Iterations

Repetition Statementswhile and do while loops

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Outline

• while statements• Example of while statement• Other repetition statements– do loop– for loop

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Online material

• Chapter one of the book is available online under the following link:

• http://introcs.cs.princeton.edu/java/home/chapter1.pdf

• Please read pages 46 – 83. • The exercises in this book are bit challenging.• Do as much as you can.

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Repetition Statements• Repetition statements allow us to execute a

statement multiple times• Often they are referred to as loops• Like conditional statements, they are controlled

by boolean expressions• Java has three kinds of repetition statements:– the while loop– the do loop– the for loop

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The while Statement• A while statement has the following syntax:

while(condition) { statement; }

• If the condition is true, the statement is executed

• Then the condition is evaluated again, and if it is still true, the statement is executed again

• The statement is executed repeatedly until the condition becomes false

Body

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Logic of a while Loop

statement

true false

conditionevaluated

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The while Statement• An example of a while statement:

int count = 1;while (count <= 5){ System.out.println(count); count= count +1;}

• If the condition of a while loop is false initially, the statement is never executed

• Therefore, the body of a while loop will execute zero or more times

Body

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The while Statement• Let's look at some examples of loop processing

• A loop can be used to maintain a running sum

• A sentinel value is a special input value that represents the end of input

• A loop can also be used for input validation, making a program more robust

Example1

• What is the output of the program?// hello.javapublic class Hello1{ public static void main(String[] arguments) { System.out.println(“hello”); System.out.println(“hello”); System.out.println(“hello”); System.out.println(“hello”); System.out.println(“hello”) ; System.out.println(“hello”); System.out.println(“hello”); System.out.println(“hello”); System.out.println(“hello”); System.out.println(“hello”); }}//end of program

Prints hello 10 times

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Example2

• What is the output of the program?// hello.javapublic class Hello2{ public static void main(String[] arguments) { int i =1; while(i<=10) { System.out.println(“hello”); }}//end of program

Prints hello 10 times

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Example3

• What is the output of the program?// hello3.javapublic class Hello2{ public static void main(String[] arguments) { int x = Integer.parseInt(args[0]); int i =1; while(i<=x) { System.out.println(“hello”); }}//end of program

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Example4

• What is the output of the program?// hello4.javaImport java.util.Randompublic class Hello3{ public static void main(String[] arguments) { Random generator = new Random(); while(i<=10) { int x = generator.nextInt(100); System.out.println(x); }}//end of program

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Example5

• What is the output of the program?// hello5.javapublic class Hello2{ public static void main(String[] arguments) { int x = Integer.parseInt(args[0]); int i =1; while(i<=x) { System.out.println(“hello”); }}//end of program

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Example6

• What is the output of the program?// sum1.javapublic class sum1{ public static void main(String[] arguments) { int i =1; int sum =0; while(i<=100) { sum = sum+i ; } System.out.println(sum);}//end of program

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Example7

• What is the output of the program?// factorial.javapublic class sum1{ public static void main(String[] arguments) { int i =1; int factorial =1; while(i<=100) { fact= fact*i ; } System.out.println(factorial);}//end of program

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Infinite Loops

• To exit the loop

– The body of a while loop has to make the control condition false

– Otherwise, the loop will on go on for ever. This is called an infinite loop.

– Press Control-C to stop the execution of an infinite loop.

– This is a common logical error

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Infinite Loops• An example of an infinite loop:

int i = 0;while (i<=0){ System.out.println (i); i = i - 1;}

• This loop will continue executing for ever.

• Control-C to stop the execution

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Nested Loops

• Similar to nested if statements, loops can be nested as well

• The body of a loop can contain another loop.

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Nested Loops• What is the output of the following program?

Int count1 = 1;Int count2;while (count1 <= 10){ System.out.println(“hello"); count2 = 1; while (count2 <= 20) { System.out.println(“lahcen"); count2++; } count1++;}

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The do-while Statement• A do-while statement (also called a do loop)

has the following syntax:

do{ statement; }while (condition )

• The statement is executed once initially, and then the condition is evaluated

• The statement is executed repeatedly until the condition becomes false

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Logic of a do-while Loop

true

statement

false

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The do Statement• An example of a do loop:

• The body of a do loop will execute for 5 times

int count = 0;do{ count++; System.out.println (count);} while (count < 5);

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Comparing while and do

statement

true false

conditionevaluated

The while Loop

true

conditionevaluated

statement

false

The do Loop

Summary

• Iterations using – while (condition) {statements}– do {statements} while(condition)

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