Decision Structures. Lecture 3 MIT 12043, Fundamentals of Programming By: S. Sabraz Nawaz

Transcription

1 Decision Structures Lecture 3 MIT 12043, Fundamentals of Programming By: S. Sabraz Nawaz

2 Chapter Topics o Relational Operators o The if Statement o The if-else Statement o Nested if statements o The if-else-if Statement o The switch Statement MIT 12043, By S. Sabraz Nawaz 3-2

3 Relational Operators Relational operators in Java are used to compare two variables. Java provides six relational operators: Equal == Greater than > Less than < Greater than or equal to >= Less than or equal to <= Unequal!= The result of any relational operator is true or false MIT 12043, By S. Sabraz Nawaz 3

4 Boolean Expressions int a = 3; b = 4; a == b will evaluate to false a!= b will evaluate to true a > b a < b will evaluate to false will evaluate to true a >= b will evaluate to false a <= b will evaluate to true MIT 12043, By S. Sabraz Nawaz 4

5 Sequence Structure MIT 12043, By S. Sabraz Nawaz 5

6 Decision Structure Programs often need more than one path of execution, however. Many algorithms require a program to execute some statements only under certain circumstances. This can be accomplished with a decision structure In a decision structure s simplest form, a specific action is taken only when a condition exists. If the condition does not exist, the action is not performed MIT 12043, By S. Sabraz Nawaz 6

7 Decision Structure Simple decision structure logic MIT 12043, By S. Sabraz Nawaz 7

8 Decision Structure Three-action decision structure logic MIT 12043, By S. Sabraz Nawaz 8

9 Selection Statements The control statement that allows alternative actions based upon conditions that are evaluated at run time are generally called selection statements One way to code a decision structure in Java is with the if statement MIT 12043, By S. Sabraz Nawaz 9

10 The if Statement The if statement is used to create a decision structure, which allows a program to have more than one path of execution. o The if statement causes one or more statements to execute only when a boolean expression is true. The if statement decides whether a section of executes or not. if (boolean expression is true) execute this statement; MIT 12043, By S. Sabraz Nawaz 3-10

11 The if Statement A block if statement may be modeled as: if (coldoutside) { wearcoat; wearhat; weargloves; } Note the use of curly braces to block several statements together. MIT 12043, By S. Sabraz Nawaz 3-11

12 if Statements and Boolean Expressions if (x > y) System.out.println("X is greater than Y"); if(x == y) System.out.println("X is equal to Y"); if(x!= y) { System.out.println("X is not equal to Y"); x = y; System.out.println("However, now it is."); } MIT 12043, By S. Sabraz Nawaz 3-12

13 Programming Style and if Statements An if statement can span more than one line; however, it is still one statement. if (average > 95) grade = A ; is functionally equivalent to if(average > 95) grade = A ; MIT 12043, By S. Sabraz Nawaz 3-13

14 Block if Statements Conditionally executed statements can be grouped into a block by using curly braces {} to enclose them. If curly braces are used to group conditionally executed statements, the if statement is ended by the closing curly brace. if (expression) { statement1; statement2; } Curly brace ends the statement. MIT 12043, By S. Sabraz Nawaz 3-14

15 Block if Statements Remember that when the curly braces are not used, then only the next statement after the if condition will be executed conditionally if (expression) statement1; statement2; statement3; Only this statement is conditionally executed. MIT 12043, By S. Sabraz Nawaz 3-15

16 Example 01 Write a Java program that takes a person s age as input and decides whether that person is eligible to vote If the person s age is above 18, he or she is eligible to vote The program should display a message whether he or she is eligible to vote MIT 12043, By S. Sabraz Nawaz 16

17 Example 01 MIT 12043, By S. Sabraz Nawaz 17

18 Example 02 Write a Java program that takes a person s age and his or her name as inputs and decides whether that person is eligible to vote If the person s age is above 18, he or she is eligible to vote The program should display a message with the Name of the person and his or eligibility to vote MIT 12043, By S. Sabraz Nawaz 18

19 Example 02 MIT 12043, By S. Sabraz Nawaz 19

20 if-else Statements The if-else statement adds the ability to conditionally execute code when the if condition is false. if (expression) statementorblockiftrue; else statementorblockiffalse; It works the same way as the if statement except that, when the condition is false, the statement within the else clause executes MIT 12043, By S. Sabraz Nawaz 3-20

21 if-else Statement Flowcharts No Wear shorts. Is it cold outside? Yes Wear a coat. MIT 12043, By S. Sabraz Nawaz 3-21

22 Example 03 Write a Java program that takes a person s age and his or her name as inputs and decides whether that person is eligible to vote or not If the person s age is above 18, he or she is eligible to vote The program should display a message with the Name of the person and his or eligibility to vote MIT 12043, By S. Sabraz Nawaz 22

23 Example 03 MIT 12043, By S. Sabraz Nawaz 23

24 Exercise A company s payment structure to its employees is as follows o o o If the Basic salary exceeds 30,000/=, the Dearness Allowance 40% the BS, otherwise 50% of the BS If the BS exceeds 25,000/=, the House Rent Allowance is 20% of the BS, otherwise 3,000 of the BS Income Tax is calculated at the rate of 12% if the Total Salary exceeds 50,000/= Write a Java program that takes BS as input, calculate DA, HRA, IT, Total Salary, Net Salary, and display them all on screen MIT 12043, By S. Sabraz Nawaz 24

25 Solution MIT 12043, By S. Sabraz Nawaz 25

26 Solution MIT 12043, By S. Sabraz Nawaz 26

27 Nested if Statements To test more than one condition, an if statement can be nested inside another if statement If an if statement appears inside another if statement (single or block) it is called a nested if statement The nested if is executed only if the outer if statement results in a true condition MIT 12043, By S. Sabraz Nawaz 3-27

28 Alignment and Nested if Statements This if and else go together. This if and else go together. if (expression) { if (expression) { statement; } else { statement; } } else { statement; } MIT 12043, By S. Sabraz Nawaz 3-28

29 Example 04 For example, consider a banking program that determines whether a bank customer qualifies for a special, low interest rate on a loan To qualify, two conditions must exist: 1. The customer s salary must be at least Rs.30, The customer must have held his or her current job for at least 02 years MIT 12043, By S. Sabraz Nawaz 29

30 Example 04 MIT 12043, By S. Sabraz Nawaz 30

31 if-else-if Statements The if-else-if statement tests a series of conditions It is often simpler to test a series of conditions with the if-else-if statement than with a set of nested ifelse statements MIT 12043, By S. Sabraz Nawaz 31

32 if-else-if Syntax if (expression_1) { statement; statement; etc. } else if (expression_2) { } statement; statement; etc. Insert as many else if clauses as necessary If expression_1 is true these statements are executed, and the rest of the structure is ignored. Otherwise, if expression_2 is true these statements are executed, and the rest of the structure is ignored. else { statement; statement; etc. } These statements are executed if none of the expressions above are true. MIT 12043, By S. Sabraz Nawaz 3-32

33 Example 05 Your lecturer has asked you to write a program that will allow a student to enter a test score and then display the grade for that score MIT 12043, By S. Sabraz Nawaz 33

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35 Logical Operators MIT 12043, By S. Sabraz Nawaz 35

36 Logical Operators Logical operators connect two or more relational expressions into one or reverse the logic of an expression. Java provides two binary logical operators, && and used to combine two boolean expressions into a single expression It also provides the unary! Operator o reverses the truth of a boolean expression MIT 12043, By S. Sabraz Nawaz 36

37 Logical Operators Operator Meaning Effect && AND Connects two boolean expressions into one. Both expression must be true for the overall expression to be true OR Connects two boolean expressions into one. One or both expressions must be true for the overall expression to be true. It is only necessary for one to be true, and it does not matter which one.! NOT Reverses the truth of a boolean expression. If it is applied to an expression that is true, the operator returns false. If it is applied to an expression that is false, the operator returns true MIT 12043, By S. Sabraz Nawaz 37

38 Logical Operators Expression Meaning x > y && a < b Is x greater than y AND is a less than b? x == y x == z Is x equal to y OR is x equal to z?!(x > y) Is the expression x > y NOT true? MIT 12043, By S. Sabraz Nawaz 38

39 Truth table for the && operator Expression true && false false && true false && false true && true Value of the Expression false false false true MIT 12043, By S. Sabraz Nawaz 39

40 Truth table for the operator Expression true false false true false false true true Value of the Expression true true false true MIT 12043, By S. Sabraz Nawaz 40

41 Truth table for the! operator Expression!true!false Value of the Expression false true MIT 12043, By S. Sabraz Nawaz 41

42 Example 06 Improved Ex 05 42

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44 The % Operator The above modulus operator returns the remainder Eg. o 3 % 2 1 (remainder is one) o 15 % 4 3 (remainder is three) Odd or Even number finder MIT 12043, By S. Sabraz Nawaz 44

45 Comparing String Objects You cannot use relational operators to compare String objects To compare, you should use the String class s equals method. The general form of the method is as follows: StringReference1.equals(StringReference2) o StringReference1 is a variable that references a String object, and StringReference2 is another variable that references a String object The method returns true if the two strings are equal, or false if they are not equal. Here is an example: if (name1.equals(name2)) The expression in the if statement will return true if they are the same, or false if they are not the same MIT 12043, By S. Sabraz Nawaz 45

46 Example 07 Note: the equal method is case sensitive; if you want to ignore the case case, use string. equalsignorecase() method MIT 12043, By S. Sabraz Nawaz 46

47 Classroom Exercise A car rental company has three types of cars; Premio, Alien, and Civic. The rental scheme is as follows. Type Premio Alien Civic Fixed Rent per day 1,500/- 2,000/- 2,500/- Rate per KM 20/- 30/- 40/- Free Allowance per day 80/- 60/- 50/- Write a Java program to input Vehicle Type, Number of Days Rented, and Kilometers run to calculate the Total Rent Payable and display on the screen. MIT 12043, By S. Sabraz Nawaz 47

48 The switch Statement For decisions involving many possible paths and values we use the switch statement The switch statement can have a number of possible execution paths At the end of each sequence we insert a break statement that means go to the end of the switch A switch works with the byte, short, char, and int primitive data types. It works with the String class as well. 48

49 The switch Statement switch case default break Reserved words 49 switch (variable) { case value1: statement; statement; break; } case value2: statement; statement; break; default: statement; break; Don t forget breaks!

50 switch - Example The same case can have two or more labels. For example: switch (num) { case 1: case 2:System.out.println ( Stand up and sing a song"); break; case 3:... } 50

51 Month Finder 51

52 Switch with Multiple case labels 52

53 Rules apply to a switch statement You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon The value for a case must be the same data type as the variable in the switch, and it must be a constant or a literal (character) When the variable being switched on is equal to a case, the statements following that case will execute until a break statement is reached 53

54 Rules apply to a switch statement When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement Not every case needs to contain a break. If no break appears, the flow of control will fall through to subsequent cases until a break is reached A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case 54

55 Looping in the next lectures MIT 12043, By S. Sabraz Nawaz 55