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Introduction to Programming

Lecturer: Steve Maybank

Department of Computer Science and Information Systems

sjmaybank@dcs.bbk.ac.ukSpring 2013

Week 6: if statement

Overview Java Lab 4, Exercises 2 and 3 Example of a class and an object if statement See Java for Everyone, Ch. 3

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JavaLab 4 Exercise 2 The data is included in the program

rather than read in from the keyboard.

int a1=28, b1=418, c1=-89, d1=-3007;

Find the width in characters of the field in which the numbers are to be printed.

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Exercise 2: first part The format specifier is %5d, where 5 is

the field width and d stands for decimal integer.

Use four print statements, one for each line of the output.

System.out.printf(“a1:%5d\n”, a1);System.out.printf(“b1:%5d\n”, b1); //etc.

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Exercise 2: second part

double a2 = 28.467, b2 = -1.2;double c2 = 0.0145, d2 = 587.2;

Find the width of the field:

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d 2 : 5 8 7 . 2 0

Format Specifier

Use %9.2f, where f: fixed floating point, 9: field width, 2: number of places to the right of the decimal point.

Note the rightmost 0. Define a format specifier that places

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d 2 : 5 8 7 . 2 0

Print Statements

System.out.printf(“a2:%9.2f\n”, a2);System.out.printf(“b2:%9.2f\n”, b2);System.out.printf(“c2:%9.2f\n”, c2);System.out.printf(“d2:%9.2f\n”, d2);

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JavaLab 4, Exercise 3

String a1=“Tom”, b1=“Jerry”;/* Print true if a1 precedes b1 in

lexicographic order, otherwise print false.Recall string1.compareTo(string2). This method returns an integer, but a boolean result is required. */

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Exercise 3

System.out.println(a1.compareTo(b1)<=0);

/* And similarly for the other pairs of words. */

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Example of a Class public class BankAccount

{

private double balance; // data held in each object

public BankAccount() // constructor to make objects

{

balance = 0; // each new object has balance=0

}

public void payIn(double payment) // pay into the account

{

balance = balance+payment;

}

}

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Creating an Object

BankAccount ba = new BankAccount();/* Create an object ba in the class

BankAccount. The object ba has its own balance which is 0.*/

double payment = 4.23;ba.payIn(payment);/* Add £4.23 to the balance in ba. */

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The if Statementint actualFloor;if (floor >13){

actualFloor = floor-1;}else}

actualFloor = floor;}

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Alternative Code

int actualFloor = floor;if (floor > 13){

actualFloor = floor-1;}

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Flow Chart for if Statement

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floor> 13?

actualFloor =floor-1

actualFloor=floor

true false

Flow Chart for if Statement with no else

Branch

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floor> 13?

actualFloor =floor-1

true false

Brackets Note the alignments

{…

} Brackets can be omitted for single

statements (not recommended).if (floor > 13)

actualFloor = floor-1;

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Avoid Code Duplicationif (floor > 13){

actualFloor = floor-1;System.out.println(“Actual floor: ”+actualFloor);

}else{

actualFloor = floor;System.out.println(“Actual floor: ”+actualFloor);

}

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Duplication Removedif (floor > 13){

actualFloor = floor-1;}else{

actualFloor = floor;}System.out.println(“Actual floor: ”+actualFloor);

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Multiple Alternatives

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The Richter Scale for Earthquakes

Value

Effect

8 Most structures fall

7 Many buildings destroyed

6 Many buildings considerably damaged. Some collapse.

4.5 Damage to poorly constructed buildings

Multiple if Statementsif (richter>=8.0)

{System.out.println(“Most structures fall”);}else if (richter >= 7.0)

{System.out.println(“Many buildings destroyed”);}else if (richter >= 6.0)

{System.out.println(“Considerable damage”);}else if (richter >= 4.5)

{System.out.println(“Damage to poorly constructed buildings”);}

else{System.out.println(“No destruction of buildings”);}

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Result As soon as one of the tests

succeeds the message is printed and no further tests are made.

If no test succeeds then the final else clause applies.

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Discussion What happens if the order of the

tests is reversed?

What happens if the all the “else” words (and the final print statement) are removed?

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Squares on a Chess Board

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8

7

6

5

4

3

2

1

a b c d e f g h

char file =`a`;int row = 3;

/* square a3 */

Nested if Statementsif (file == `a` || file == `c` || file == `e` || file == `g`){

if (row%2 == 1){colour = “black”;}

else{colour = “white”;}

}else{

if (row%2 == 0){colour = “black”;}

else{colour = “white”;}

}

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Dangling else Problem

double shippingCharge = 5.00;if (country.equals(“USA”))

if (state.equals(“HI”))shippingCharge = 10.00;

elseshippingCharge=20.00;

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Problem Avoided by Using Brackets

double shippingCharge = 5.00;if (country.equals(“USA”)){

if (state.equals(“HI”)){

shippingCharge = 10.00; // Hawaii is more expensive}

}else{

shippingCharge = 20.00; // As are shipments outside the USA

}

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