spacer.gif spacer.gif


Being able to repeatedly execute statements of interest is a powerful programming mechanism. In this laboratory you will gain practice with the Java while looping statement.

Key Concepts

  • while statement
  • Common looping problems

Getting Started

Using the procedures in the previous laboratories (i.e. right-click on the link and select "save as"), copy the files,, and for your manipulation.  All three of the Java files need to be completed for this lab.

Determining sums of numbers

The first two while examples are concerned with computing sums of consecutive numbers. Although simple in nature, they illustrate nicely while loop behavior.

  • Open the file
  • Consider the following while loop-based program that displays the sum of the integers in the range 1 to maxNumber, where maxNumber is a user-supplied positive value. If maxNumber is 4, then the program displays the sum of 1 + 2 + 3 + 4,, which is 10. The program contains one logic error (bug). Attempt to find it. A common debugging technique is to step through code manually (i.e., pretend you are the computer). Try three stepping through times – using in turn the values 1, 3, and 4.
  • If you cannot find the error in the program, you should run it using different input values to see what is actually happening (e.g., 1, 3, and 4).  If that does not help, as a TA for assistance.
  • Once the bug is found, fix it.
  • Run the program using the values 1, 3, 4, -5, and 0. Does the program work correctly now? Once it is correct, close program
  • Open the file The intended purpose of is to calculate the sum of integers from a user-specified interval minNumber to maxNumber. Thus, is a generalization of, whose minNumber is always 1.
  • As written, just prompts its users for the two values and checks that the extracted inputs are sensible. The program does not define the index variable whose value repeatedly takes on the numbers in the interval nor does it contain the loop that sums those numbers. Using the code from as a model, add a while loop that sums the numbers from the user-specified interval.
  • Complete the program and compile it. Test the program to make sure it is working correctly.
  • To test your program some more – complete the comments at the end of the program to show the result of runs using the following inputs. In your comments report both the expected and actual results
    • 1 and 4
    • -4 and -1
    • -3 and 3
    • 5 and 5
    • 8 and 6
  • Close program

Text conversion

Often programs analyze text to see if it has some particular property. The next example program counts the number of uppercase letters in the text that it extracts. The program extracts its values from the standard input, which by default comes from the keyboard.  You will need to add the code so that the program also counts the total number of characters in the input.

  • Open the program
  • Examine the program to get a sense of how it accomplishes its task.
    • The outer while loop condition is stdin.hasNext().  This means that while there is still input to be read in from the keyboard, this method will return true.  When there is no more input to be read in (i.e. when the user enters Ctrl-Z), it will return false.
    • The first line in the outer while loop creates an input object which holds the current line read in from the keyboard.
    • An inner while loop iterates through this input String so that it can analyze each individual character.
      • For each character, if it is upper case, then the numberUpperCase variable is incremented.  This is done via a call to the Character.isUpperCase() method.
      • Either way, i is incremented, as it is the counter variable for the inner while loop.
  • Compile and run the program. It is important to observe that the program does not issue a prompt to the user to start supplying text. The program just waits silently for its input.  This is why we always put a legend in our programs (note that you don't have to for this lab).
  • Provide the following text as input. When finished, you need to signal the program that you are done supplying standard input. On Windows-based PCs, a line consisting solely of a Control-Z followed by enter produces the signal; most other systems often use Control-D followed by enter.

    Java is great!
    This course is FANTASTIC.
    This Line Has a Lot of Uppercase Letters
  • Modify the program to count also the total number of characters (including spaces and punctuation). The modification requires an additional counter variable. When defining the variable, provide a comment that justifies its initial value. This new variable should be incremented once per non-end of stream value.
  • Have your program print out the total number of characters
  • Test your program. Once it is completed correctly, close program

Finishing up

  • Copy any files you wish to keep to your home directory.
  • Delete all of the files you copied or created on the laboratory machine.
  • Have a great spring break!
spacer.gif footer-middle.gif spacer.gif