Programming Tutorials

for loop in java

By: Abinaya in Java Tutorials on 2007-09-07  

Here is the general form of the 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 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, i, is declared inside the 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 / 2; 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 {
    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

Note : If you are familiar with C/C++, then you know that in those languages the comma is an operator that can be used in any valid expression. However, this is not the case with Java. In Java, the comma is a separator that applies only to the for loop.