statement nested inside another. i.e. true part and/or false part is another if else statement. E.g. Depending on noofpeopletoinvitetothestreetparty get one of three messages. if (noofpeopletoinvitetothestreetparty > 300) else System.out.println("We will need a Mega master 500 Watt amplifier!"); if (noofpeopletoinvitetothestreetparty > 100) else System.out.println("We will need a Maxi Master 150 Watt amplifier!"); System.out.println("We should be okay with a normal HiFi."); Notice indentation following usual nesting rule, but... October 25, 2017 Java Just in Time - John Latham Page 19(0/0)

23 Statement: if else statement: nested Make exception to usual rule for if else statements nested in false parts: if (noofpeopletoinvitetothestreetparty > 300) System.out.println("We will need a Mega master 500 Watt amplifier!"); else if (noofpeopletoinvitetothestreetparty > 100) else System.out.println("We will need a Maxi Master 150 Watt amplifier!"); System.out.println("We should be okay with a normal HiFi."); Reflects abstraction: is one selection offering one choice from three. Don t have to increase indentation if using many nested if else statements as long as nested in false parts. October 25, 2017 Java Just in Time - John Latham Page 20(0/0)

24 Oldest spouse 2 Coffee time: If we wanted some program code to execute one out of N choices, how many if else statements would we need? October 25, 2017 Java Just in Time - John Latham Page 21(0/0)

25 Oldest spouse 2 001: public class OldestSpouse 002: { 003: public static void main(string[] args) 004: { 005: int husbandsage = Integer.parseInt(args[0]); 006: int wifesage = Integer.parseInt(args[1]); 007: 008: if (husbandsage > wifesage) 009: System.out.println("The husband is older than the wife"); 010: else if (husbandsage == wifesage) 011: System.out.println("The husband is the same age as the wife"); 012: else 013: System.out.println("The husband is younger than the wife"); 014: } 015: } October 25, 2017 Java Just in Time - John Latham Page 22(0/0)

26 Trying it Console Input / Output $ java OldestSpouse The husband is older than the wife $ java OldestSpouse The husband is older than the wife $ java OldestSpouse The husband is the same age as the wife $ java OldestSpouse The husband is younger than the wife $ java OldestSpouse The husband is younger than the wife $ _ Run October 25, 2017 Java Just in Time - John Latham Page 23(0/0)

27 . if. if. System.out.println("The. System.out.println("The. System.out.println("The Trying it Coffee time: What do you think of the following code as an alternative to the if else statements we wrote in our program? Would it work? Is it as easy to read? (husbandsage <= wifesage) (husbandsage!= wifesage) husband is younger than the wife");. else husband is the same age as the wife");. else husband is older than the wife"); October 25, 2017 Java Just in Time - John Latham Page 24(0/0)

28 . if. System.out.println("The. else. System.out.println("The. System.out.println("The Trying it Coffee time: What about the following code instead? What makes it better than the code above? (husbandsage < wifesage) husband is younger than the wife"); if (husbandsage == wifesage) husband is the same age as the wife");. else husband is older than the wife"); October 25, 2017 Java Just in Time - John Latham Page 25(0/0)

29 . if. System.out.println("The. else. System.out.println("The. System.out.println("The Trying it Coffee time: And finally, what about the following code? What makes it less good than the one above? (husbandsage == wifesage) husband is the same age as the wife"); if (husbandsage < wifesage) husband is younger than the wife");. else husband is older than the wife"); October 25, 2017 Java Just in Time - John Latham Page 26(0/0)

30 Coursework: DegreeCategory (Summary only) Write a program to report the degree category of a given mark. October 25, 2017 Java Just in Time - John Latham Page 27(0/0)

31 Section 4 Example: Film certificate age checking October 25, 2017 Java Just in Time - John Latham Page 28(0/0)

32 Aim AIM: To introduce the if statement without a false part. October 25, 2017 Java Just in Time - John Latham Page 29(0/0)

33 Statement: if statement Sometimes want nothing done if condition is false. E.g. could have empty false part: if (noofpeopletoinvitetothestreetparty > 500) System.out.println("You may need an entertainment license!"); else ; An empty statement between else and semi-colon! Instead can use if statement has no else, nor false part. if (noofpeopletoinvitetothestreetparty > 500) System.out.println("You may need an entertainment license!"); October 25, 2017 Java Just in Time - John Latham Page 30(0/0)

34 Film certificate age checking 001: public class FilmAgeCheck 002: { 003: public static void main(string[] args) 004: { 005: int minimumage = Integer.parseInt(args[0]); 006: int ageofperson = Integer.parseInt(args[1]); 007: if (ageofperson < minimumage) 008: System.out.println("The person is too young to watch the film!"); 009: } 010: } October 25, 2017 Java Just in Time - John Latham Page 31(0/0)

35 Trying it Console Input / Output $ java FilmAgeCheck The person is too young to watch the film! $ java FilmAgeCheck The person is too young to watch the film! $ java FilmAgeCheck $ java FilmAgeCheck $ java FilmAgeCheck $ _ Run October 25, 2017 Java Just in Time - John Latham Page 32(0/0)

36 . if. System.out.println("You Trying it Coffee time: Under what condition will the following code print the message? (noofpeopletoinvitetothestreetparty > 500); may need an entertainment license!"); (Hint: count the semi-colons.) October 25, 2017 Java Just in Time - John Latham Page 33(0/0)

37 . if. if. System.out.println("You. System.out.println("It Trying it Coffee time: When do we get the quiet party message with this code? (noofpeopletoinvitetothestreetparty > 100) (noofpeopletoinvitetothestreetparty > 500) may need an entertainment license!");. else will be a fairly quiet party."); (Hint: which if does the else match?) October 25, 2017 Java Just in Time - John Latham Page 34(0/0)

38 Coursework: PassFailDistinction (Summary only) Write a program to report the pass or fail status of an exam candidate, giving a message of distinction if appropriate using an if statement. October 25, 2017 Java Just in Time - John Latham Page 35(0/0)

39 Section 5 Example: Absolute difference October 25, 2017 Java Just in Time - John Latham Page 36(0/0)

40 Aim AIM: To introduce the compound statement. October 25, 2017 Java Just in Time - John Latham Page 37(0/0)

41 Statement: compound statement The compound statement list of statements between { and }. E.g. body of a method? Semantics sequential execution. Most useful with statements containing one other statement but want more than one. E.g. conditionally get three messages: if (noofpeopletoinvitetothestreetparty > 500) { System.out.println("You may need an entertainment license!"); System.out.println("Also hire some street cleaners for the next day?"); System.out.println("You should consider a bulk discount on lemonade!"); } Note: no indent between if statement and compound, only within it. October 25, 2017 Java Just in Time - John Latham Page 38(0/0)

42 Statement: compound statement Less useful, compound can be empty: if (noofpeopletoinvitetothestreetparty > 500) { System.out.println("You may need an entertainment license!"); System.out.println("Also hire some street cleaners for the next day?"); System.out.println("You should consider a bulk discount on lemonade!"); } else {} October 25, 2017 Java Just in Time - John Latham Page 39(0/0)

43 Absolute difference 001: public class AbsoluteDifference 002: { 003: public static void main(string[] args) 004: { 005: double firstnumber = Double.parseDouble(args[0]); 006: double secondnumber = Double.parseDouble(args[1]); 007: 008: double absolutedifference; 009: 010: if (firstnumber > secondnumber) 011: { 012: System.out.println("The first number is larger than the second"); 013: absolutedifference = firstnumber - secondnumber; 014: } October 25, 2017 Java Just in Time - John Latham Page 40(0/0)

44 Absolute difference 015: else if (firstnumber == secondnumber) 016: { 017: System.out.println("The two numbers are equal"); 018: absolutedifference = 0; 019: } 020: else 021: { 022: System.out.println("The second number is larger than the first"); 023: absolutedifference = secondnumber - firstnumber; 024: } 025: System.out.println("The absolute difference is " + absolutedifference); 026: } 027: } October 25, 2017 Java Just in Time - John Latham Page 41(0/0)

45 Trying it Console Input / Output $ java AbsoluteDifference The second number is larger than the first The absolute difference is $ _ Run Coffee time: Are you surprised by the inaccuracy of the above result? Console Input / Output $ java AbsoluteDifference The two numbers are equal The absolute difference is 0.0 $ java AbsoluteDifference The first number is larger than the second The absolute difference is $ _ Run October 25, 2017 Java Just in Time - John Latham Page 42(0/0)

46 Trying it Note: equality of reals is a dangerous notion... Console Input / Output $ java AbsoluteDifference The two numbers are equal The absolute difference is 0.0 $ _ Run October 25, 2017 Java Just in Time - John Latham Page 43(0/0)

47 Concepts covered in this chapter Each book chapter ends with a list of concepts covered in it. Each concept has with it a self-test question, and a page reference to where it was covered. Please use these to check your understanding before we start the next chapter. October 25, 2017 Java Just in Time - John Latham Page 44(0/0)