Lab 7: Bug Hunt

In this lab, you will identify and fix bugs in several programs.

Instructions

We encourage you to work with a partner.

Start by downloading bug-hunt.zip, which contains the starter files for this assignment.

Each of the programs has at least one bug. To identify the bugs:

• Read through the program’s code
• Try running the program with different inputs
• Read stack traces carefully. Stack traces are the messages displayed if the program crashes.
• Add print statements to show variable values
• Compare the program’s output to calculations you perform by hand
• Trace the program by hand (e.g., by drawing a call tree)

CodeCount.java

This program should return the number of times that the string co*e appears anywhere in the given string. Note that * is the wildcard character, so any character should be accepted in this place. For example, code, cone, and cope should all count, but CODE (uppercase) shouldn’t count. For example:

> java-introcs CodeCount code
1
> java-introcs CodeCount aaaCODEcodebbb
1
> java-introcs CodeCount codebbbcone
2

Hint: If you can’t find the bug in the program, write code to test countCode() on randomly generated strings until the program crashes. This technique is known as “fuzz testing.”

PairStar.java

Given a string, this program should recursively compute a new string where identical chars that are adjacent in original string are separated from each other by a *. For example:

> java-introcs PairStar hello
hel*lo
> java-introcs PairStar xxyy
x*xy*y

Note: Don’t try to fix the program by avoiding recursive programming!

Turtle.java

This interactive program allows you to control the movement of a virtual turtle. For example:

> java-introcs Turtle
Initial coordinates: (0.00, 0.00)
Turn how many degrees? 0
Move how far? 10
Updated coordinates: (10.00, 0.00)
Turn how many degrees? -90
Move how far? 10
Updated coordinates: (10.00, -10.00)

Note: The program won’t exit until it encounters the “end of file” character. On MacOS and Linux, you can type this with “Control+D”, or on Windows “Control+Z” then “Enter”.

Note: If you try to run Turtle before compiling it, you may accidentally open the textbook authors’ Turtle class, which opens the Standard Draw window.

Submit

Upload the following files to Gradescope:

• CodeCount.java
• PairStar.java
• Turtle.java
• A completed readme.txt file, including your names, etc.
  • Also, describe each of the bugs you discovered!

Finally, be sure to indicate your group member on Gradescope.

Learning Goals

• Practice debugging
• Practice recursive programming

Acknowledgements

Some of these programs were based on CodingBat.