Repetition Structure

Outlines
 Repetition Statements
 while statement
 do..while statement
 for statement
 Nested loops
 Repetition Control Structures

Repetition Statements
 Repetition statement (or loop) a block of code to be
executed for a fixed number of times or until a certain
condition is met.
 In JAVA, repetition can be done by using the
following repetition statements:
a) while
b) do…while
c) for

The while statement
 The while statement evaluates
expression/condition, which must return a boolean
value. If the expression/condition is true, the
statement(s) in the while block is/are executed.
 Syntax:
while (expression/condition)
Statement(s);
 It continues testing the expression/condition and
executing its block until the expression/condition
evaluates to false

Output ? 1 2 3 4
Example 1
int i=1;
while (i<5){
System.out.print(i + “”);
i++;
}

Output ? SUM : 1
SUM : 7
Good Bye
Example 2
int sum=0, number =1;
while (number <= 10)
{
sum+=number;
number = number + 5;
System.out.println(“SUM :” + sum);
}
System.out.println(“Good Bye”);

The do…while statement
 It is similar to while loops except it first executes the
statement(s) inside the loop, and then evaluates the
expression/condition. If the expression is true, the
statement(s) in the do loop is/are executed again.
 Syntax
do
statement(s);
while (expression);
 It continues executing the statement until the
expression/condition becomes false.
 Note that the statement within the do loop is always
executed at least once.

Output ? 0 1 2 3
Example 3
int i=0;
do{
System.out.print(i +
“”);
i++;
}while(i<=3);

Output ? SUM : 2
SUM : 9
Example 4
int sum=0, number =2;
do{
sum+=number;
number = number + 5;
System.out.println(“SUM :” + sum);
} while (number <= 10);

The for statement
 Usually used when a loop will be executed a set
number of times.
 Syntax:
for(initial statement; loop condition; update statement)
statement(s);
 The for loop executes as follow:
1) The initial statement is executed.
2) The loop expression/condition is evaluated. If it is
TRUE, the loop statement is executed followed by the
execution of the update statement
3) Repeat step 2) until the loop condition evaluates to
FALSE.

Output ? 1 2 3 4
Example 5
for (i=1; i<5; i++)
System.out.print(i);

Output ? YAHOO ***YAHOO ***
Example 6
for (i=1; i<3; i++){
System.out.print(“YAHOO” + “”);
System.out.print(“***”);
}

Output ? YAHOO YAHOO YAHOO YAHOO YAHOO ***
Example 7
for (i=1; i<=5; i++)
System.out.print(“YAHOO”);
System.out.print(“***”);

Nested Loops
 The placing of one loop inside the body of
another loop is called nesting.
 When you "nest" two loops, the outer loop
takes control of the number of complete
repetitions of the inner loop.
 While all types of loops may be nested, the
most commonly nested loops are for loops.

Nested for Loops
 When working with nested loops, the outer loop
changes only after the inner loop is completely
finished (or is interrupted.).

Example 8
int num1, num2;
for(num2 = 0; num2 <= 2; num2++)
{
for(num1 = 0; num1 <= 1; num1++)
{
System.out.println(num2 + " " + num1);
}
}
Output ?
0 0
0 1
1 0
1 1
2 0
2 1

Infinite Loop
 By using any repetition statements, make sure that
the loop will eventually terminate.
 An infinite loop occurs when a condition always
evaluates to true and continues to execute endlessly.
int product =0;
for (product = 0;product < 10;)
{ product = product * 2;
System.out.println(product);
}

Repetition Control Structures
Repetition can be controlled by:
 Counter controlled loop
 Sentinel controlled loop
 Flag controlled loop
Exercise

Counter Controlled Loop
 Know exactly how many times a set of statements
needs to be executed.
Output ? 1 3 5 7 9
int num =1;
while (num < 10)
{
System.out.print (num);
num = num +2;
}
Example 10:
back

Sentinel Controlled Loop
 You might not know exactly how many times a set of
statements needs to be executed.
 It uses a "special" or sentinel value to indicate that the
loop is to end.
 This must be a value that doesn't occur normally in
the data.

Example 11 (complete program)
import java.util.Scanner;
class sentinelLoop {
public static void main (String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter an Integer, or -1 to stop: ");
int choice= input.nextInt();
while (choice!=-1)
{
System.out.println("INSIDE LOOPING");
System.out.print("Enter an Integer, or -1 to stop: ");
choice= input.nextInt();
}
System.out.println("Sentinel value detected. Good Bye.");
}
}

Example 11 ….
Enter an Integer, or -1 to stop: 2
INSIDE LOOPING
INSIDE LOOPING
INSIDE LOOPING
Enter an Integer, or -1 to stop: -1
Sentinel value detected. Good Bye.
OUTPUT?
back

Flag Controlled Loop
 Use a boolean variable to control the loop
boolean found = false;
while (!found){
:
:
if(expression)
found = true;
}
back

Exercise 1:
What is the output of the following program?
public class LoopExercise1
{
public static void main (String args[])
{
int choice=1, total=0;
while (choice <4){
total = choice++;
System.out.print(total); }
}
}

Exercise 2:
What is the output of the following program?
public class LoopExercise2
{
public static void main (String args[]){
for (int number =2; number <20; number++)
{
number = number *2;
if (number <15)
System.out.println(number);}
}
}

Exercise 3:
How many times is the following loop body repeated?
public class LoopExercise3 {
public static void main (String args[])
{
int i=1;
do {
if ((i % 2)== 0)
System.out.print(i);
i++;
} while(i<5);
}
}

Repetition Structure

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Repetition Structure