While Loops (page 1)

There are two types of loops: for loops and while loops. Some languages have a couple of variations on the while loop (e.g. until, do while).

• for loops are used when you want to repeat something a certain number of times
• while loops are when you need to repeat something an unknown number of times.
  e.g. until you reach the end of a file, your player gets killed, or until something happens.
• You can write a while loop as a for loop and vice versa, but it's not really good practice.
• Both while loops and for loops can easily be made into infinite loops

Syntax

while (condition) {
   ...
   ...
}

while (lives > 0) {
   movePlayer();
   resolveAttacks();
   ...
}

while (true) {
   ... 
}

for (;;) {
   ...
}

do {
   ...
   ...
} while (condition);

boolean done = false;
do {
   num = myScanner.nextInt();
   if (num > 0) done = true;
} while (!done);

Note that in the do-while format, you MUST end the last line with a ; (semicolon).
Remember: all lines must end in { } or ;

Example:

public static void main(String[] args) {
   int n=0;
   while (n < 15) {
      System.out.println(n + " still looping ...");
	  n++;
   }
   System.out.println("loop is over");
}

Break, Continue, and other ways of exiting loops

I've written about this in the page on for loops
Basically, . . . break; exits the loop,
continue; stops that iteration of the loop and goes back to the top.
You can also exit while loops by making the loop test condition false.

Example:

import java.util.Scanner;
/* Program to add up to 100
  by Mr. Harwood
  Sept 18, 2014
*/
public class WhileLoop {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int num, count, total;
		total = count = 0;
		
		while (total < 100) {
			System.out.print("Type in a number: ");
			if (sc.hasNextInt()) { 	//check for "int"				
				num = sc.nextInt();				
			} else {	//get rid of junk (rest of line)
				sc.nextLine();				
				continue;	//go to beginning of loop
			}
			if (num > 100) continue; //no numbers more than 100
			if (num < 0) break; //stop loop immediately for negative numbers
			if (num == 0) System.exit(0); //terminate!
			count++;
			total += num;
		}
		System.out.printf("Your total is %d, after %d numbers",total,count);
	}

}

READ THIS! you should never use System.exit(0) unless you have a very specific and good reason to do so. It is not good programming technique and should be avoided.
If this were an important program, I'd write this instead: if (num == 0) return . This is a better way, but we haven't learnt about "return" yet.

While loops page 2