Lab 1: Message Decryptor

Welcome to CSCI 0111 Lab! We're so excited to work with you. Now that you're with your partner(s), discuss one/some/all of these questions:

• What is your favorite food?
• What do you do for fun?
• What would you name your [hypothetical] pet cow?

Part 1 - Practice reading error messages

When writing code, it is common to encounter error messages. This doesn't only happen when learning; because programming languages are very particular in what they allow, it's common for professional programmers to encounter and deal with errors while working on a piece of code.

Just like anything else we learn in CS111, learning how to read an error message is a skill we can practice. Let's do that now. For example, let's say we want to figure out the number of digits that 111 has. If we remember that string-length tells us how many characters the input has, we might run the expression string-length(111) and see what happens:

This error message tells us that the first (and only) input to string-length should be a String, which 111 does not satisfy (it's okay if this is not immediately obvious to you from the error message. As you work more with Pyret, you'll pick up on some of the patterns in the wording of error messages.) Take a few moments to chat with your neighbor and identify how to fix this error before reading the answer below.

The fix

Because string-length expects a String input, we can put quotes around 111 so that Pyret interprets the value as a String. Try running string-length("111") and see that the error is fixed.

Each of the following lines of code will result in an error. One by one, run them in Pyret, discuss the error, and fix the code so that it runs (you can make assumptions about functionality or order of operations if necessary). For some of the examples, you or your partner might see the issue with the code before even running it – talk about the error message anyway. One of the points of this task is to start getting used to individual error messages so that it's less scary to encounter them when working on more complex code later on. If you get stuck, browse the Pyret documentation on the Number, String, and Image functions to see if that helps guide you in the right direction before asking for help.

1. Draw a small hexagon: regular-polygon("orange", "solid", 6, 100)
2. Compute a complex math expression: num-sqrt(3*3+4*4)
3. Get the last two characters of a word: string-substring("bees", 3, 5)
4. Get the last letter of a word: string-char-at("Frankenstein", string-length("Frankenstein"))
5. Get the absolute difference of the length of two words: num-abs(string-length("Frankenstein", "monster"))

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CHECKPOINT:

Call over a TA after you have finished problems 1-5.

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Part 2 - Edge Cases

Code can often be brittle in the face of inputs unforeseen by the programmer (edge cases).

1. With your lab partner(s), have one partner read this short article from the Verge detailing how a novelty license plate exposed a huge flaw in the ticketing system and another partner read this article about how ID requirements create unique obstacles for women, people of color, and low-wage workers.
2. Discuss the main takeaways from each article. Then talk about the importance of edge cases in real life and how you can use them to test a function with unknown behavior.

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CHECKPOINT:

Call over a TA and share your takeaways from your discussion.

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Part 3 – Cracking Encryptors

You're on a top secret mission and need to communicate an encrypted message containing important code. However, the instructions to your encryption machine were accidentally thrown into a hay bale, and now it’s up to you to figure out how the machine works!

The encryptor machine has 10 black-box functions that encrypt a String. Your first task is to test various inputs and figure out how each function changes the input based on the output.

Once you have an idea of what an encryption function does, write a version of the function that encrypts a String in the same manner and run it through the checks in the support file. If all checks pass, then you’ve successfully built an encryptor for covert communications!

Hint: The encryptors are composed of functions from the Pyret Strings library. Start by reading through it.

Here is an example of what an encryptor looks like under the hood:

provide *
provide-types * 

fun encryptor0(s :: String) -> String:
  doc: "returns the first letter of the string" 
  string-substring(s, 0, 1)
end

check "encryptor0: empty str":
  encryptor0("") raises "index"
end

check "encryptor0: general":
  encryptor0("a") is "a"
  encryptor0(" ") is " "
  encryptor0("hello") is "h"
  encryptor0("1, ") is "1"
end

We created an encryptor called encryptor0 that takes in a string and returns the first letter only. We use check blocks to ensure that it works correctly given empty and non-empty strings.

Instructions

Include this at the top of your program (before any other code, including code from the previous parts):
import shared-gdrive("lab1-support-2022.arr", "1kS_-rd0eE9-DLndIYw-0Bez8c6J73MRK") as support

Now you will be able to access all of the encryptors and their testers in your Pyret file.

The encryptor functions are named

support.encryptor1,
support.encryptor2,
support.encryptor3,
...
support.encryptor10

The encryptor functions take in Strings as input (e.g: support.encryptor1("mystery")) and produce an encrypted result.

Please keep a record of what Strings you use to test each encryptor – you should be prepared to explain your choice of Strings to a TA. They will be useful for you to reference, too!

The tester functions are named:

support.test-encryptor1,
support.test-encryptor2,
support.test-encryptor3,
...
support.test-encryptor10

To use them, you must use the name of your function as an input. For example, after giving some inputs to support.encryptor1, you may think it always returns the String "cs0111" (HINT: it doesn't do this – just an example). Then you would write the function:

​fun my-cool-encryptor(str :: String) -> String:
​    "cs0111"
end

You would test whether your function is correct (meaning it does the same thing as encryptor1 for all inputs) by running:

​support.test-encryptor1(my-cool-encryptor)

Hint: The Pyret Strings library will be very helpful when writing your functions!

More Hints!

• When choosing Strings to use as inputs to the encryptors, it will be useful to vary your inputs – think about why this could be. In what ways can you vary your Strings?
• Think about scenarios where we might see strings of text, such as a book or report. What symbols might you commonly see in that text, other than alphabetical letters?
• Remember that Strings in Pyret are 0-indexed. This means that the first character in a string will be assigned index 0 (not 1).
• Encryptor 9 is one of the trickiest. Try strings of various lengths that start with the same letters. What do you notice?

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CHECKPOINT:

1. Call over a TA after you have finished encryptors 1-4.
2. Call over a TA after you have finished encryptors 5-8.
3. Call over a TA once you have finished encryptors 9-10.

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Challenge Task:

Now that you're an encryptor expert, let's try making your own! Create an encryptor function in the style of the ones you've explored so far and name it new-encryptor. Now, share it with your classmates to let them take a crack at decrypting it. Link to shared spreadsheet.

To share your encryptor in the shared spreadsheet, press 'Publish' in your Pyret file and copy the code that begins with 'import shared-gdrive'. (If you don't see the 'Publish' button, make sure you're logged into Pyret with your Brown Google account.) Add that to the sheet. Once you've successfully added yours, try solving another group's from that sheet!

Task 1: Import another group's encryptor from the encryptor swap spreadsheet using the same format you used to import support.arr – call over a TA if you get stuck.

Task 2: Write a function called new-decryptor that takes in the output of the other group's new-encryptor and decrypts it. When you think your decryptor works, consult with the other group.

Success!

Nice! Your new encryptor functions worked! You successfully sent the code and the mission was completed.

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Brown University CSCI 0111 (Spring 2025)
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