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Mobius strip

Assignment 11


Write a program that reads in an arbitrary number of numbers and then displays the total and average.

Concepts: while loops, for loops; Integer.parseInt(), Double.parseDouble()
Textbook: 5.5, 5.8; 3.8


Use a loop to allow the user to enter as many doubles as they like. Keep a running total (and count) of the numbers entered. Display the current total after each number entered. If the user enters anything other than (or in addition to) a double, print an error message and ignore that input, but keep reading numbers. If the user enters nothing, the program should stop reading numbers.

Once the user has indicated that they are done entering numbers, print the sum of all the entered numbers and their average.

Sample Output

D:\TA\grading\A11>java ZtomaszeA11
This program will average a series of numbers.
Enter a number (or nothing to stop): 10
Total: 10.0
Enter a number (or nothing to stop): 5.5
Total: 15.5
Enter a number (or nothing to stop): 1o
Bad input: please enter only numbers (or, to quit, just hit enter).
Enter a number (or nothing to stop): oops
Bad input: please enter only numbers (or, to quit, just hit enter).
Enter a number (or nothing to stop): total ?
Bad input: please enter only numbers (or, to quit, just hit enter).
Enter a number (or nothing to stop): 9.8
Total: 25.3
Enter a number (or nothing to stop): 1.1
Total: 26.400000000000002
Enter a number (or nothing to stop):

Total: 26.400000000000002
Average: 6.6000000000000005

D:\TA\grading\A11>java ZtomaszeA11
This program will average a series of numbers.
Enter a number (or nothing to stop): 5
Total: 5.0
Enter a number (or nothing to stop):

Total: 5.0
Average: 5.0

D:\TA\grading\A11>java ZtomaszeA11
This program will average a series of numbers.
Enter a number (or nothing to stop):

Total: 0.0
Average: 0

Note how the user might not enter a number at all; or they many enter multiple errors in a row. Your program should still handle this gracefully. Also, note that whenever you do math with doubles, there's always the possibility that the result will not be exact. Don't worry about that.


  • You won't be able to use nextDouble() to read in the numbers for this one. If you did, the program would hang until the user entered one or more non-white-space characters.
  • Instead, use nextLine() to read in everything they typed. If this is an empty string, they mean to quit; otherwise, turn it into an double using Double.parseDouble(). Remember that Double.parseDouble() throws a NumberFormatException if it cannot turn the given string into a double.

What to Submit

Upload your file to Tamarin.

Grading [4 points]

1 - Compiles + Coding Standards
Your program compiles successfully (no errors). Your code follows Java coding standards.
1.5 - Read loop
Your program reads in an arbitrary number of doubles (0.5). The program does not crash if given something other than a double (0.5). The loop ends if the user enters nothing (0.5).
1.5 - Results
You print the total (0.5) and average (0.5), clearly labelled. Results are still correct even when incorrect input is handled (0.5).


Problem set from lab
Practice for students (individually or in small groups) in lab. Use a loop to do the following:
  1. Parrot: echo the user's input until the user enters nothing.
  2. Print all the words to the song "99 Bottles of Beer"
  3. Print all the powers of 2 that are less than 5000
  4. Print backwards all the positive even integers starting with 100. (That is: 100, 98, 96, 94.... 2.)
  5. Print the numbers 1 through 10 on one line, with a comma after each except the last one. So, the list should look exactly like: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.
  6. Hi-Lo number game: The computer picks an int between 1 and 100 and then tells the user whether each guess is too high or low until the user guesses the number correctly.

Challenge problems (for more practice or if you finish the above early):
  • Without looking at an ASCII chart, print a conversion table showing the numerical ASCII values of all the lowercase letters. (Hint: int letter = 'a'; letter++;)
  • Print the full lyrics of the song "12 Days of Christmas" (Hint: a fall-thru switch)
  • The reverse of #6: the user picks the number and tells the computer whether its guesses are too high or too low until the computer guesses it right. Done correctly, the program should be able to determine the number within 7 guesses (assuming the human doesn't cheat). (Hint: binary search)
Sample solutions:

I suggest looking at the solutions only after you've done the problems (or can't figure out how to do them). Remember, there's always more than one way to do it, so you may come up with some solutions that differ wildly from those given in this file--that's fine, as long as they produce correct output. You should also be able to understand how my solutions work too, though.

How do I tell if the user entered nothing?
Compare the String that they entered to the empty String: "". (That's a String with zero characters in it.) Or see if the length of the string is 0. (Please do not use the isEmpty() method, though; it's not available in Java 5, so your code won't compile on Tamarin.)
Argh, I'm stuck!
First, have a look at the sample solutions to the problem set. In particular, the first problem (which we did in lab) involves a loop that needs to stop when the user enters nothing.

It usually helps me to write a hard program one step at a time, where I compile, run, and test after each step. With loops, I often find myself working from the inside out. Ignore the loop to start with and just try to get your program to compile/work for only a single number: read in the user's input as a String and then convert it to a number. Maybe then add the error handling for times when the String cannot be turned into a number.

Now this is the code you want to repeat over and over, so see if you can wrap a while loop around it. You want the loop to stop when the String input (before you convert it to a number) is empty. Of course, you may find that you read the string in half-way through the loop body, so you'll probably also need to use an if/else somehow in the while loop body to avoid trying to convert the empty string into a number and adding it to the total.
How does parseDouble work again?
String input = ...;
double num = Double.parseDouble(input);
Remember that it might throw a NumberFormatException if input cannot be converted to a meaningful number. Both the exception and the Double class are in java.lang, so no import is required to use them.
My average comes out as NaN if the user enters no numbers.
This is short for "not a number" because you're dividing by 0. (This happens with doubles; remember that dividing ints by 0 causes a runtime crash--an ArithmeticException.) This is acceptable output. If you want to avoid it, just use a conditional to check that your number count isn't 0 before you try dividing.
Things to note:
• Please declare your Scanner before the loop and not in the loop. This way, you only create one Scanner object, rather than creating a new Scanner for each number. Either way will work for you, but creating a new Scanner each time is inefficient.

~ztomasze Index : TA Details: ICS111: A11
Last Edited: 10 Oct 2009
©2008 by Z. Tomaszewski.