Java Loops - Break? Continue?

Question

I've read through a bunch of threads on using break and continue and I suspect the problem isn't necessarily my use of those, but the layout of my loops. In the following code, I am trying to iterate through the chars in a string that is input by the user to find any - symbols. If found, it will throw an error to the user that a negative number was found and exit. Otherwise, if it does not find a - symbol, it should print out all of the chars in the string.

I used break at the end of my first loop to find the - symbol, but it is not continuing on to the next loop. I tried continue as well but that didn't work. Loops are new to me so I may have this completely wrong, all I know is my first loop is working OK and will throw the error when it finds a - in the string.

strNum1 = JOptionPane.showInputDialog ("Enter Number String");
for (int i = 0; i < strNum1.length(); i++) {
  char c = strNum1.charAt(i);
  if (c == '-') {
    System.out.println("Negative Digit Found - Exiting");
    break;
  }
}

for (int i = 0; i < strNum1.length(); i++) {
  char c = strNum1.charAt(i);
  if (c <= 9) {
    System.out.println(c);
  }
}

Answer 1

The break statement breaks you only from the first loop. In order to skip running the second loop in the event of finding a - character, you can use some boolean variable to indicate whether the second loop should run :

strNum1 = JOptionPane.showInputDialog ("Enter Number String");
boolean isValid = true;
for (int i=0; i<strNum1.length(); i++) {
        char c = strNum1.charAt(i);
        if (c == '-'){
            System.out.println("Negative Digit Found - Exiting");
            isValid = false;
            break;
        }
}
if (isValid) {
    for (int i=0; i<strNum1.length(); i++) {
        char c = strNum1.charAt(i);
        if (c <= '9'){
            System.out.println(c);
        }
    }
}

Answer 2

If you replace the break with a return it will exit the whole method. It sounds like this is probably what you want.

Answer 3

'break;' will stop the loop that it is in from running, where 'continue;' will skip the current 'iteration' in the loop.

 for(int x = 0; x < 10; x++)
 {
     if (x == 5)
        break;
      // other code
 }
 // more other code

This will exit the loop once x == 5, and not do the 6th through 10th iterations.

 for(int x = 0; x < 10; x++)
 {
     if (x == 5)
        break;
      // other code
 }
 // more other code

This will do every iteration, besides the 6th iteration.

But if you want to skip the '// more other code', then you would need to use a 'return;', provided your code is in a function, and it will skip the rest of the function, which in this case is the '// more other code'.

Answer 4

Use the return statement instead of break if you dont want to execcute the second loop after the first one.

Answer 5

You don't say if the number should be an integer, so I'm assuming yes. If so, instead of using loops, a better way of validating the input would be:

int num1;

try {
  num1 = Integer.parseInt(strNum1);
catch (NumberFormatException e) {
  //not a valid number, complain about it
}

if (num1<0) {
  //negative number not allowed, complain about it
}

Answer 6

 for (int i=0; i<strNum1.length(); i++) {
    char c = strNum1.charAt(i);
         if (c == '-'){
            System.out.println("Negative Digit Found - Exiting");
        break;
        }
 }

can be modified as

 if(strNum1.charAt(0) != '-'){
   for (int i=0; i<strNum1.length(); i++) {
    char c = strNum1.charAt(i);
    if (c <= 9){
        System.out.println(c);
    }
   }
 }
else {
 //negative number found...
 }

In This way, unnecessary for loop and break statements can be avoided

Answer 7

All answers are good, but if you want to repeat prompt until you get a valid value, you will need another loop for that, using labels:

negative: while(true) {
    strNum1 = JOptionPane.showInputDialog ("Enter Number String");
    for (int i=0; i<strNum1.length(); i++) {
        char c = strNum1.charAt(i);
        if (c == '-'){
            System.out.println("Negative Digit Found - Exiting");
        continue negative;
        }
    break negative;
    }
}