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Java do-while Loop



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The do-while loop is mainly used in the menu-driven programs. As you just saw in the previous chapter, if the conditional expression controlling the while loop is initially false, then the body of the loop will not be executed at all. However, sometimes it is desirable to execute the body of the loop at once, even if the conditional expression is false to start with. In other words, there are times when you would like to test the termination expression at the end of the loop rather than at the beginning to run the body of the loop at once if the conditions becomes false at first. Fortunately, Java supplies a loop to overcome this problem, i.e., the do-while loop.

The do-while loop always executes its body at least once, as its conditional expression is at the bottom of the loop.

Java do-while Loop Syntax

Following is the general form of the do-while loop :

do
{
	// body of the loop
}while(conditional-expression);

Each iteration (looping) of the do-while loop first executes the body of the loop and then evaluates the conditional expression. If the expression becomes true, the loop will repeat. Otherwise, the loop will terminates. As with all of the Java's loop, condition must be a Boolean expression.

Java do-while Loop Example

Following is a reworked version of the "tick" program (as we saw earlier in the while loop chapter) which demonstrates the do-while loop here. It generates the same output as before :

/* Java Program Example - Java do-while Loop
 * This program demonstrates the do-while loop 
 */

public class JavaProgram
{   
    public static void main(String args[])
    {
        
        int n = 10;
        
        do
        {
            System.out.println("tick " + n);
            n--;
			
        }while(n > 0);
        
    }
}

When the above Java program is compile and executed, it will produce the following output:

java do while loop

The loop in the above program, while technically correct, can be written more efficiently as follows :

do
{
	System.out.println("tick " + n);
	
}while(--n > 0);

In this example, the (--n > 0) expression combines the decrement of n and the test for zero into one expression.

Here is how it works.
First, the statement (--n) executes, decrementing the n and returning the new value of n. This value is then compared with zero. If it will greater than zero, then the loop continues, otherwise, terminates.

The do-while loop is especially useful when you process a menu selection program, as you will usually want the body of a menu loop to execute at least once. Consider the program given here, which implements a very simple help system for the Java's selection and iteration statements :

Java do-while Loop Example - Menu Selection

Here is a simple menu-driven program using do-while loop in Java:

/* Java Program Example - Java do-while Loop 
 * Using a do-while loop to process a menu selection 
 */

import java.util.Scanner;

public class JavaProgram
{   
    public static void main(String args[])
    {
        
        Scanner scan = new Scanner(System.in);
        
        char choice;
        
        do
        {
            System.out.println("Help on : ");
            System.out.println("1. if");
            System.out.println("2. switch");
            System.out.println("3. while");
            System.out.println("4. do-while");
            System.out.println("5. for\n");
            
            System.out.println("Choose any one : ");
            choice = scan.next().charAt(0);
        }while(choice < '1' || choice > '5');
        
        System.out.println("\n");
        
        switch(choice)
        {
            case '1' : System.out.println("The if :\n");
                System.out.println("if(condition)\n{\n\tstatement\n}");
                System.out.println("else\n{\n\tstatement\n}");
                break;
            
            case '2' : System.out.println("The switch :\n");
                System.out.println("switch(expression)\n{");
                System.out.println("\tcase constant: statement sequence\n\tbreak;");
                System.out.println("\t//...\n}");
                break;
                
            case '3' : System.out.println("The while :\n");
                System.out.println("while(condition)\n{");
                System.out.println("\t// body of loop\n}");
                break;
                
            case '4' : System.out.println("The do-while :\n");
                System.out.println("do\n{");
                System.out.println("\t// body of loop\n\n}while(condition);");
                break;
            
            case '5' : System.out.println("The for :\n");
                System.out.println("for(initialization; condition; iteration)\n{");
                System.out.println("\t// body of loop\n}");
                break;
        }
        
    }
}

When the above Java program is compile and executed, it will produce the following output. Output when none selected (fresh menu):

java do while

Here is the output produced when 1 is selected

do while loop in java

Output produced when 2 is selected:

do while in java

Output produced when 3 is selected:

java do while example

Output produced when 4 is selected:

java do while syntax

Output produced when 5 is selected:

do while loop java

In the above Java program, the do-while loop is used to verify that the user has entered a valid choice. If not, then the user is re-prompted. As the menu must be displayed at least once, the do-while is the perfect loop to accomplish this.


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