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Lab 2 -- String Encryptors

Purpose​

Learn how to define functions, and about string library

Introduction​

You’ve stumbled upon a malfunctioning encryption device that scrambles secret messages into code. Unfortunately, the documentation went missing! Your mission is to reverse-engineer each of the device’s 10 black-box functions so that you can replicate their behavior in Pyret.

How it works​

The device exposes ten functions, named support.encryptor1 through support.encryptor10. Each encryptor takes in a String and returns a new, “encrypted” String. Alongside each encryptor, there’s a corresponding tester routine — support.test-encryptor1, support.test-encryptor2, and so on that — checks whether your replication matches the original for all possible inputs.

Instructions​

  1. Experiment
    1. Call each support.encryptorN with a variety of sample strings.
    2. Observe how the output changes and identify the pattern or rule it applies.
    3. Keep a written log of the inputs you tried and the outputs you observed (your TA will ask you to justify these choices).
  2. Recreate
    1. Write your own function, for example,
    fun my-encryptor1(s :: String) -> String: ... end
    that implements the same transformation you discovered. 3. Use Pyret’s String library functions (e.g. string-length, string-substring, string-index-of, etc.) to build your version.
  3. Verify
    1. Pass your function to the provided tester:
support.test-encryptor1(my-encryptor1)

If the tester completes without errors, your encryption logic is correct!

Here’s how we built an encryptor that returns only the first character of any string:

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

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?