Code in while loop doesn't execute

Question

I am playing with Java and want to do a simple while loop that keeps going until the user presses ctrl+z.

I have something like this:

public static void main(String[] args) {

    //declare vars
    boolean isEvenResult;
    int num;

    //create objects
    Scanner input = new Scanner(System.in);
    EvenTester app = new EvenTester();

    //input user number
    System.out.print("Please enter a number: ");
    num = input.nextInt();

    while() {

        //call methods
        isEvenResult = app.isEven(num);

        if(isEvenResult) {
            System.out.printf("%d is even", num);
        } else {
            System.out.printf("%d is odd", num);
        }

    }//end while loop

}//end main

I tried while( input.hasNext() ) { ... but the code inside the while loop wouldn't execute.

True Soft · Accepted Answer · 2009-11-04 17:17:20Z

3

//input user number
System.out.print("Please enter a number: ");

do {
    try {
        num = input.nextInt();
    } catch (Exception e) {
        break;
    }
    // call methods
    isEvenResult = app.isEven(num);

    if (isEvenResult) {
        System.out.printf("%d is even", num);
    } else {
        System.out.printf("%d is odd", num);
    }
} while (true);

When the user writes something non-numeric, the loop breaks.

answered Nov 4, 2009 at 17:17

True Soft

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2 Comments

Joe Over a year ago

this is what I was looking for, except it won't let me enter <ctrl> z to exit the loop

ChadNC Over a year ago

if you want to respond to multiple keys being pressed then it's best to implement KeyBindings. You must listen to what keys are being pressed to achieve what you are asking.

Gandalf · Accepted Answer · 2009-11-04 17:10:16Z

1

while (num != 'z')

Although if you are expecting a 'z' why are doing input.getInt()?

You may want to check out the Console class too.

answered Nov 4, 2009 at 17:10

Gandalf

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2 Comments

rsp Over a year ago

I think he wants to end with control-Z, thus ending the input stream.

Gandalf Over a year ago

He changed the question after I answered it unfortunately.

JRL · Accepted Answer · 2009-11-04 17:18:36Z

1

If you want to loop until the user has to force break via Ctrl+Z, then just do while(true). But you want your nextInt() to be inside the loop, and maybe also your prompting statement.

answered Nov 4, 2009 at 17:18

JRL

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Comments

Carl Smotricz · Accepted Answer · 2009-11-04 21:33:16Z

TrueSoft's solution is dead on. The reasons it may not be working for the asker is are kinda outside the scope of the program.

The program works for me: I'm running it under Linux and enter Ctrl-D as the first thing on a line. Ctrl-D is end-of-file for Linux the same way that Ctrl-Z is for Windows. Program stops dead in its tracks, perfectly.

The Windows console (the black DOS box, whatever you want to call it) has a wrinkle: It reads input line-by-line. It won't see the Ctrl-Z until it's read the line, so it needs an Enter keyin before it will see the Ctrl-Z.

I'm unwilling to fire up Windows just to try this, but my guess is that CTRL-Z followed by the Enter key (just like after the number entries) should cause the program to stop cleanly.

There are system-y ways to make a Java program work on a character-by-character basis so you can handle any characters directly and respond immediately to Ctrl-Z. But that's advanced stuff and doesn't belong in a simple programming exercise like this. I think Ctrl-Z / Enter is an acceptable way to have the program end.

ChadNC · Accepted Answer · 2009-11-04 17:18:44Z

0

You need to implement KeyBindings. Then you can make the determination to exit based off of what keys were pressed.

answered Nov 4, 2009 at 17:18

ChadNC

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Comments

Kushal Paudyal · Accepted Answer · 2009-11-04 17:30:29Z

0

you are doing the input outside the loop,and it will run for only once.

System.out.print("Please enter a number: ");
num = input.nextInt();

Put your above code inside the loop.

Since you are having a system out inside the loop you will also come to know whether the control went inside the loop, obviously it should.

Also, try

while(true)

I wonder if while() alone is working.

answered Nov 4, 2009 at 17:30

Kushal Paudyal

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Comments

Jim · Accepted Answer · 2012-07-23 22:26:59Z

This looks like Exercise 6.16 out of Deitel's book Java How to Program, 9th Edition.

The CTRL-Z charcter does, indeed, end input on a Windows platform just as CTRL-D ends input on most any UNIX or Linux platform.

Also, there are logic errors in the construction of the program that indicate Scanner methods and the System.in byte stream (i.e. the standard input from the console) are not well understood.

In your posted program, the statement:

    num = input.nextInt();

executes unconditionally. It will block execution until some kind of input is received. If the input is not an integer, it will throw an exception. If the input received is an integer, then num will be assigned the integer value and the integer in the input stream (input) will be discarded from the input stream. There may be remainaing stuff on the input line up to the end of line, depending on what the user typed in before hitting the enter key that ended the input line and placed it into the System.in byte stream that Scanner is scanning.

If you were to leave your program as written except for putting input.hasNext() into the while statement's test condition, it would block until more input was in the input stream after the integer that nextInt() processed.

Some answer(s) suggest using KeyBindings as a solution. Whilst that may work, it gets into waiting for keypress events at nearly the hardware level and is NOT friendly to platform independence. It is a potential rabbit-hole into Alice's Wonderland for having to figure out all kinds of event processing and the code having to know what platform it is running on. Using the hasNext() boolean false return to indicate the end of the input stream should work on any platform and will avoid potentially non-portable gee-whiz code for processing the keyboard and key presses at nearly the hardware event level.

The following program is one that does what you (and the exercise) intended and will end the input if the user presses CTRL-Z on a Windows platform or a CTRL-D on a UNIX/Linux platform without you having to determine the platform on which the code is executing.

// Exercise 6.16: EvenOrOddTest.java
// Write a method isEven that uses the remainder operator (%)
// to determine whether an integer is even. The method should
// take an integer argument and return true if the integer is
// even and false otherwise. Incorporate this method into an
// application that inputs a sequence of integers (one at a time)
// and determines whether each is even or odd.
import java.util.Scanner;

public class EvenOrOddTest {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        int integer;
        System.out.println("Odd even integer test.");
        System.out.printf("Input CTRL-Z on Windows or CTRL-D on UNIX/Linux to end input\n"
            + "or an integer between values\n"
            + "%d and %d\n"
            + "to test whether it is odd or even: ",
            Integer.MIN_VALUE, Integer.MAX_VALUE);
        // the input.hasNext() will block until
        // some kind of input, even a CTRL-Z,
        // arrives in the stream
        // the body of the while loop will execute
        // every time input appears for as long as the input
        // is not a CTRL-Z
        while (input.hasNext()) { // repeat until end of input
            // prompt user
            // now see if the input we did get is an integer
            if (input.hasNextInt()) { // we got an integer...
                integer = input.nextInt();
                System.out.printf("\n%d is "
                        + (EvenOrOdd.isEven(integer) ? "even.\n\n" : "odd.\n\n"), integer);
            } else { // we got a non-integer one too large for int
                System.out.printf("\nInput %s invalid! Try again...\n\n", input.next());                
            } // end if...else
            // white space (i.e. spaces and tabs) are separators
            // next and nextInt get only to the first separator
            // so it is possible for the user to enter an integer
            // followed by tabs and/or spaces followed by more
            // input, integer or not up to the end of the input line
            // input.nextLine() flushes everything not processed 
            // by the nextInt() or next() to the input line end 
            // won't block execution waiting for input
            // if there is nothing left on the input line
            input.nextLine();
            // prompt for user input again
            System.out.printf("Input CTRL-Z to end input\n"
                    + "or an integer between values\n"
                    + "%d and %d\n"
                    + "to test whether it is odd or even: ",
                    Integer.MIN_VALUE, Integer.MAX_VALUE);
        } // end while
    } // end main

    static boolean isEven(int integer) {
        // integer modulus 2 is zero when integer is even
        return ((integer % 2) == 0);
    } // end isEven
} // end class EvenOrOddTest

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