130707833146508191

Repetition Statements Lecture#5

Aleeha Iftikhar

Repetitions Statementswhile Loops do-while Loopsfor Loops break and continue

The while Statement

Syntax for the while Statementwhile ( <boolean expression> )

<statement>

while ( number <= 100 ) {

sum = sum + number;

number = number + 1;

}

Statement(loop body)


Boolean ExpressionBoolean Expression

Control Flow of while

int sum = 0, number = 1int sum = 0, number = 1

number <= 100 ?number <= 100 ?

falsesum = sum + number;


sum = sum + number;


true

int age;

Scanner scanner = new Scanner(System.in);

System.out.print("Your Age (between 0 and 130): ");

age = scanner.nextInt( );

while (age < 0 || age > 130) {

System.out.println(

"An invalid age was entered. Please try again.");

System.out.print("Your Age (between 0 and 130): ");

age = scanner.nextInt( );

}

Example: Testing Input DataPriming ReadPriming Read

For Integer inputFor Integer input

CautionDon’t use floating-point values for equality checking in a loop control. Since floating-point values are approximations, using them could result in imprecise counter values and inaccurate results. This example uses int value for data. If a floating-point type value is used for data, (data != 0) may be true even though data is 0.

Make sure the loop body contains a statement that will eventually cause the loop to terminate.

Make sure the loop repeats exactly the correct number of times.

If you want to execute the loop body N times, then initialize the counter to 0 and use the test condition counter < N or initialize the counter to 1 and use the test condition counter <= N.

Loop Pitfall - 1

Infinite Loops Both loops will not terminate because the boolean expressions will never become false.

Infinite Loops Both loops will not terminate because the boolean expressions will never become false.int count = 1;

while ( count != 10 ) {

count = count + 2;

}

22

int product = 0;

while ( product < 500000 ) {

product = product * 5;

}

11

Loop Pitfall - 2• Goal: Execute the loop body 10 times.

count = 1;

while ( count < 10 ){

. . .

count++;

}

11

count = 0;

while ( count <= 10 ){

. . .

count++;

}

33

count = 1;

while ( count <= 10 ){

. . .

count++;

}

22

count = 0;

while ( count < 10 ){

. . .

count++;

}

44

The do-while Statement

do {

sum += number;

number++;

} while ( sum <= 1000000 );

do

<statement>

while ( <boolean expression> ) ;



Boolean ExpressionBoolean Expression

Control Flow of do-while

int sum = 0, number = 1int sum = 0, number = 1

sum += number;

number++;

sum += number;

number++;

sum <= 1000000 ?sum <= 1000000 ?

true

false

The for Statement

for ( i = 0 ; i < 20 ; i++ ) {

number = scanner.nextInt();

sum += number;

}

for ( <initialization>; <boolean expression>; <increment> )

<statement>

InitializationInitialization Boolean Expression

Boolean Expression IncrementIncrement



The for Statementint i, sum = 0, number;

for (i = 0; i < 20; i++) {

number = scanner.nextInt( );

sum += number;

}

These statements are executed for 20 times ( i = 0, 1, 2, … , 19).

These statements are executed for 20 times ( i = 0, 1, 2, … , 19).

Control Flow of for

i = 0;i = 0;

false

number = . . . ;sum += number;

number = . . . ;sum += number;

true

i ++;i ++;

i < 20 ? i < 20 ?

More for Loop Examplesfor (int i = 0; i < 100; i += 5)11

i = 0, 5, 10, … , 95 i = 0, 5, 10, … , 95

for (int j = 2; j < 40; j *= 2)22

j = 2, 4, 8, 16, 32 j = 2, 4, 8, 16, 32

for (int k = 100; k > 0; k--) )33

k = 100, 99, 98, 97, ..., 1k = 100, 99, 98, 97, ..., 1

Which Loop to Use?

The three forms of loop statements, while, do, and for, are expressively equivalent; that is, you can write a loop in any of these three forms.

I recommend that you use the one that is most intuitive and comfortable for you. In general, a for loop may be used if the number of repetitions is known, as, for example, when you need to print a message 100 times. A while loop may be used if the number of repetitions is not known, as in the case of reading the numbers until the input is 0. A do-while loop can be used to replace a while loop if the loop body has to be executed before testing the continuation condition.

CautionAdding a semicolon at the end of the for clause before the loop body is a common mistake, as shown below:

for (int i=0; i<10; i++);{ System.out.println("i is " + i);}

Similarly, the following loop is also wrong:int i=0; while (i<10);{ System.out.println("i is " + i); i++;}In the case of the do loop, the following semicolon is needed to end the loop.int i=0; do { System.out.println("i is " + i); i++;} while (i<10);

Wrong

Correct

Loop-and-a-Half Repetition Control

• Loop-and-a-half repetition control can be used to test a loop’s terminating condition in the middle of the loop body.

• It is implemented by using reserved words while, if, and break.

Example: Loop-and-a-Half Control

String name;

Scanner scanner = new Scanner(System.in);

while (true){

System.out.print("Your name“);

name = scanner.next( );

if (name.length() > 0) break;

System.out.println("Invalid Entry." +

"You must enter at least one character.");

}

The break Keyword

false

true

Statement(s)

Next Statement

Continuation condition?

Statement(s)

break

The continue Keyword

false

true

Statement(s)

Next Statement

Continue condition?

Statement(s)

continue

Chapter 4 Methods

Introducing MethodsBenefits of methods, Declaring Methods, and Calling

Methods

Passing ParametersPass by Value

Introducing MethodsMethod Structure

A method is a collection of statements that are grouped together to perform an operation.

Introducing Methods, cont.•parameter profile refers to the type, order, and number of the parameters of a method.

•method signature is the combination of the method name and the parameter profiles.

•The parameters defined in the method header are known as formal parameters.

•When a method is invoked, its formal parameters are replaced by variables or data, which are referred to as actual parameters.

Declaring\Definning Methodspublic static int max(int num1, int num2) {

if (num1 > num2) return num1; else return num2;}

Calling Methods, cont.

public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println( "The maximum between " + i + " and " + j + " is " + k); }

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

pass i pass j

Calling Methods, cont.

The main method i: j: k:

The max method num1: num2: result:

pass 5

5

2

5

5

2

5

pass 2 parameters

THE END

130707833146508191

Engineering

statement syntax

repetitions statements

aleeha iftikhar