Homework 1A (CS15): Map, Filter, and Writing Tests

Due: Tuesday, Feb 1, 2022 at 11:59pm ET

Collaboration Policy: You may collaborate as much as you want on this assignment (this is more flexible than the normal course policy). The goal is to get everyone up to speed on functional programming. You are required to turn this in, but it will be weighted lightly in final grades. That said, we strongly encourage you to actively try writing these on your own while collaborating, as assignments after we come back together will assume you can do these sorts of problems on your own.

Need help? Find us, as well as questions and answers from other students, on Ed. Here's our office hours schedule.

Handin Instructions are at the end of the document.

Learning Objectives

• Practice with filter and map, two functions built into many programming languages for performing common operations on lists
• Practice passing functions as arguments
• Practice writing test suites

Stencil Code and Assignment Setup

• Go to code.pyret.org; this requires logging in with any Google account (like your Brown account).
• Make a new file in code.pyret.org called hw1a-15-code.arr.
• Do not put your name anywhere in the file. Before you start, paste this statement at the top of your code.

provide {unread-messages-from: unread-messages-from, messages-about: messages-about, unread-from: unread-from, rainfall: rainfall, checkout: checkout} end

Style Expectations

Follow these style guidelines when writing your code:

• Types and doc strings for all functions (you can look in the class notes for examples of these)

When defining functions make sure to name them exactly what is given in the handout. If you do not do this, the autograder will fail.

The handout guides you through developing the code in stages. You will turn in one set of files with your cumulative work on all tasks. You do not need to maintain versions of your code from each task separately.

Relevant Documentation

If you'd like help contextualizing Pyret through the lens of Java, check out our Pyret crashcourse for those with Java experience.

Once you have opened the editor, the Pyret documentation is accessible from the Pyret logo button in the top left corner of code.pyret.org. The list documentation may be particularly useful for this assignment.

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Problem Setup

Note: Click on blue headers throughout for a fun surprise :)

It's the early 2000s! There's no social media. Cell phones are just becoming popular. People rely on email to communicate electronically. Let's use email as a sample domain to practice manipulating lists of data in a lightweight way.

Since U Been Gone…

…we've been swapping a lot of emails. Start with the following data definition for email messages:

# assume the sender string is an email address like [e-mail]
data Email:
  | message(sender :: String, content :: String, read :: Boolean)
end

Check out the Pyret textbook for more info on data definitions.

In OOP speak, we can think of message as a constructor. The way we actually make one of these data is by calling message with three parameters; for instance:

msg = message("[e-mail]", "jough", true)

In order to access a parameter's value, you can use .. For example, in order to access the content of Joe's message, you would write msg.content.

Task: Make a list of email message objects to use as test data. Stick to short contents, but have a mix of read and unread messages from a small number of email addresses. Also include multiple emails from the same sender (needed for later).

Task: We want to see which emails we haven't read yet. Use filter to write an expression that gets all of the unread messages from your test data.

Task: Now we want to mark all emails as 'unread'. Use map to write an expression that produces a new copy of the test data in which all of the messages are now unread.

Note: Expressions are NOT the same as functions. x + y and 1/5 are examples of expressions.

Task: Write a function unread-messages-from that takes a list of email messages and a sender's email and returns a list of unread messages from that sender.

Task: Write a function messages-about that takes a list of email messages and a word or phrase (string) and returns a list of all messages that contain that phrase.

Task: Write a function unread-from that takes a list of email messages and returns a list of unique usernames (without the "@..." suffix) for whom there are unread messages. Use the built-in method distinct that takes a list and returns a list of the unique elements to trim duplicates.

Under My Umbrella..ella..ella…

A weather station needs to compute average rainfall over a period of time. Their data are available as a list of numbers. Negative numbers in the data are ignored as noise (ie. sensor malfunctions). One negative number, -999, is special: it may be used to indicate the end of the time period of interest (so all values that follow -999 are to be ignored, whether positive or negative). There is always at least one non-negative number before either -999 or the end of the list.

Here's an example of the function that they need:

rainfall([list: 1, -2, 5, -999, 8]) is 3 (the average of 1 and 5)

Task: Write the rainfall function as described above.

Task: Write a check block with a thorough set of test cases for rainfall. Thorough tests catch reasonable logic errors or functionality omissions that someone might make in writing this program.

Useful built-ins:

• take-while, like filter, takes in a predicate (a function that maps Item -> Boolean) and a list as arguments. It splits the list into a tuple denoted by {<sub-list-1>; <sub-list-2>}. The function takes elements in the list until it encounters the first element where the predicate is false. The 'taken' elements are returned in a list as <sub-list-1>, and the rest of the elements are returned in a list as <sub-list-2>. For example:

>>> take-while(lam(e): e < 5 end, [list: 4, 6, 2])
{[list: 4]; [list: 6, 2]}

• To extract an item from a tuple, you use the syntax .{<index>}.

>>> {[list: 1, 2, 3]; [list: 4, 5, 6]}.{0}
[list: 1, 2, 3]

• To extract an item from a list, you use the syntax .get(<index>).

>>> [list: 4, 6, 2].get(0)
4

• sum takes a list of numbers and adds them up. To use sum, you must have include math at the top of your file.
• length takes a list and returns its length.

Bills, bills, bills

An online clothing store applies discounts during checkout. A shopping cart is a list of the items being purchased. Each item has a name (a string like “shoes”) and a price (a real number like 12.50).

Each item is represented as follows:

data Item:
  | item(name :: String, cost :: Number)
end

Task: Write a function called checkout that takes a list of Item (this list represents the shopping cart) and produces the total cost of the cart, reduced (where applicable) by applying the following two discounts:

• If the cart contains at least $100 worth of shoes, take 20% off the cost of all shoes (match only items whose exact name is "shoes").
• If the cart contains at least two hats, take $10 off the total of the cart (match only items whose exact name is "hat").

Example:

• checkout([list: item("shoes", 25), item("bag", 50), item("shoes", 85), item("hat",15)]) is (25 + 50 + 85 + 15) - ((25 + 85) * 0.2)

Task: Write a check block with a good set of tests for checkout. Annotate each with a description of what situation the test is designed to check.

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How to Hand-in

For this homework, you MUST submit your solutions in a file called: hw1a-15-code.arr

For autograder compatibility, please check that all required function names exactly match the functions named in this handout, and that the file is named correctly. Our autograder will break and you will lose points if your function names are not correct.

You may submit as many times as you want before the deadline. Only your latest submission will be graded (though you will spend late days if the latest submission is after the due date.)

To hand in your solution, you must upload them to Gradescope. Do not zip or compress them. Read our Gradescope Submission Guide for help on submitting to Gradescope.

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