Getting Started –
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DrScheme, Design Recipe
Original Author: Dr. John Greiner
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Outside of lab, don't forget to get a homework partner, if you haven't already. If you're having trouble getting a partner, try using the sign up sheet. Partners are encouraged, but not required, to attend the same lab section.
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DrScheme Basics
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- Open a terminal.
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- Make a new folder (directory) called
Comp211
, for this course. The command is for making a new folder on Linux ismkdir
. - Make a sub-folder in
Comp211
calledLabs
. - Make a sub-folder in
Labs
calledLab01
. Save all your Lab01 work to this folder. For submitting homeworks, see the Owlspace page for Comp 211. #Start - Start DrScheme by typing the command
drscheme
.
If #If this is the first time you've run DrScheme, it will ask you your preferred language, and the Scheme language level; the latter should be "Beginning Student with List Abbreviations". This language level provides the expected set of primitive operations and enables DrScheme to reject "legal" Scheme constructions that are
not part of our introductory dialect.
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Simple example of using DrScheme
- Type (or cutcopy/paste) the following blocks of code expressions into the definition interactions window , including:
Code Block (+ 2 7)
Code Block (* (+ 2 7) 3)
- Type (or copy/paste) the following blocks of code into the definition window, including:
Observe that DrScheme helps you with the indentation if you use the Return key in appropriate places.Code Block ;; contract: ;; owe: nat -> num ;; Returnspurpose: ;; (owe n) returns how much money you owe for eating the given number of ;; n slices of pizza. ;; definition: (define (owe slices) (* (/ 12 8) slices))
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- It also "bounces" the cursor to visually match the closing parenthesis with the corresponding opening parenthesis.
- Use the definition of
owe
by typing in the interaction window some Scheme expressions using those definitions, e.g.,Code Block (owe 5) (owe 7)
- We included comments before each function definition, explaining what it is supposed to do. DrScheme ignores the comments, but you, your grader, your boss, etc., can read them. Soon, we'll provide more guidance on what to say in your comments.
{{
Code Block ; One form of comments is anything following a semicolon
; up to the end of the line.
;; However, you'll often see two semicolons starting a comment.
#|
Another form of comments is anything between these
two matching markers.
# |
You can also go to the Special menu, choose Insert comment box. You are welcome to do this, but one caveat: if DrScheme saves a file which includes a fancy comment box, the file will not be saved in plain-text format. (This is only an issue if you want to open or print the file with a different program later.)#| Another form of comments is anything between these two matching markers. |#
- Edit your definitions or comments some. You can use the mouse, the arrow keys, backspace, and standard keyboard shortcuts to move the cursor around. Also, the PC, Mac, and Unix versions of DrScheme each use their respective standard keyboard shortcuts. (You can look in the Edit menu under Keybindings for the complete list.)
- Load the definitions into the interaction window by clicking on the Run button. If DrScheme detects any syntactic errors, it will give you an error message in the interactions window and highlight the error in the definitions window.
- Use those definitions by typing in the interaction window some Scheme expressions using those definitions. E.g.,
{{
(owe 5)
(owe 7)
}}
- Save your work in your
Comp211\Labs\Lab01
directory. Be sure to use a filename that has a.ss
extension. You don't need to turn in your lab work. In fact, you don't have to save it, but we recommend it so that you can look at it again. - Finally, intentionally introduce an error or, what you think will be an error, press Run, and see what error message is given. We'll go around the room to see what different error messages people get. Do the error messages tend to make sense?
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- Place an example use or two of your functions in the definitions window, after the appropriate definitions. For example,
{{
Code Block (owe 10)
}} - Click on the Step button. This brings up a new stepper windows which will show how DrScheme calculates the answer.
- Use the stepper's button to look through the evaluation. At each step, it shows what part of the code needs to be evaluated next, and what the result is.
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- Hand-evaluate a few example expressions, and type in each of the small steps you would make to calculate the result. Type these in the definitions window after your definitions. For example,
{{
Code Block ;; Hand-evaluation:
(owe 10)
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(* (/ 12 8) 10)
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(* 3/2 10)
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This is just high school algebra, successively simplfying expressions, until you reach a final, simple value.15
- What do you hope to see, when you Run all these expressions? What would you see if there were a mistake?
- While you are doing this, it is often helpful to copy the previous step, and edit this copy for the current step. This can save you some typing, and it helps eliminate mistakes. You'll want the Copy and Paste features in the Edit menu.
- Verify your steps by clicking Run.
- Once you've used this to verify your steps, put the steps in comments, rather than deleting them. (Later, you might change the code, and you want to re-use these as test cases.)
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- Write the function's contract, purpose, and header. The contract specifies the function name and what kind of values the function will use as input and output. The purpose is a short statement describing what (not how) the function will calculate. The header is the part of the function definition specifying the function and parameter names. Type these in the definition window. Put the contract and purpose in comments, as in the following examples:
{{
;; owe : nat -> num
;; Returns how much money you owe for eating the given
;; number of pizza slices.
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We won't use DrScheme to verify that our contracts are correct, although that is a very useful thing to do. Look for that in COMP 212.Code Block ;; contract: ;; owe : nat -> num ;; ;; purpose: ;; (owe n) returns how much money you owe for eating n pizza slices. ;; ;; header: ;; (define (owe slices) ...)
- Make some examples of what the function should docompute. You need to make sure you understand what the function is supposed to do before you define it. When applicable, use some of the example data. We recommend the following form for these examples:
{{{
Code Block ;; Examples:
0;; (owe 0) =>
0 ;; (owe 10) => 15
}}
The first line provides an explanation of the following, making it more readable. (We haven't introduced symbols yet officially -- we will see them in class soon.) The next two lines each give an example, asking DrScheme to compare (with the numeric equality function =) the actual result with the expected result. You should pick enough examples to test the function adequately. We'll say more about that later, but this should include any interesting inputs like zero. - Write the function body. Soon, we will have lots to say about this step. For now, for programs on "unstructured" data, this is very straightforward, because we are typically given an equation like …
{{
(define (owe slices)
(* (/ 12 8) slices))
}}
For sets of intervals or other conditional functions, there should be exactly one condition clause per option. - Test the function. i.e., make sure the previous examples really work. If you wrote the examples as suggested above, you can use them to test the function. Add a test section after your function definitionexamples, like this:
All of these tests should returnCode Block "Testing;; oweTests:" (equal?check-expect (owe 0) 0)) (equal?check-expect (owe 10) 15))
true
pass , assuming that the tests themselves are correct. If your program doesn't work, either fix it now or put that in comments too, to remind yourself to fix it later, and to let your grader/boss/customer/whomever know also. The better code forowe
is correct. The more elaborate your test, the more errors you'll catch sooner and the less time it will take to write a correct function. Note that at this pointowe
has not been written yet. Still we write the test code first! This is one of the key steps in modern software development: test before coding. It is called test-driven development.
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- This is where we deviate from the design recipe describe in HTDP (the textbook). We write the test code first!
- Write the function body. Soon, we will have lots to say about this step. For now, for programs on "unstructured" data, this is very straightforward, because we are typically given an equation like …
For sets of intervals or other conditional functions, there should be exactly one condition clause per option.Code Block ;; definition: (define (owe slices) (* (/ 12 8) slices))
Using the design recipe, define a function calculating the area of a right isosceles triangle.
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Simple Data: Booleans and Symbols
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Numbers are old hat, so let's explore Booleans and symbols some.
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Exercises: Using built-in data
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- Try calling some the built-in
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Code Block |
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<= |
,
Code Block |
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= |
,
Code Block |
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and |
,
Code Block |
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or |
,
Code Block |
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not |
,
Code Block |
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symbol=? |
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- operations like
<=
,=
,and
,or
,not
,equal?
on various data values, e.g., is 17 times 18 bigger
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- than 256?
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- What are the contracts of these built-in functions?
*Choose any two, and write down the contract.
*Verify your guess by asking the labby
or by using DrScheme's Help Desk to look at the
manual for the Beginning Student language.
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- *Practice writing a function which returns a Boolean:
Following the design recipe,
write
which takes in two numbersCode Block within-two?
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m
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</var> |
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<var> |
- and
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n
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</var> |
- , and returns true if
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<var> |
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<nobr>
m
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</var> |
-
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<var> |
n
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</var> |
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</nobr> |
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<nobr> |
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<var> |
- and
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n
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</var> |
-
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<var> |
m
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</var> |
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</nobr> |
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- are both
less than2
.
Otherwise, it should return false.
(For learning purposes only, don't use
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abs
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- ,
which computes the absolute value. Also, don't use
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cond
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- or
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if
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- .)
Your examples/tests can look likeCode Block (
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check-expect (within-two? 99.8 101) true) (
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check-expect (within-two? 5 -5) false)
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</ol>
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Other Basics
We assume that you already know how to use email and a Web browser. If you have any trouble, ask a labbie during office hours, or contact the staff of the information desk in Mudd.
You'll also need to know how to use SVN. On the SVN Turnin Page we have basic instructions for using SVN to turn in your homeworks.
Always "log out" from your account when you are done and leaving. Otherwise, someone can use your account, e.g., to copy or delete your assignments. Be sure to save everything before you log out!
!! Access Permissions
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