Homework 1: Scheme Intro 1

You'll work either in one of the Olin CS labs (310, 304, or 308) or on your own computer. Even if you plan to primarily work in the Olin labs, I recommend getting your own computer set up for this class using the Docker approach if possible. Follow the instructions on Moodle entitled "Development setup (personal computer)" before you begin this assignment.

At the Guile prompt, type

(+ 3 5)

In Scheme, + is a function. To call a function in Scheme, you place the name of the function and its arguments, separated by spaces, inside parentheses. This takes a little getting used to! In most programming languages, function calls look something like this:

function(arg1, arg2, arg3)

In Scheme, function calls look like this:

(function arg1 arg2 arg3)

So the above code that you typed in should add 3 to 5, and so you should see output that looks like:

$1 = 8

This means that the return value from the above expression is 8, and Guile is assigning it to a history variable called $1. You can ignore these history variables; they're not especially useful for us.

Back at the regular Scheme prompt, change your program in some way. For example, re-type it as

(+ 3 7)

Experiment with using the up and down arrow keys, which let you move backwards and forwards through your command history.

If you make a mistake and type in some invalid syntax, Guile Scheme presents a sub-prompt indicating that you've had an error. Perhaps this has already happened to you. The error prompt looks looks like this:

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]>

The key way to notice that you're there is somewhat subtle: it's the [1] that appears in the prompt. It means that you're 1 level deep into the error system. We'll play with the debugging system more later on, but for now, when you see this, get out of by typing =,q= (don't miss the comma in front).

Let's try it, just so you can see how it works. At the Guile prompt, just type x, and hit enter. You should see an error about an unbound variable, and then you'll find yourself at the error prompt. Type =,q= to return to the main Guile prompt.

Notice that if you type =,q= at the main Guile prompt, it will exit Guile and you'll end up back at the regular bash prompt. If that happens, type ./scheme again to start it up.

Write sequences of cars and cdrs that will pick the symbol koala out of the following expressions:

(koala eel cheetah okapi)
(cheetah koala)
(((cheetah) (eel) (koala) (okapi)))
(cheetah (eel) ((koala)) (((okapi))))

Here's an example of what I might write if I was trying to pick koala out of the expression (koala):

(car '(koala))

It's ok if you have trouble getting this to work: the key aspect to this assignment is to make the effort before the deadline so that you can ask questions.

In the (initially empty) file main.scm, paste in your code to pick out koala from each expression. You should end up with four lines in main.scm, one for each of the four expressions above.

Additionally, create a file named CollaborationsAndSources.txt within the starter folder. In this file, you should describe (a) your collaborations with anyone on the assignment and (b) your use of any outside sources on the assignment. For example, I might say something like "I worked through a few related examples on the board with Grace Hopper to better understand car and cdr, and then I wrote my code for the examples in the assignment on my own. I talked with Alan Turing about the assignment to help him understand list. I didn't refer to any outside sources except the Dybvig book." With every assignment you submit, you'll need to include a file named CollaborationsAndSources.txt that has these two parts. Your file should be specific about your collaborations and sources - it shouldn't be empty or just list names of people.

Zip up the starter folder with your modified files (i.e., your modifications to main.scm and your CollaborationsAndSources.txt) and upload it to Moodle.

For this assignment only, you'll earn a grade of M (meets expectations; 3) if you've clearly given it a solid attempt, even if you didn't always get the koala, and if you have CollaborationsAndSources.txt. For assignments going forward, you'll need to have a series of tests run successfully on your code to earn an M. You'll earn an E (exemplary; 4) if your four lines of code always pick out koala and you have CollaborationsAndSources.txt. The graders will copy and paste your code from main.scm into Scheme to confirm that they correctly get the koala.