Selection statements Repetition statements

Lesson plan

• Main concepts• Practice session

– If- then– Switch– Nested if

Sequential sequence and Control Statement

• Sequential sequences are insufficient to solve most problems.– Get in the Car.– Drive to the supermarket.

• Tank nearly empty?– Get in the Car.– If we need gas, then fill up.– Drive to the supermarket.

Dependent Actions

• A question is asked.– A condition is checked.

• The dependent actions are only performed if the answer is Yes (the condition is true).

• Sometimes the dependent actions are performed, and sometimes they are not.

Example: Logging In• Computer access:

– Request a login name from the user.

– Request a matching password.– If the password matches the login name, then grant access.

if (<boolean expression>) <block>;else <block>;

Simple Choice Statement

if (<boolean expression>) single statement;else single statement;

Boolean expression• Boolean expression: is a conditional

expression that is evaluated to either true or false.

• Conditional expression: is a three part expression:<exp.> <relational operators> <exp.>

• Boolean expressions can be combined by boolean operators

Relational Operators

Expression Meaninga == b Is a equal to b?a != b Is a not equal to b?a > b Is a greater than b?a < b Is a less than b?a >= b Is a greater than or equal to b?a <= b Is a less than or equal to b?

Boolean operators

&& means AND|| means OR! means NOT

Boolean OperatorsOperator a b Result && false any-value false

&& true false false

&& true true true

|| true any-value true

|| false false false

|| false true true

Example of Boolean expression

testScore < 62age > 35Height > 6.5(testScore < 62) && (age > 35)

testScore = 95 ???????

Block

- Represents compound statement{

<statement 1><statement 2><statement 3>

}

ExampleIf (age < 35) {

coPay = 5;System.out.println(“….”);

} Else {

coPay = 10;System.out.println(“….”);

}

Practice• Identify invalid boolean

expression

• Identify invalid If-then-else statement

Answers for last practice exercise

I.(i) (x+1>=0) && (x==1)

Valid(ii) x2+2x-1

Invalid. Arithmetic expression(iii) ((y%2) != 0)

Valid

Answers to last practice exercise

if (x==y) {System.out.println(“ x=y”);

}Else {

System.out.println(“ x does not equal to y”);}

Should be Lowercase

Answers to last practice exercise

if (x=y) then {

grade = “A”;}else {

Grade = “B”;}

If we actually ask (is x equal y) then it should be (x==y)

Answers to last practice exercise

if (x==y) {grade = “A”;

else grade = “B”;

}

Hanging “else”

if (<boolean expression>) <block>;[else <block>;] // optional

Simple Choice Statement

if (<boolean expression>) single statement;[else single statement;] // optional

Answers to some questions• Java machine code is generated

and stored in *.class files. For example:– LoanCalculator.java – LoanCalculator.class

• When we compile a java program outside of Textpad, we need to use:

javac.exe <source_name>• When we run a java program

outside of Textpad, we need to use:java.exe <class_name>

Answers to some questions

• How to clear screen from DOS:for (int i=0; i<25; i++)

System.out.println();

Nested If Statement

if (<boolean expression>)if (<boolean expression>) if (<boolean expression>) {

statement 1}

else if (<boolean expression>) {statement 2

} else {

statement 3 }

else ….

Example

if (score >= 95) grade= “A”;

else if (score >=90)grade=“A-”;else if (score>=85)

grade =“B+”;

SWITCH statementswitch (variable) {<case label 1>: <case body 1>……<case label 2>: <case body 2>}

<case label i>: has the formcase <constant>

or: default<case body i>: is a block or a single statement,

with break variable: of integer, or char data type

Example switch (month) {

case 1: System.out.println("January"); break;

case 2: System.out.println("February"); break;

case 3: System.out.println("March"); break;

case 4: System.out.println("April"); break;

case 5: System.out.println("May"); break;

case 6: System.out.println("June"); break;

……………..

default: System.out.println("Not a month!"); break;

}

Char data type & boolean data type

• Char:– It is 2 bytes– Char constant is denoted by single

quote:for example: ‘s’, ‘h’

Special characters:\b backspace, \t tab

• Boolean:- Can be true or false

Examples

char aLetter;aLetter =‘k’;…aLetter =‘k’;

boolean flag;flag = true;…flag = false;

PracticeEvaluate boolean expressionsIdentify invalid Switch statement

Project 1 (d)

• Exit the system normally:– System.exit(0);

• Exit the system abnormally:e.g: System.exit(1);– a nonzero status code indicates

abnormal termination.

Repetition statements• Repetition of statements is a further

common facet of everyday life and object behavior:– Repeat every day:

• Get up; have breakfast; go off to work; have lunch; do more work; have an evening meal; relax; go to bed.

• Java has three types of loop for repetition:– while, do-while, for.

The While Loop

while(<boolean expression>){ // Repeat multiple statements. statement 1 statement 2 statement 3 ...}

Example

// Print the numbers 1 to maximum.Assuming that maximum is 100

int maximum = 100;int nextToPrint = 1; while(nextToPrint <= maximum){ System.out.print(nextToPrint+" "); // Get ready to print the next number. nextToPrint = nextToPrint +1; } System.out.println();}

The Do Loopdo{ // Repeat multiple statements. statement 1 statement 2 statement 3 ...} while(<boolean expression);

•Note that the statements in the body of the loop are always executed at least one.•Note the final semicolon, which is required.

Printing a Sequenceint maximum = 100;int nextToPrint = 1;

do{ System.out.print(nextToPrint+" "); // Get ready for the next number. nextToPrint = nextToPrint + 1; } while(nextToPrint <= maximum); System.out.println();}

The For Loop• Used to repeat statements a fixed

number of times:– Add three teaspoons of sugar.– Knock twice and then give the

password.• Used very often with arrays and

collections of objects • Similar in effect to the while loop.

The For-Loop Outline// Repeat multiple statements.for(initialization; condition; post-body update){ // Statements to be repeated. statement 1 statement 2 statement 3 ...}

•Commonly used with increment and decrement operators.

Increment and Decrement

• Used as a shorthand for add-one-to and subtract-one-from:

•value = value+1;•value += 1;•value++;

• Prefix and postfix forms:•++value;•--value;•value--;

A For Loop to Print a Sequence

int maximum = 100;int nextToPrint;for(nextToPrint = 1; nextToPrint <= maximum; nextToPrint++){

System.out.print(nextToPrint+“$“+” ”);}System.out.println();

The variable is not available outside the loop12345678910111$ 2$ 3$ 4$ 5$ 6$.

Practice

Develop a java class called:SumCalculator.java

which computes a sum of all integer from 1 to 100 and displays the result to the screen