Restarting While Loop after false boolean in Java

Question

I'm doing an assignment for school that requires us to find the largest of ten numbers. The numbers are between 0-9. I believe I got that part down. My problem is I'm trying to add an extra feature that is not required for the assignment. I am trying to get the loop to completely restart after the boolean statement is false and gives an error message. After I type the invalid value in, it gives the error message, but after I press "ok" it continues on to the next number. I want it to start back at the beginning of the loop.

Here's the code:

package Largest;

import java.util.Scanner;
import javax.swing.JOptionPane;

public class LargestMain {

    public static void main(String[] args) {

    int number = 0;
    String numStr = "";
    int []myArray = new int[10];
    int count = 1;
    int largest = 0;
    boolean valid = false;

    while(valid == true); {  // Loop to check validity  

        for(int i = 0; i < myArray.length; i++) {

            myArray[i] = i + 1;
            numStr = JOptionPane.showInputDialog("Please enter number " + count++ + ":");
            number = Integer.parseInt(numStr); // Converts string value to integer  

            if(number >= largest) {
                largest = number;
            }

            // If-Else if statements checks if values entered are equal to 0-9
            if(number >= 0 && number <= 9) {
                valid = true;
            }

            else if ((!(number >= 0 && number <= 9))) {
                valid = false;
            }   

            if (valid == false) {
                JOptionPane.showMessageDialog(null, "INVALID INPUT...Try Again!!!", "Results", JOptionPane.YES_OPTION);
                continue;
            }
        }

        JOptionPane.showMessageDialog(null, "The Largest Number Is: " + largest, "Results", JOptionPane.PLAIN_MESSAGE);
    }           
    }
}

I could just end the loop here by adding return:

if (valid == false) {
    JOptionPane.showMessageDialog(null, "INVALID INPUT...Try Again!!!", "Results", JOptionPane.YES_OPTION);
    return;
}

I just really want to learn how to restart the loop from the beginning. I tried search different topics, but none helped me solve my problem. Thanks for the help in advance!

Answer 1

Since you're a beginner and trying to learn, I have done a review of your code and enclosed some comments that might help you. I have posted updated code below.

1. Declarations: You should declare a variable in the innermost closure that requires it. Except largest, all other can go inside the for.

2. Your array variable did not make sense to have. Since you're keeping track of the largest as you go and not finding it at the end.

3. Control: Your /loop to check validity/ needs to be strictly around the input part, not your whole program, so you can repeat just the input statements till you're satisfied.

   public static void main(String[] args)
   {
       int largest = 0;
       for(int i = 1; i <= 10; i++)
       {
           boolean valid = false;      
           while (!valid)
           {
               String numStr = JOptionPane.showInputDialog("Please enter number " + i + ":");
               int number = Integer.parseInt(numStr);  //Converts string value to integer
   
               if (number >= 0 && number <= 9)
               {
                   valid = true;
               }
               else
               {
                   JOptionPane.showMessageDialog(null, "INVALID INPUT...Try Again!!!", "Results", JOptionPane.YES_OPTION);
               }           
           }
   
           if (number > largest)
           {
               largest = number;
           }
       }   
       JOptionPane.showMessageDialog(null, "The Largest Number Is: " + largest, "Results", JOptionPane.PLAIN_MESSAGE);
   }
   

Answer 2

To restart a loop, you would use the continue keyword. continue will skip to the next loop iteration.

When using a while loop, it'll simply restart the loop, since the loop doesn't end til valid is true. When using a for loop, itll skip to the next iteration (so if you're currently on index 5 and use continue, it'll move onto index 6 instead of staying at 5).

For nested loops, you can label the loop to specify which loop the statement is for:

firstloop:
while(valid) {

    secondloop:
    while(true) {
         continue firstloop;
    }
}

Also, no need for == true when checking a boolean. It could be represented as

while(valid) {

}

As for checking for false, valid == false, you'd use

while(!valid) {

}

Answer 3

You can use a labeled continue:

firstloop:
while(valid){
  secondloop:
  for(int i=0; i<10; i++{
    //something
    if(something){
      continue firstloop;
    }
  }
}

See the documentation for more details.

UPDATE: Also... there is no need for the condition in the else part of your if/else statement. Just doing a else{ valid=false } would be equivalent to what you're doing right now. Another option would be to simplify it even further and skip the if/else part alltogether and just have:

valid = number >= 0 && number <= 9

Answer 4

Write a recursive function, that is, a function that calls itself.

public static void DoSomething()
{
  // optionally ask the user for input at this point before processing begins.

  while(yourCondition)
  {
    // do your stuff.
  }

  // when execution reaches here the condition is no longer valid; start over.
  if(conditionToStartOver)
      DoSomething();  // start again
}

Answer 5

Your logic is almost right but you shouldn't be looping on valid in the outer loop. Remember, you want to stop the inner loop when an input is invalid. Normally the outer loop would give the user an option to exit the program.

So for example:

while(true) {
    boolean valid = true;

    for(...; ...; ...) {
        ...

        if(number < 0 || 9 < number) {
            valid = false;
            break;
        }
    }

    if(valid) {
        // show largest input dialog
    } else {
        // show invalid input dialog
    }

    // optionally ask the user if they want to exit
    // if so, break
}