Problem 51

Counting with a While Loop

Type in the following code, and get it to compile. This assignment shows you how we can abuse a while loop to make something repeat an exact number of times.

import java.util.Scanner;

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

        System.out.println("Type in a message, and I'll display it five times.");
        System.out.print("Message: ");
        String message = keyboard.nextLine();

        int n = 0;
        while (n < 5) {
            System.out.println((n+1) + ". " + message);
            n++;
        }
    }
}

Normally, while loops are best for repeating as long as something is true:

• Keep going as long as they haven’t guessed it.
• Keep going as long as you haven’t got doubles.
• Keep going as long as they keep typing in a negative number.
• Keep going as long as they haven’t typed in a zero.

But sometimes, we know in advance how many times we want to do something.

• Do this ten times.
• Do this five times.
• Pick a random number, and do it that many times.
• Take this list of items, and do it one time for each item in the list.

We can do that sort of thing with a while loop, but we have to use a counter. A counter is a number variable (usually int) that starts with a value of 0, and then we add 1 to it whenever something happens. So, here, we’re going to be adding 1 to the counter everytime we repeat the loop. And when the counter reaches a predetermined value, we’ll stop looping.

What You Should See

Type in a message, and I'll display it five times.
Message: All work and no play makes Jack a dull boy.
1. All work and no play makes Jack a dull boy.
2. All work and no play makes Jack a dull boy.
3. All work and no play makes Jack a dull boy.
4. All work and no play makes Jack a dull boy.
5. All work and no play makes Jack a dull boy.

What You Should Do on Your Own

Assignments turned in without these things will receive no credit.

1. What does n++ do? Remove it and see what happens. (Then put it back.)
2. Change the code so that the loop repeats ten times instead of five.
3. See if you can change the code so that the message still prints ten times, but the numbers in front count by tens, like so:

Type in a message, and I'll display it ten times.
Message: I'm sending out an S.O.S.
10. I'm sending out an S.O.S.
20. I'm sending out an S.O.S.
30. I'm sending out an S.O.S.
40. I'm sending out an S.O.S.
50. I'm sending out an S.O.S.
60. I'm sending out an S.O.S.
70. I'm sending out an S.O.S.
80. I'm sending out an S.O.S.
90. I'm sending out an S.O.S.
100. I'm sending out an S.O.S.

4.Change the code so that it asks the person how many times to display the message. Then, print it that many times. Still count by tens.

Type in a message, and I'll display it several times.
Message: HELLO! My name is Inigo Montoya. You killed my father; prepare to die.
How many times? 3
10. HELLO! My name is Inigo Montoya. You killed my father; prepare to die.
20. HELLO! My name is Inigo Montoya. You killed my father; prepare to die.
30. HELLO! My name is Inigo Montoya. You killed my father; prepare to die.