Comp 210 Lab 3: Dr. Scheme; Data Definitions

Table of Contents

Dr. Scheme, an alternative to donkey

Dr. Scheme combines a Scheme interpreter with an environment to write and debug Scheme programs. To start Dr. Scheme, either go to the button labelled Comp210 near the top of your workstation's screen and select ``Dr Scheme'', or type drscheme & (all lower case) and press return. After a quite a few moments (even more than donkey needs) the Dr. Scheme window will pop up.

Before we forget, there's a one-time task to do: you should set your Language level to "beginner": pull down the Language menu, select ``Configure Language", wait, and where it says "Language V Quasi-R4Rs", click on the triangle and instead select "Beginner". (There is no need to re-start Dr. Scheme.) This setting causes Dr. Scheme to warn you of some common errors that are technically legal, but almost certainly not what you want. And to repeat a warning on the newsgroup: you will get 0 credit for homeworks which use begin or any function with an exclamation point in its name, up until they are introduced in lecture.

The Dr. Scheme window is divided into two subwindows:

Let's try it: In the definition window (top half), type in (+ 1 2 3), then click the Execute button. Sure enough, in the interaction window you see the result 6 printed. Now click back in the definition window, and type something more complicated:

(define inverse-of
  (lambda (x)
     (/ 1 x)))

When Dr. Scheme, When Donkey?

As with Donkey, Dr. Scheme's File menu contains options ``Save'', ``Save As'', and ``Print''. (Remember, Scheme files always end in the suffix .ss.)

Locating and Fixing Mistakes

If you have an error, fix it by editing your program, up in the definitions window.

You can significantly increase your speed (and decrease frustration) by taking a few moments to practice the keyboard shortcuts which work in Donkey and Netscape also work in Dr. Scheme. I particularly recommend C-e (end of line), C-a (start of line), and C-M-f (forward one S-expression).

Using Libraries in Dr. Scheme

To confirm that the desired library is loaded, try one of the promised functions:
evaluate (word-defined? 'mellifluous).

quotient and remainder

(quotient  20 3)  =  6
(remainder 20 3)  =  2

Documenting Your Code

This means that to write a function template, you can first write it outside a comment (which will have Dr. Scheme help you indent automatically), and then comment it out all at once, so that the template's ``...'' won't trigger the doctor's error-flagging. To fill in the template, you can copy-and-paste it (see the Edit menu), uncomment it, and replace the template's ``...''s with honest-to-goodness code.

When writing code in any language, an excellent habit to get into early is to have a comment for each function. The comment should explicitly mention the type and meaning of each of the function's arguments. Writing these comments doesn't take long if you're clear on exactly what the function should do; if you aren't so clear, then writing these comments (before you write the function) helps delineate the problem for you.2

;; (fahr c)
;; c (number) the temperature in celsius
;; Returns a number, the corresponding temperature in Fahrenheit
;;
(define fahr
   (lambda (c)
      (+  (* 9/5 c)  32)))

How much you want to say about how the function works is up to you; for the programs in the first few weeks, the individual functions will be simple enough (once you've written them!) that you'll probably feel they don't require explanations of how they work, which is fine. Your comments can assume that the reader is familiar with Scheme.

A Note on Formatting

Some particular points:

... (cond [(null? argument)  (some-function other-argument)]
          [(complicated-expression (car argument)) (complicated-function argument other-argument)]
          [else (another-complicated-function (cdr argument))])

... (cond [(null? argument)
           (some-function other-argument)]
          [(complicated-expression (car argument))
           (complicated-function argument other-argument)]
          [else
           (another-complicated-function (cdr argument))])