Class - X , L - 9 Loops in Java

Notes and Exercise programs of L-9

In computer programming, loops are used to
repeat a specific block of code until a certain
condition is met (test expression is false). For
example,

Imagine we need to print a sentence 50 times

on your screen. Well, we can do it by using the
print statement 50 times (without using loops).
How about you need to print a sentence one
million times? You need to use loops. With
loops, we can simply write the print
statement one time and run it for any number
of times.

It's just a simple example showing the

importance of loop in computer programming.

There are 3 types of loops in Java:

1. for loop(fixed or entry control loop,
2. while loop(user control or entry control), and
3. do-while loop(user control or exit control
loop).

(1) for ( ) Loop

The syntax of for Loop in Java is:

for (initialization; testExpression; update)

{
// codes inside for loop's body
}
Working of for loop/Dry run

1 - The initialization expression is executed only

once.

2 - Then, the test expression / condition is

evaluated. Here, test expression is a boolean
expression.

3 - If the test expression is evaluated to

true, Codes inside the body of for loop
is executed. 4- Then the update
expression is executed.
Again, the test expression is evaluated.
If the test expression is true, codes inside the
body of for loop is executed and update
expression is executed.
This process goes on until the test expression
is evaluated to false.

If the test expression is evaluated to false, for

loop terminates.

for Loop Flowchart

Working of for loop

Example 1: for Loop

// Program to print a sentence 10 times class

Loop
{
public static void main(String[] args)
{
for (int i = 1; i <= 10; ++i)
{
System.out.print("Line " + i+" ");
}
}
}

Output:

Line 1 Line 2 Line 3 Line 4 Line 5 Line 6 Line 7

Line 8
Line 9 Line 10

In the above example, we have

initialization expression: int i =

1.e test expression: i <=10

update expression: ++i

Here, initially, the value of i is 1. So the test

expression evaluates to true for the first time.
Hence, the print statement is executed. Now
the update expression is evaluated.

Each time the update expression is evaluated,

the value of i is increased by 1. Again, the test
expression is evaluated. And, the same process
is repeated.

So, i is the control variable here which is used

to control the execution of loop.
This process goes on until i is 11. When i is
11, the test expression (i <= 10) is false and
the for loop terminates.
To learn more about test expression and how it
is evaluated, visit relational and logical
operators.

Example 2: for Loop

// Program to find the sum of natural

//numbers from 1 to
1000. class Number
{
public static void main(String[] args)
{
int sum = 0;
for (int i = 1; i <= 1000; ++i)
{
sum += i; // sum = sum + i
}
System.out.println("Sum = " + sum);
}
}

Output:

Sum = 500500

Here, we have a variable named sum. Its

initial value is 0. Inside for loop, we have
initialized a variable named iwith value 1.

In each iteration of for loop,

the sum variable is assigned value:

sum + i the value of i is increased by 1

The loop continues until the value of i is

greater then 1000. For better visualization,
Dry run
1st iteration: i = 1 and sum = 0+1 = 1 2nd
iteration: i = 2 and sum = 1+2 = 3 3rd
iteration: i = 3 and sum = 3+3
= 6 4th iteration: i = 4 and sum = 6+4 = 10 ...
.. ... 999th iteration: i = 999 and sum = 498501
+ 999 = 499500 1000th iteration: i = 1000 and
sum = 499500 + 1000 = 500500

infinite for Loop

We should be always careful while working with

loops. It is because if we mistakenly set test
expression in such a way that it is never false,
the for loop will run forever.

This is called infinite for loop. For example,

// Infinite for Loop

class Infinite
{
public static void main(String[] args)
{
int sum = 0;
 for (int i = 1; i <= 10; - -i)
{
System.out.println("Hello");

}
}
}

Here, the test expression (i <= 10) is

never false and hello is printed infinite number
to times (at least in theory).

Note: The initialization, update and test

expression used in for statement is optional.
Here's another example of the infinite for loop.

for ( ; ; )
{ }
(2)while ( ) Loop

The syntax of while loop in Java

is: while (testExpression)

{
//codes inside the body of while loop
}

How while loop works?

In the above syntax, the testexpression inside

parenthesis is a boolean expression. If the test
expression is evaluated to true,

statements inside the while loop are

executed. then, the test expression is

evaluated again.

This process goes on until the test expression

is evaluated to false. If the test expression is
evaluated to false,

the while loop is terminated.

Flowchart of while Loop

Working of while Loop
Example 1: while Loop

// Program to print line 10 times

class Loop
{
public static void main( )
{
int i = 1;
while (i <=
10)
{
System.out.print("Line " + i+" ");
++i;
}
}
}

Output:

Line 1 Line 2 Line 3 Line 4 Line 5 Line 6 Line 7

Line 8
Line 9 Line 10

In the above example, we have a test

expression (i
<= 10). It checks whether the value of i is less
than or equal to 10.

Here initially, the value of i is 1. So the test

expression evaluates to true for the first time.
Hence, the print statement inside while loop is
executed.
Inside while loop notice the statement

++i;

This statement increases the value of i by 1 in

each iteration. After 10 iterations, the value of
i will be
11. Then, the test expression (i <= 10)
evaluates to false and whileloop
terminates.

Example 2: Java while Loop

// Program to find the sum of natural

//numbers from 1 to
100. class
AssignmentOperator
{
public static void main( )
{
int sum = 0, i =
100; while (i != 0)
{
sum += i;
- -i;
}
System.out.println("Sum = " + sum);
}
}

Output:
Sum =

5050
Here, we have two variables named sum and i
whose initial values are 0 and 100
respectively.

In each iteration of the while loop,

the sum variable is assigned value:

sum + i the value of i is decreased by

The loop continues until the value of i is equal

to 0. For better visualization,

1st iteration: i = 100, sum = 0+100 = 100,

and --i = 99 2nd iteration: i = 99, sum =
100+99 = 199, and --i = 98 3rd iteration: i =
98, sum = 199+98 = 297, and --i = 97 ... .. ...
... .. ... 99th iteration: i = 2, sum = 5047+2 =
5049, and --i = 1 100th iteration: i = 1, sum =
5049+1 = 5050, and --i = 0

(3) do...while ( ) Loop

The do...while loop is similar to while loop with

one key difference. The body of do...while loop
is executed for once before the test expression
is checked.
Here is the syntax of the

do...whileloop. do
{
// codes inside body of do while loop
} while

(testExpression); How

do...while loop works?

The body of do...while loop is executed once

(before checking the test expression). Only
then, the test expression is checked.

If the test expression is evaluated to true,

codes inside the body of the loop are executed,
and the test expression is evaluated again.
This process goes on until the test expression
is evaluated to false.

When the test expression is false, the do..while

loop terminates.

Flowchart of do...while Loop

Working of do...while

Loop Example 3:

do...while Loop

The program below calculates the sum of

numbers entered by the user until user enters
0.

To take input from the user, we have used

the Scanner object. To learn more about
Scanner, visit Java Scanner.

import java.util.Scanner;
class Sum
{
public static void main(String[] args)
{
double number, sum = 0.0;
// creates an object of Scanner class Scanner
input = new Scanner(System.in); do
{
// takes input from the user
System.out.print("Enter a number: "); number
= input.nextDouble();
sum += number;
} while (number != 0.0); // test expression
System.out.println("Sum = " + sum);
}
}
Output:
Enter a number: 2.5 Enter a number: 23.3
Enter a number: -4.2 Enter a number: 3.4 Enter
a number: 0 Sum = 25.0

Infinite while Loop

We should be always careful while

working with loops. It is because if we
mistakenly set the test expression in such
a way that it is never false, the while and
do...while loop will run forever.

This is called infinite while and do...while loop.

For example,

// Infinite while loop

while (true==true)
{
// body of while loop
}

Let's take another example,

// Infinite while loop

int i = 100;
while (i == 100)
{
System.out.print("Hey!");
}
Jump statements
While working with loops, it is sometimes
desirable to skip some statements inside the
loop or terminate the loop immediately
without checking the test expression.

In such cases, break and continue statements

are used.

The break statement in Java terminates the

loop immediately, and the control of the
program moves to the next statement
following the loop.

It is almost always used with decision-making

statements (Java if() Statement).

Here is the syntax of the break statement in

Java: break;

How break statement works?

Working of Java break

Statement Example 1: Java

break statement class Test

{
public static void main(String[] args)
{
// for loop
for (int i = 1; i <= 10; ++i)
{
// if the value of i is 5 the loop
terminates if (i == 5)
{ break; }
System.out.println(i);
}}}
Output:

1234

In the above program, we are using the for loop

to print the value of i in each iteration. To know
how for loop works, visit the Java for loop. Here,
notice the statement,

if (i == 5)
{
break;
}

This means when the value of i is equal to 5,

the loop terminates. Hence we get the output
with values less than 5 only.

Example 2: Java break statement

The program below calculates the sum of

numbers entered by the user until user enters
a negative number.

To take input from the user, we have used

the Scanner object. To learn more about
Scanner, visit Java Scanner.

import java.util.Scanner;
class UserInputSum
{
public static void main(String[] args)
{

double number, sum = 0.0;

// create an object of Scanner
Scanner input = new Scanner(System.in); while
(true)
{
System.out.print("Enter a number: ");
// takes double input from user
number = input.nextDouble();
// if number is negative the loop
terminates if (number < 0.0)
{ break; }
sum += number;
}
System.out.println("Sum = " + sum);
}}

Output:

Enter a number: 3.2 Enter a number: 5 Enter a

number:
2.3 Enter a number: 0 Enter a number: -4.5 Sum
= 10.5

In the above program, the test expression of

the while loop is always true. Here, notice the
line,
if (number < 0.0)
{
break;
}

This means when the user input negative

numbers, the while loop is terminated.

Java break and Nested Loop

In the case of nested loops, the break

statement terminates the innermost loop.

Working of break Statement with Nested Loops

Here, the break statement terminates the

innermost whileloop, and control jumps to
the outer loop.

Lesson 10
Labeled break Statement
Till now, we have used the unlabeled break
statement. It terminates the innermost loop and
switch statement. However, there is another
form of break statement in Java known as the
labeled break.

We can use the labeled break statement to

terminate the outermost loop as well.

Working of the labeled break statement in Java

As you can see in the above image, we have

used the label identifier to specify the outer
loop. Now,
notice how the break statement is used (break
label;).

Here, the break statement is terminating the

labeled statement (i.e. outer loop). Then, the
control of the program jumps to the statement
after the labeled statement.
Here's another example:

while (testExpression)
{ // codes second:
while (testExpression)
{ // codes
while(testExpression)
{ // codes break second;
}
} // control jumps here
}

In the above example, when the statement

break second; is executed, the while loop
labeled
as second is terminated. And, the control
of the program moves to the statement
after the second whileloop.

Example 3: labeled break

Statement class LabeledBreak

{
public static void main(String[] args)
{
// the for loop is labeled as
first first:
for( int i = 1; i < 5; i++)
{
// the for loop is labeled as second second:
 for(int j = 1; j < 3; j ++ )
{
System.out.println("i = " + i + "; j = " +j);
// the break statement breaks the first
for loop if ( i == 2)
break first;
}
}
}
}

Output:

i = 1; j = 1 i = 1; j = 2 i = 2; j = 1

In the above example, the labeled break

statement is used to terminate the loop
labeled as first. That is,

first: for(int i = 1; i < 5; i++) {...}

Here, if we change the statement break first; to

break second; the program will behave
differently. In this case, for loop labeled as
second will be terminated. For example,

class LabeledBreak
{
public static void main(String[] args)
{
// the for loop is labeled as first
 first:
for( int i = 1; i < 5; i++)
{
// the for loop is labeled as second
second: for(int j = 1; j < 3; j ++ )
{
System.out.println("i = " + i + "; j = " +j);
// the break statement terminates the loop
labeled as second
if ( i == 2)
break
second;
}
}
}
}

Output:

i = 1; j = 1 i = 1; j = 2 i = 2; j = 1 i = 3; j = 1 i
= 3; j = 2 i = 4; j
= 1 i = 4; j = 2

Note: The break statement is also used to

terminate cases inside the switch statement.

Java continue Statement

The continue statement in Java skips the

current iteration of a loop (for, while,
do...while, etc) and the control of the program
moves to the end of the loop.
And, the test expression of a loop is evaluated.

In the case of for loop, the update statement is

executed before the test expression.

The continue statement is almost always used

in decision-making statements (if...else
Statement). It's syntax is:

continue;

How continue statement works?

Working of Java continue statement

Example 1: Java continue statement

class Test
{
public static void main(String[] args)
{
// for loop
for (int i = 1; i <= 10; ++i)
{
// if value of i is between 4 and 9, continue is
executed if (i > 4 && i < 9)
{
continue;
}
System.out.println(i);
}
}
}

Output:

1 2 3 4 9 10

In the above program, we are using for loop to

print the value of i in each iteration. To know
how forloop works, visit Java for loop. Here,
notice the statement,

if (i > 5 && i < 9)

{ continue; }
This means when the value of ibecomes more
than 4 and less then 9, the print statement
inside the loop is skipped. Hence we get the
output with values 5, 6, 7, and 8 skipped.

Example 2: Java continue statement

The program below calculates the sum of 5

positive numbers entered by the user. If the
user enters negative number or zero, it is
skipped from the calculation.

To take input from the user, we

have used the Scanner object.

import
java.util.Scanner; class
AssignmentOperator
{
public static void main(String[] args)
{
double number, sum = 0.0;
// create an object of Scanner
Scanner input = new Scanner(System.in); for
(int i = 1; i
< 6; ++i)
{
System.out.print("Enter a number: ");
// takes double type input from the user
number = input.nextDouble();
// if number is negative, the iteration is
skipped if (number <= 0.0)
{ continue; }
sum +=
number;
}
System.out.println("Sum = " + sum);
}
}

Output:

Enter a number: 2.2 Enter a number: 5.6 Enter

a number: 0 Enter a number: -2.4 Enter a
number: -3 Sum
= 7.8

In the above program, notice

the line, if (number < 0.0)

{ continue; }

This means when the user input negative

numbers, the current iteration of the loop is
skipped. And the next iteration is started.

Java continue and Nested Loop

In the case of nested loops, the continue skips

the current iteration of the innermost loop.
Working of the continue statement with Nested
Loops

Lesson 10
Labeled continue Statement

Till now, we have used the

unlabeled continue statement. It is used to
terminate the innermost loop . However, there
is another form of continue statement in Java
known as labeled contine.

We can use the labeled continue statement to

terminate the outermost loop as well.
Working of the labeled continue Statement in
Java

As you can see in the above image, we have

used the label identifier to specify the outer
loop. Now, notice how the continue statement
is used (continue label;).

Here, the continue statement is skipping the

current iteration of the labeled statement (i.e.
outer loop).
Then, the control of the program goes to the
next iteration of the labeled statement (outer
loop)

Example 3: labeled continue

Statement class LabeledContinue

{
public static void main(String[] args)
{
// the outer for loop is labeled as
label first: for (int i = 1; i < 6; ++i)
{
for (int j = 1; j < 5; ++j)
{
if (i == 3 || j == 2) // skips the iteration of label
(outer for loop)
continue first;
System.out.println("i = " + i + "; j = " + j);
}
}
}
}

Output:

i = 1; j = 1 i = 2; j = 1 i = 4; j = 1 i = 5; j = 1

In the above example, the labeled continue

statement is used to skip the current iteration
of the loop labeled as first.

if (i==3 ||
j==2)
continue
first;

Here, we can see the outermost for loop is

labeled as first,

first:
for (int i = 1; i < 6; ++i)
{..}
Hence, the iteration of the outer for loop is
skipped if the value of i is 3 or the value of j is
2.

Note: The use of labeled continue is often

discouraged as it makes your code hard to
understand.

Exercise programs
Write the programs With comments and
Variable Description.

Where ever
InputStresmReader and BufferedReader
classes are use use Scanner class and it's
methods for input.
Q15
class Q15
{
public static void main( )
{
for( int i=150; i<=250;i++)
{

//If the number is divisible by 5 and 6

//don't display it if(i
%5==0 && i
%6==0)
;
else if(i%5==0)
System.out.println( i);
else if(i%6==0)
 System.out.println( i);
}
}
}

Q16
// Reversing a number by using while
loop import java.util.Scanner;
class Q16
{
public static void main( )
{
Scanner sc= new Scanner(
System.in); int n=0;
System.out.println( " Enter a number ");
n=sc.nextInt( );
int rev=0,t=n;
while( n>0)
{
int r=n%10;
rev=(rev×10)+r;
n=n/10;
}
System.out.println( "Original no is : "+ t);
System.out.println( "Reverse no is :"+ rev);
}
}

Q17
// Removing 0 from the entered number.
import java.util.Scanner;
class Q17
{
public static void main( )
{
Scanner sc= new Scanner(
System.in); String n="", st="";
System.out.println( " Enter a number
"); n=sc next( );

for( int i=0; i<n.length( );i++)

{
char
ch=n.charAt(i);
if(ch!='0')
st=st+ch;
}

System.out.println("New number is : "+ st);

}
}
Q18
//Tribonic series
import java.util.Scanner;
class Q18
{
public static void main( )
{
Scanner sc= new Scanner(
System.in); int n=0, a=0,b=1,c=2,
s=0; System.out.println( " Enter a
number ");
 n=sc next( );
System.out.print( a+" "+b +"
"+c); for( int i=4; i<=n;i++)
{
s=a+b+c;
System.out.print( "
"+s); a=b;
b=
c;
c=s
;
}
}
}
Q20
//Menu driven program
import java.util.Scanner;
class Q20
{
public static void main( )
{
Scanner sc= new Scanner(
System.in); int ch=0;
System.out.println(" (1) for Pattern1\n (2
)for Pattern2");

System.out.println( " Enter your

choice :"); n=sc nextInt( );
switch(ch)
{
case 1:
String s=" ICSE";
 for( int i=0; i<s.length( );i++)
{
char ch=s.charAt(i);
for( int sp=0; sp<=i;sp++)
System.out.print(" ");
System.out.println(ch);
}
break;
case
2:
String s=" ICSE";
for( int i=0; i<s.length( );i++)
{
char ch=s.charAt(i);
for( int sp=s.length()-1; sp>=i;sp--)
System.out.print(" ");
System.out.println(ch);
}
break;
default:
System.out.println( " Wrong choice ");
}
}
}
Q22
//Pronic number
import java.util.Scanner;
class PronicNumber
{
public static void main( )
{
 Scanner sc= new Scanner(
System.in); int n=0;
System.out.println( " Enter a
no."); n=sc nextInt( );
for( int i=1; i<=n/2;i++)
{
if(i × ( i+1)==n)
{
System.out.println((i × ( i+1)+"= "+ i +"
× "+(i+1)); break;
}
}
if(i>n/2)
System.out.println(n+" is not a Pronic no ");
}
}

Q(24)
Use Scanner class for accepting the number.
Answer:
class test
{
public static void main (int n)
{
int S = 0, R;
int c = n;
while (n! =
0)
{
R = n % 10;
S = S + R;
 n = n/ 10;
}
if (c % S == 0)
System.out.println (“it is Niven
number"); else
System.out.println (“It is Not a Niven No.");
}
}

Q(26)
Q(27)
Q30
(ii)
import java.util.Scanner;
class Series
{
public static void main( )
{
Scanner sc= new Scanner(
System.in); int n=0,s=0, x=3;
System.out.println( " Enter the value of n
= "); n=sc nextInt( );
for( int i=1; i<=n;i++)
{
if(i %2==0)
s=s-(int)Math.pow(x,i);
else
s=s+(int)Math.pow(x,i);
}
System.out.println("Sum of the series is : "+s);
}
}
(vi)
import java.util.Scanner;
class Series
{
public static void main( )
{
Scanner sc= new Scanner(
System.in); int n=0,s=0, x=0;
System.out.println( " Enter the
value of n = ");
n=sc nextInt( );
 System.out.println( " Enter the
value of x = ");
x=sc nextInt( );
for( int i=1; i<=n;i=i+1)
{
s=s+(float)(x+i)/(i+2);
}
System.out.println(" Sum of the series is :"+ s);
}
}

(vii)
class Series
{
public static void main ( )
{
float
s=0f; int
f=1;
for( int i=2; i<=20;i++)
{
f=f× i; // calculating the
factorial s=s+(float)x/f;
}
System.out.println(" Sum of the series is : "+ s);
}
}

Q(31
)
Last modified: 9:31 am