cond

In this lab, you'll practice with some of the things introduced in class, including functional decomposition, and functions on Booleans and symbols. You'll explore conditionals in greater depth. Based upon this work, we'll revise the design recipe.

Instructions for students & labbies: Students should read and use DrScheme on the exercises at their own pace, while labbies wander among the students, answering questions, bringing the more important ones to the lab's attention. Students shouldn't look at the sample solutions until finishing the exercise. Depending on their pace, students won't necessarily have time to finish all exercises. That's perfectly OK, in which case, students should skip ahead during lab to do at least one exercise in each section. For the last 25 minutes or so, have a group discussion on the points listed and consider the modified design recipe.

Design Recipe Reminder

4. Function contract, purpose, header
5. Function examples
6. Function body
7. Function testing

Programming is not tinkering on a program until it passes some tests. That approach leaves people wandering, and provides little confidence in the program correctness, and none in its maintainability. There is Process in creating a program.

Functional Decomposition

• Each function is simpler. Each is easier to understand, write, test, and debug.
• Code is re-used. Each computation is written once, instead of multiple times -- a single point of control. Thus, if we need to change/fix, say, the computation of the average of a bunch of numbers, we only need to change one piece of code. This generally leads to shorter code with fewer mistakes.

; hypotenuse : real real -> positive-real
; Returns the length of the hypotenuse of a right triangle having
; the given height and width.
(define (hypotenuse height width)
   (sqrt (+ (* height height) (* width width))))

'hypotenuse_examples
(= (hypotenuse 0 0) 0)
(= (hypotenuse 3 4) 5)
(= (hypotenuse 6 8) 10)
      

Conditionals

     (cond
        [(symbol=? 'Sam 'Sandy) 'wrongo]
        [(symbol=? 'Sam 'Sam)   'yep]
        [else                   'hmmmm])

This starts with the cond keyword. It then has a series of "clauses", here, three of them. Here, each clause is grouped with brackets, but you can use parentheses instead, too. Each clause has a question and an answer. The whole cond expression evaluates to the answer of the first true question. The last clause's question can be else, which is always true.

Of course, writing code like the above, where we test the equality of two known things, is kind of silly. A more realistic example would be the following:

     ; number-to-day : nat in 1...7 -> 'symbol
     ; Purpose: Returns the name of the day corresponding to the
     ; given number, assuming that the week starts with Monday.
     (define (number-to-day num)
        (cond
           [(= 1 num) 'Monday]
           [(= 2 num) 'Tuesday]
           [(= 3 num) 'Wednesday]
           [(= 4 num) 'Thursday]
           [(= 5 num) 'Friday]
           [(= 6 num) 'Saturday]
           [(= 7 num) 'Sunday]))

The previous example is one way we use conditionals. The following two have a slightly different flavor that we'll discuss.

Discussion

1. In fastest-runner, we could have shortened the sample solution by eliminating the first part of the and in each of the 2nd and 3rd cases. Why is this a bad idea?

2. In the running and color examples, we could have replaced the last clause question with else. Is this a good idea?

3. Is there anything in common about the fastest runner and color examples? Yes! We find there are often several times that we'd like to create or our categories or types of data. E.g.,

        ; A running-distance is one of
        ;  - an integer in [0,300)
        ;  - an integer in [300,1000)
        ;  - an integer in [1000,10000)
        ;  - an integer in [10000,+inf.0)
       

   Aside

   This is "half-open interval" notation, standard for mathematicians: the square-bracket means include the number in the interval, and the round-bracket means exclude the number from the interval. These are not meant to be scheme-parentheses. We'll use "+inf.0" to mean positive-infinity for now.

   This data definition better reflects our thinking of the program.

   We might want to use this user-defined type several times. E.g., to write functions for female runners, male runners, Junior Olympics, whatever.

4. Once we've given this name to this type of data, what would the contract for your fastest-runner function be?

5. How would we define a color type? How would we use it for the color complement and primary color tester?

6. There are several ways to write the primary color tester. What are their tradeoffs?

In summary, this "x, y, or z" structure is a very common way to define data, and you see that the code reflects this way of thinking about the data: There is one cond branch for each branch of data.

Stepping Back: Recipe Redux

• The data is just one of the built-in data types. Any particular function dealing with that data will use the corresponding built-in functions.
• The data is of the form "x, y, or z". Any particular function dealing with that data will use a cond, with one branch for each type of data.
• ... There will be a third case next week. ...

Our code mirrors our data, which in turn mirrors how we conceive of the problem. This is no coincidence. More than half the work of writing a good program lies in choosing good data definitions. We'll see how the shape of the data often guides you to a bug-free solution.

We'll close by updating our design recipe.

1. new Data definition: How, exactly, do you represent the problem in terms of Scheme data?
2. new Data examples.
3. Function contract, purpose, header.
4. Function examples: Use your previously-written data examples.
5. Function body.
   • new If your data has various cases, include a cond -- one branch per case of data. (You can write all the cond questions first, to distinguish the data, and come back later to worry about the answer for a particular function.)
6. Function testing: Use your previously-written examples.

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