Loops and the Do-While Loop

You've seen for and while loops before, but this node will review when to use each one and discuss a variety of the while loop.

for & while loops review

If you know how many times a loop will run, you usually should use a for-loop, since you can setup the index variable, condition and increment all in the loop header. For example, the loops below will both print the numbers from 1 to 10, but the for loop has clearer code:

While Loop

int index = 1;
while(index <= 10){
  System.out.println(index);
  index = index + 1;
}

For Loop

for(int index = 1; index <= 10; index=index+1){ 
  System.out.println(index);
}

While Loop Needed

In cases that do not depend on a index variable being incremented, you'll need a while loop instead of a for loop. This is often the case when your loop needs to run as long as an external condition is true, such as:

• while a game character is alive
• while a key or button is pressed
• while a user-set variable is an incorrect value.

For example, here's some code for a really simple guessing game. As long as the user does not guess 5 the program will prompt him for another number. Since it will loop until that number is guessed, it needs a while loop:

public static void guessGame()
{
    System.out.println("Welcome to the guessing game.");
    System.out.println("Guess a Number");
    int num = getInput(); //a method which gets user's number input
    while(num != 5){
       System.out.println("Guess A Number");
       num = getInput();
    }
    System.out.println("5 is Correct!");
}

The above code is pretty simple, so click on the "Menu Example" button below for a similar but larger example. The challenge on the bottom will be to improve some similar code.