Explain For Loop In Java, For Loop Program And Examples With Explanation
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For Loop
Beginning with JDK 5, there are two forms of the
for
loop. The first is the traditional form
that has been in use since the original version of Java. The second is the new “for-each” form.
Both types of
for
loops are discussed here, beginning with the traditional form.
Here is the general form of the traditional
for
statement:
for( initialization ; condition ; iteration ) {
// body
}
If only one statement is being repeated, there is no need for the curly braces.
The
for
loop operates as follows. When the loop first starts, the initialization portion of
the loop is executed. Generally, this is an expression that sets the value of the loop control
variable , which acts as a counter that controls the loop. It is important to understand that
the initialization expression is only executed once. Next, condition is evaluated. This must be
a Boolean expression. It usually tests the loop control variable against a target value. If this
expression is true, then the body of the loop is executed. If it is false, the loop terminates.
Next, the iteration portion of the loop is executed. This is usually an expression that increments
or decrements the loop control variable. The loop then iterates, first evaluating the conditional
expression, then executing the body of the loop, and then executing the iteration expression
with each pass. This process repeats until the controlling expression is false.
Here is a version of the “tick” program that uses a
for
loop:
// Demonstrate the for loop.
class ForTick {
public static void main(String args[]) {
int n;
for(n=10; n>0; n--)
System.out.println("tick " + n);
}
}
Declaring Loop Control Variables Inside the for Loop
Often the variable that controls a
for
loop is only needed for the purposes of the loop and
is not used elsewhere. When this is the case, it is possible to declare the variable inside the
initialization portion of the
for
. For example, here is the preceding program recoded so that
the loop control variable
n
is declared as an
int
inside the
for
:
// Declare a loop control variable inside the for.
class ForTick {
public static void main(String args[]) {
// here, n is declared inside of the for loop
for(int n=10; n>0; n--)
System.out.println("tick " + n);
}
}
When you declare a variable inside a
for
loop, there is one important point to remember:
the scope of that variable ends when the
for
statement does. (That is, the scope of the variable
is limited to the
for
loop.) Outside the
for
loop, the variable will cease to exist. If you need
Chapter 5:
Control Statements
89
to use the loop control variable elsewhere in your program, you will not be able to declare
it inside the
for
loop.
When the loop control variable will not be needed elsewhere, most Java programmers
declare it inside the
for
. For example, here is a simple program that tests for prime numbers.
Notice that the loop control variable, , is declared inside the
i
for
since it is not needed elsewhere.
// Test for primes.
class FindPrime {
public static void main(String args[]) {
int num;
boolean isPrime = true;
num = 14;
for(int i=2; i <= num/i; i++) {
if((num % i) == 0) {
isPrime = false;
break;
}
}
if(isPrime) System.out.println("Prime");
else System.out.println("Not Prime");
}
}
Using the Comma
There will be times when you will want to include more than one statement in the initialization
and iteration portions of the
for
loop. For example, consider the loop in the following program:
class Sample {
public static void main(String args[]) {
int a, b;
b = 4;
for(a=1; a<b; a++) {
System.out.println("a = " + a);
System.out.println("b = " + b);
b--;
}
}
}
As you can see, the loop is controlled by the interaction of two variables. Since the loop is
governed by two variables, it would be useful if both could be included in the
for
statement,
itself, instead of
b
being handled manually. Fortunately, Java provides a way to accomplish
this. To allow two or more variables to control a
for
loop, Java permits you to include multiple
statements in both the initialization and iteration portions of the
for
.
Each statement is separated
from the next by a comma.
Using the comma, the preceding
for
loop can be more efficiently coded as shown here:
// Using the comma.
class Comma {
90
Part I:
The Java Language
public static void main(String args[]) {
int a, b;
for(a=1, b=4; a<b; a++, b--) {
System.out.println("a = " + a);
System.out.println("b = " + b);
}
}
}
In this example, the initialization portion sets the values of both
a
and
b
. The two comma-
separated statements in the iteration portion are executed each time the loop repeats. The
program generates the following output:
a = 1
b = 4
a = 2
b = 3