In Class: Week 8 Tuesday:
Top-Down Design Practice

Create a week08 subdirectory in your cs21/class directory by running update21:

$ update21
$ cd cs21/class/week08

Topics

• Iterative refinement
• Bottom-up implementation
• Unit testing using python interactive mode

Top Down Design

1. Start with General Description of Problem
2. Break it into several high-level steps
3. Iteratively break the steps into smaller steps until you have steps that are easy to solve

In-class work

1. Last week, we started working on the program to play Blackjack. Open up blackjack.py to review our design. Note that I have added a deck of cards to our design to make the problem more realistic
2. To begin, we will analyze our first level of design and see which tasks need to be further broken down. This process of breaking subproblems into a set of even smaller subproblems is known as iterative refinement. Specifically, we will see how playUser() requires a function for flipping a single card as well as a function for getting a yes-or-no response from the user.

3. Together, we will begin implementation on the bottom-level functions. First, we will look at my implementation of generateDeck(). We will use the python interpretor to perform unit testing to notice that my implementation was no complete.

4. On your own, work on implementing the getYesOrNo() function. When finished, let's try some unit testing on getYesOrNo() (see below).
5. Once we are satisfied with getYesOrNo(), work individually on implementin other functions in blackjack.py. Start with flipCard() and then move on to playUser().
6. Perform unit-testing on flipCard() multiple times to see if you get numbers that you expect.
7. If you finish flipCard(), move up a level in implementation and work on playUser(). Since flipCard() is implemented and tested, you can concentrate on the larger problem of executing one user turn.

Unit Testing

Unit testing is the process of validating functions as individual pieces, in isolation from other large portions of the program. This allows a developer to focus solely on the (small) subproblem at hand and think carefully about potential errors. To perform unit testing:

• You can write calls to getYesOrNo() from main() and run your program to see the results
• You can also write a special testing function, called testGetYesOrNo() whose sole purpose is to call the function multiple times with different arguments (if applicable).
• In python, you can open up python in interactive mode and make calls to the function getYesOrNo().

if __name__ = "__main__":
  main()

This allows python to import your program without running it. To do so, run python in interactive mode:

$ python

>>>import blackjack

>>>blackjack.flipCard()
in function flipCard()
5

If you make changes to your program, you can reload your file in the python interpretor by calling the reload command:

>>>reload(blackjack)

>>>help(blackjack)