Annoucements

1
Annoucements

• Copy of textbook (1st edition) is in the library
  on two hour reserve
• Reminder, homework 0 due Friday
• You should be starting programming assignment 1
• Shelby Rollins - tell me your e-mail address

2
CS 162 - Spring 2005

• Chapter 7, 8 - Problem Solving, Testing and
  Debugging

3
How do you go about solving complex problems?

• Abstraction -- dont get bogged down in details
  until you must
• Take small steps (at the right level of
  abstraction!)
• Sometimes look at the problem top down, sometimes
  bottom up, usually a little bit of both

4
Your Programming Assignment

• Take, for example, your first programming
  assignment.
• Write a program that reads lines from the input.
• Which are then broken into words
• ... Which are then scrambled and output

5
Top Down Design

• Top down development -- first address the problem
  of how to read a line of text from the standard
  input. Test by writing to the standard output
• Note that the test code (often called a stub)
  isnt going to be part of the final application.
  But it helps you get this part right.
• How do you test? What are some boundary cases?
• When you have this part working, go on ..

6
Next Step, Break into Words

• Next step. After reading a line of text, break
  into words.
• Questions -- how to find start? How to find end?
  How to display the result so you know it is
  correct?
• Testing? What are some exceptional conditions?
  Boundary conditions?

7
Next Step, Scramble Words

• Next Step. Having found a word, how to scramble?
• Loop over all but first and last character,
  select another character, and swap.
• Again, test to make sure you are doing right
  thing. What are good test cases?

8
Bottom Up Development

• We described an approach that was top down.
  Sometimes it is useful to address a problem
  bottom up. (Maybe you have only a part of a
  larger task, maybe a particular problem seems
  difficult and/or interesting)
• Example, suppose you want to address the scramble
  aspect by itself.

9
Word Scramble as separate problem

• So now we are thinking of word scramble as a
  problem that is independent of the larger task.
  First, describe the problem in terms of input/out
• String wordScramble(string word)
• Result has same size as argument, same letters
  but in different positions, first and last letter
  unchanged.

10
Write and Test Independently

• Develop your version of wordScramble.
• Create a main program that finds or creates input
  to wordScramble, calls the function, and prints
  or tests the result.
• Called a test harness, sits around the code under
  test. Can be in the same class, or another class.
• Process of testing individual methods is called
  unit testing

11
Testing, Deja Vu

• Testing with test harness has same issues as
  testing with stubs.
• What are exceptional (or boundary) cases? What
  are normal cases? How do you tell if you have
  the right answer?

12
Integration Testing/ Regression Testing

• You test various pieces by themselves. Put them
  together into larger pieces. Test the pieces, put
  them into larger pieces, test again. This is
  termed integration testing
• You will likely still have bugs. After you fix
  each bug, you must run all the tests that you
  have tried previously to make sure they still
  work. This is termed regression testing.

13
Debugging

• Despite the best efforts at design, development,
  and testing, your code will still likely contain
  errors. Programming is one of the most difficult
  things people do!
• When you have errors, how do you find them?

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Steps in Debugging

• Reproduce the error. Make sure you can show it
  repeatedly.
• Simplify the error. Find the simplest test case
  or set of values that show the error.
• Divide and conquer. Use breakpoints, print
  statements, to locate where the error occurs
• Know what your program should do.

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Steps in Debugging, cont

• Keep an open mind. "When you have eliminated the
  impossible, whatever remains, however improbable,
  must be true"
• Look around. If you don't see anything at the
  point the error is manifest, the mistake is
  probably earlier.
• Look at the details. The simple errors are the
  hard ones to see.

16
For Next Time

• Read Chapter 9 (Chap 11 in 2nd edition)
• Start Programming Assignment 1
• Do homework 0 (due on Friday in class)