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Objective:
Learn
about:
Concept:
Loops
Often
when writing a program, it is necessary
to repeat a statement or statement
block many times. This objective is
accomplished with a repetition statement.
These repetition statements are called
loops. Three repetition statements
are the for, while and do-while statements.
Every
repetition statement has three parts:
-
Initializing
loop variable
-
Loop
terminating statement
-
Incrementing
loop variable
for
loop
The
for statement is a repetition statement
that is particularly well suited for
executing the statements of a loop
for a specific number of times. A
for loop is usually used when we have
to execute some statements for a fixed
number of times.
Syntax
for
(initialization; condition; increment)
{
statement / body;
}
Example
for
(int i = 0; i < 10; i++) {
System.out.println(“Your number
is: “ + i);
}
while
loop
The
while statement is a repetition statement
that is particularly used to execute
the body of a loop, subject to a certain
condition. A while loop is usually
used when we want specified statements
to be continuously executed to a certain
condition.
Syntax
While
(condition) {
Statement / body;
}
Example
int
i = 0;
while (i < 10) {
System.out.println(“Your number
is “ + i );
i++;
}
Difference
between for and while Loops
The
while loop is good when you don’t
initially know how many times you
want to execute the loop. The for
loop is suitable when executing the
body of a loop for a specific number
of times.
Do-while
loop
The
do-while statement is similar to the
while statement, except that it’s
termination condition is at the end
of the body of the loop. The do-while
loop executes the statement in the
loop until the condition becomes false.
The condition is evaluated at the
end of the loop; therefore, the body
of the loop is always executed at
least once. The syntax of do-while
loop is:
Syntax
do
{
statement;
}
while (condition);
Difference
between while and do-while loops
The
difference between the while and do-while
loops is obvious from the syntax.
In the while loop the loop condition
is evaluated first and then the body
of the loop executes. In the do while
loop the body is executed once and
then the condition is checked for
the loop execution. In the do-while
loop there is a semicolon at the end
of the while (condition), statement.
Example
int
i = 10;
do {
System.out.println(“The Number
is: “ + i);
i++;
} while (i < 10);
Nested
for statement
In
the nested for loop, just like in
the nested if statement, we declare
a for loop in the body of another
loop. This forms a block of for loops.
We can declare as many for loops in
a block as we want.
Syntax
for
(initialization; condition; increment)
{
for (initialization; condition;
increment) {
statement / body;
}
statement;
}
Mode
of Execution
First
of all the outer for loop will execute
one time. If the condition is true,
then program control will shift to
the next for loop. If the conidtion
in the second for loop is true then
the body of the inner for loop will
execute. It is to be noted there that
until the inner loop has not executed
completely, the control will not shift
to the outer loop. When the program
control shifts to the outer loop then
there will be increment / decrement
in the variable as specified in the
outer loop.
Example
for
(int i = 0; i < 5; i++) {
for (int k = 0; k < 5; k++)
{
System.out.println(“The number
of k is: “ + k);
}
System.out.println(“The number
of i is: “ + i);
}
Special
Loop Flow Controls or Jump Statements
Continue
The
continue statement is used to skip
over and jump to the end of the loop
body. Some times it is useful to force
an early iteration of a loop. That
is, you might want t continue running
the loop, but stop processing the
remainder of the code in its body
for this particular iteration. In
while and do-while loops, a continue
statement causes control to be transferred
directly to the conditional expression
that controls the loop. In the for
loop, control goes first to the iteration
portion of the for statement and then
to the conditional expression.
Example
class
Continue {
public static void main(String
args[]) {
for (int x = 0; x < 10;
x++) {
System.out.println(x + " :");
if (x % 2 == 0) continue;
System.out.println("
:");
}
}
}
Break
In
java, the break statement has
three uses. First it terminates a
statement sequence in a switch statement.
Secondly, it can be used to exit a
loop. Thirdly, it can be used as a
civilized for of goto.
By
using break, you can force immediate
termination of a loop, bypassing the
conditional expression and any remaining
code in the body of the loop. When
a break statement is encountered inside
a loop, the loop is terminated and
program control resumes at the next
statement following the loop.
Example
class
BreakLoop {
public static void main(String
args[]) {
for (int i = 0; i < 100;
i++) {
if (i == 10) break;
System.out.println("i:
" + i);
}
System.out.println("Loop
complete");
}
}
When
used inside a set of nested loops,
the break statement will only break
out of the innermost loop.
Example
class
BreakLoop2 {
public static void main(String
args[]) {
for (int i = 0; i < 3; i++)
{
System.out.println("Pass
" + i);
for (int j = 0; j < 100;
j++) {
if (j == 10) break;
System.out.println(j + "
");
}
System.out.println();
}
System.out.println("Loop
Complete");
}
}
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