Problem 56

do-while Swimming

So far you have only worked with one type of loop: the while loop. But there is another type: the “do-while” loop.

The do-while loop works almost exactly like a while loop. In fact, most of the time they are equivalent. Examine the program below to see if you can figure out the tiny difference.

Files Needed: DoWhileSwimming.java

import java.util.Scanner;

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

        String swimmer1 = "GALLANT";
        String swimmer2 = "GOOFUS ";

        double minimumTemperature = 79.0; // degrees Fahrenheit

        System.out.print("What is the current water temperature? ");
        double currentTemperature = keyboard.nextDouble();
        double savedTemperature = currentTemperature; // saves a copy of this value so we can get it back later.

        System.out.println("\nOkay, so the current water temperature is " + currentTemperature + "F.");
        System.out.println(swimmer1 + " approaches the lake....");

        int swimTime = 0;
        while (currentTemperature >= minimumTemperature) {
            System.out.print("\t" + swimmer1 + " swims for a bit.");
            swimTime++;
            System.out.println(" Swim time: " + swimTime + " min.");
            Thread.sleep(600); // pauses for 600 milliseconds
            currentTemperature -= 0.5; // subtracts 1/2 a degree from the water temperature
            System.out.println("\tThe current water temperature is now " + currentTemperature + "F.");
        }

        System.out.println(swimmer1 + " stops swimming. Total swim time: " + swimTime + " min.");

        currentTemperature = savedTemperature; // restores original water temperature

        System.out.println("\nOkay, so the current water temperature is " + currentTemperature + "F.");
        System.out.println(swimmer2 + " approaches the lake....");

        swimTime = 0;
        do {
            System.out.print("\t" + swimmer2 + " swims for a bit.");
            swimTime++;
            System.out.println(" Swim time: " + swimTime + " min.");
            Thread.sleep(600);
            currentTemperature -= 0.5;
            System.out.println("\tThe current water temperature is now " + currentTemperature + "F.");
        } while (currentTemperature >= minimumTemperature);

        System.out.println(swimmer2 + " stops swimming. Total swim time: " + swimTime + " min.");
    }
}

What You Should See

Goofus and Gallant are both going swimming. They hate to swim in cold water; once the water temperature drops below 79°F, they stop.

Run the program, and type in 80.5 for the water temperature.

What is the current water temperature? 80.5

Okay, so the current water temperature is 80.5F.
GALLANT approaches the lake....
        GALLANT swims for a bit. Swim time: 1 min.
        The current water temperature is now 80.0F.
        GALLANT swims for a bit. Swim time: 2 min.
        The current water temperature is now 79.5F.
        GALLANT swims for a bit. Swim time: 3 min.
        The current water temperature is now 79.0F.
        GALLANT swims for a bit. Swim time: 4 min.
        The current water temperature is now 78.5F.
GALLANT stops swimming. Total swim time: 4 min.

Okay, so the current water temperature is 80.5F.
GOOFUS  approaches the lake....
        GOOFUS  swims for a bit. Swim time: 1 min.
        The current water temperature is now 80.0F.
        GOOFUS  swims for a bit. Swim time: 2 min.
        The current water temperature is now 79.5F.
        GOOFUS  swims for a bit. Swim time: 3 min.
        The current water temperature is now 79.0F.
        GOOFUS  swims for a bit. Swim time: 4 min.
        The current water temperature is now 78.5F.
GOOFUS  stops swimming. Total swim time: 4 min.

What You Should Do on Your Own

Assignments turned in without these things will receive no credit.

1. Run the program, and type in 80.5 for the current water temperature. Do Goofus and Gallant swim for the same amount of time? Put your answer in a comment.
2. Run the program again, but this time enter 78 for the starting temperature. What changes?
3. Does Gallant check the water temperature first, or does he just dive right in?
4. What about Goofus? Does he check the water temperature first or just dive in?
5. What is the difference between a while loop and a “do-while” loop?
6. One of these loops is sometimes called a “pre-test loop”, and the other is called a “post-test loop”. Which one is which?