---
tags: Homeworks-S25, 2025
title: Homework 2
---

# Homework 2: Ads Take Aim

### Due: Tuesday, February 4th at 11:59 pm


## Setup
- Create a new file named `hw2-code.arr` in [Pyret](https://code.pyret.org).
- Unlike with Homework 1, we are writing examples (test cases) for this assignment! Write examples for each function you implement using a `where` block. Here's an example:

fun add-one(n :: Number) -> Number:
  doc: ```Function adds one to the input number```
  n + 1
where:
  add-one(1) is 2
  add-one(3) is 4
  add-one(-1) is 0
end

- Write examples before you write your code! This will ensure you understand the question (and this habit will help you significantly later on).
- You will be graded on having examples that cover a variety of situations, so take a look at the [Testing and Style Guidelines](https://hackmd.io/@cs111/pyret-design-clarity-guide).
- **Do *not* put your name anywhere in any file.** We grade anonymously, and submitting online helps us keep track of your submissions without your names on the file!

## Helpful Things

### Documentation

The [Pyret documentation](https://www.pyret.org/docs/latest/) is accessible from the pirate icon on the top left corner of code.pyret.org.

For this assignment, you will find the [Strings documentation](https://www.pyret.org/docs/latest/strings.html) useful. We recommend briefly browsing through the documentation before working on these problems to get a sense of what functions are available! But don't worry about reading the page in detail – you can always come back for more information on a function you need.

### Getting Help If You Need It

You can post small questions on [Edstem](https://edstem.org/us/courses/73794). Check the HW2 FAQ that will be pinned on Edstem first, as some questions may have already been asked and answered. *Make sure you're following our guidelines for Edstem, including not posting any solution code (or anything close to it) publically.*

If you have a longer question or want to go over something with a friendly staff member, come to [office hours](https://brown-csci0111.github.io/pages/calendar).

### Collaboration Policy

Starting with this homework, you are required to work on most assignments on your own and to make sure that the work you turn in is your own. If you are unsure of what this means, refer to the [syllabus](https://hackmd.io/@cs111/syllabus-s25#Collaboration-Policy) or ask! According to the official Brown policy on academic dishonesty, ignorance of a course's collaboration policy is not an excuse for plagiarism.

### How You'll Be Graded

Just as the syllabus lays out, having "working code" is only one means of demonstrating understanding of course concepts. You will be graded on a rubric that encompasses the following:

* The code passes *our* examples/tests (this is done by autograding, which is explained in a pinned post on [Edstem](https://edstem.org/us/courses/73794))
* The functions you write have examples/tests written by you
* Your code follows our [Testing and Style Guidelines](https://hackmd.io/@cs111/pyret-design-clarity-guide)
* Your reflections are complete and thoughtful
* You show understanding of the specific learning goals of the assignment (for example, reusing functions you have previously written where possible in this assignment)

Make sure you follow directions. For example, for Task 1, make sure to write down 6-8 elements *and* put stars by some of them.

## The Big Picture of this Assignment

Targeted advertising is a real-world example of how data, computation, and social impacts come together. Over the first few assignments, we want you to develop an understanding of how these systems work. The coding components of the first few homeworks will explore different ways of matching ads to potential customers, where the methods get more realistic as we go (and highlight some important program design points).

This week, we start with the simplest form of matching ads: writing explicit conditional expressions to see whether a person is in the target audience for a single, fixed ad.

## The Assignment
Cows often have to decipher the ads that they come across to see if there is any information on good grazing or drinking spots. In this assignment, we are going to help our cows by learning how to build functions that determine ad targeting.

<img src="https://hackmd.io/_uploads/HkM6PTLwJl.jpg" alt="sailor bee" width="50%" style="display: block; margin: 0 auto;">

*Image Source: [iStock Photo](https://www.istockphoto.com/vector/cow-graze-gm133971699-10745938)*

## Part 1: Representing Ads

The first step in processing something with a program is to *represent* that thing in terms of focused, discrete pieces of data. Consider the following ad for an outdoor adventure meetup:


**Task 1:** Imagine that this ad was posted around town. Write down a list of 6-8 elements that are in this ad that are designed to attract people to it.

Then, **put stars** (***) next to those elements that you think a program might use to decide whether to show the ad to someone.

Write your answer as a block comment in your code file. It should look something like the following (but in terms of ads, not calendars):

#|
My calendar includes dedicated times for
- looking for bears
- sleeping ***
- going to class ***
- exploring Rhode Island
|#

**Task 2:**
Data plays a big role in your life whether you are aware of it or not. For example, the information that a search engine has on you comes from a combination of your profile and predictions that the search engine makes about you.

Look at some of the information that Google gathers and infers about users. You can check out your own data by visiting https://myadcenter.google.com/ or look at the picture below for an example.


**In a separate document that you will save as `hw2-src.pdf`, write your responses to the following question(s).**
1. Using your data gathered by Google, *only if you’re comfortable*, what is the most surprising thing about your targeted ads?
* If you don't have your Google personalized ads turned on or aren't comfortable with using your own data, you may skip this question. Remember to answer question 2!
2. In **2-3 sentences**, discuss how Google may have come to infer these things about you and how this information might be used when making ad targeting decisions.
* If you don't have your Google personalized ads turned on or aren't comfortable with using your own data, use the information on the rightmost phone screen in the image above to answer this question.

**Note:** A summary of the examples submitted in this task may be shared after submission and grading. This may include paraphrased versions of your response. Everything will remain anonymous. **If you do *NOT* wish to share your response in any capacity, please write “DO NOT SHARE” at the beginning of the document.**

**Task 3:** For this assignment, we will use three pieces of information to represent an ad: the age of person it targets, the location of the event, and the kinds of activities it involves. Because we are working with a single fixed ad this week, we will store the information about the ad in three constants (values that we will access in the expressions that we write, using their names):

TARGET-AGE = 40
TARGET-TOWN = "Rockville"
TARGET-HOBBIES = "running, biking, walking, swimming, water polo"

Copy and paste these constants into the top of your code.

## Part 2: Writing Expressions

:::info
Learning Goals:
- To practice writing expressions.
:::

We're going to practice writing **[expressions](https://dcic-world.org/2023-02-21/getting-started.html#%28part._expressions%29)** (not functions, we'll get to those later in the assignment). It might seem a little strange to write down an expression when you could also just see what the answer is, but we are starting with these warmup tasks in order to transition to reasoning about functions in the later tasks. Think of this task as writing out a full mathematical expression in place of an answer, like `2 * 2` instead of `4`, but with programming! The expressions should refer to the names above (`TARGET-TOWN`, `TARGET-AGE`, `TARGET-HOBBIES`) instead of using their values (do not directly use the number `40`, the string`"Rockville"` etc).

When you hit "Run" and your expressions are evaluated, they might print out in the interactions window on the right-hand side of the screen. That's fine (for this assignment). If you want to stop this from happening, you can give the expression a name (e.g., `task1 = ...`).

**Task 4:** Write an expression to that evaluates to true if and only if Vanessa (age 21) is within 5 years (inclusive) of the constant `TARGET-AGE`.

*Hint: You can use the `>` and `<` symbols to compare two values with exclusive bounds (think "greater than") and `>=` and `<=` symbols to compare two values with inclusive bounds (think "greater than or equal to"). You might have to combine two comparisons with a Boolean operator (such as `and` or `or`).*

**Task 5:** Write an expression that evaluates to true if either `"Providence"` is *exactly* the same as `TARGET-TOWN` or `"reading"` is *exactly* the same as `TARGET-HOBBIES`.

*Note:* You can you use `==` or `string-equal` to do the comparison (the former works on any type of data, the latter works only on strings). For this homework, we are concerned only with exact matches (for example, we will treat "providence" and "Providence" as different strings).

**Task 6:** This time, instead of producing a Boolean, we want to produce a String that indicates whether the target town, target hobbies, both, or neither *exactly* match `"Rockville"` and `"reading"` respectively (evaluating to either `"town"`, `"hobbies"`, `"both"`, or `"neither"`). Use nested `if` expressions (where one `if` is within the answer of the other) to write an expression that evaluates to the appropriate string.

*Hint: Remember that* `if` *expressions finish with* `end`.

:::spoiler More Detailed Explanation
* If the target town matches `"Rockville"` but the target hobbies do not match `"reading"`, your expression should produce `"town"`.
* If the target town does not match `"Rockville"` but the target hobbies match `"reading"`, your expression should produce `"hobbies"`.
* If both match, your expression should produce `"both"`.
* If neither match, your expression should produce `"neither"`.
:::

<br>

**Task 7:** Contrast the expressions for the previous two tasks (the one that returned the `Boolean` and the one that returned a `String`). Write 1-2 sentences in a comment comparing boolean logic and nested `if` expressions. When can we use each form? When must we use each form?

*Note: You can create multi-line comments when you go over 80 characters on one line as follows:*

# This is a regular comment.

#| This is a
   multi-line
   comment! 
|#

## Part 3: Writing Functions for Targeted Ads

:::info
Learning Goals:
- To practice writing functions and their annotations
- To practice using documentation
:::

So far, our expressions have used fixed information about a person to compare to the constants that describe the ad. Now, we want to check whether the ad is suitable for an arbitrary person. To do this, we need to write functions that take information about the person as inputs.

:::warning
**Note:** Remember to include type annotations, doc-strings, and examples with all of your functions! Refer to the [Testing and Style Guidelines](https://hackmd.io/@cs111/pyret-design-clarity-guide) if you need help with this.
:::

**Task 8:** Convert your expression from Task 4 (checking the age) into a function called `within-5`. This function should take in a `Number` representing a person's age and produce a `Boolean`. It should return `true` if `TARGET-AGE` is within five years of the input age (inclusive), and `false` if not. Make sure to write type annotations, a docstring, and examples.

*Note: For this task and the next few tasks, you should be able to re-use a lot of the logic from tasks 4-6. Think about the examples from class: what specific value from the expression can you replace with an input name to make the computation work on any input value?*

**Task 9:** Previously, our expressions checked for an *exact* match of a hobbies string to `TARGET-HOBBIES`. In practice, we might want to match an ad if a given hobby was *within* the string (what we call a "substring"), rather than an exact match. For instance, if our `TARGET-HOBBIES` is `"running, biking, walking, swimming, water polo"`, we might want to return a match if our input hobby string is `"walking"`.

Write a function called `hobby-relates`, which takes in a `String` representing a person's hobby and produces a `Boolean` which is `true` when `TARGET-HOBBIES` *contains* the input hobby string, and `false` otherwise. Take a look at Pyret's [Strings documentation](https://www.pyret.org/docs/latest/strings.html) for an operation on strings that would help you do this (practicing using documentation is a key goal of this question, and is an important skill to develop in general).

**Task 10:** Consider Sporty Cow who has hobbies that include ``“mountain biking”``, ``“polo”``, and ``“swiming”`` (which they misspelled by accident). In your code file under `hobby-relates`, write a multi-line comment about why each of these hobbies might lead to a misleading output from `hobby-relates`.

**Task 11:** **Take a look at** this [article](https://www.brookings.edu/articles/solving-the-problem-of-racially-discriminatory-advertising-on-facebook/) and then **read** this [article](https://www.npr.org/2019/03/19/704831866/after-lawsuits-facebook-announces-changes-to-alleged-discriminatory-ad-targeting) about Facebook ads and ad targeting options, such as zip code and gender. In your **`hw2-src.pdf` file**, write **4-5 sentences** that discuss how these options could intentionally or unintentionally lead to harmful outcomes.

**Task 12:** The articles above provide just some examples of issues that may have been avoided had more diverse voices been included in the decision making process. Research another issue (having to do with programming or otherwise) that may have been prevented with more intentional efforts for diversity, equity, and inclusion. **Write about your researched example in your `hw2-src.pdf` file** - feel free to be creative with presenting the issue and how it could have been avoided. A good starting point would be to think about how your own background or identity might lead to you to make limiting assumptions or overlook certain edge cases. There is no specific length of response we are looking for, but make sure to cite your sources, summarize the issue, and include your own views and experiences.

**Note:** A summary of the researched examples submitted in this task may be shared after submission and grading. This may include paraphrased versions of your response. Everything will remain anonymous. **If you do *NOT* wish to share your response in any capacity, please write “DO NOT SHARE” at the beginning of the document.**

**Task 13:** Our current approach to matching towns/locations for ads is a bit restrictive: someone who lives in a neighboring town might still be able to get there, and we'd like to target them as well. This motivates our next function.

Write the function `is-nearby`, which takes in a `String` representing the name of a town and produces a `Boolean`. Produce `true` if the input is *exactly* one of `"Forest Dale"`, `"Waterville"`, `"Newtown"`, or `"Oldport"`; produce `false` otherwise.

**Task 14:** Now, let's practice calling functions in the body of another function. Write the function `in-range`, which takes in a`String` representing a person's town and a `Boolean` representing whether the person has a car. Produce `true` if the place is `TARGET-TOWN` *or* both the input place is nearby (one of the 4 towns in Task 13) and the person has a car. Otherwise, the function should produce `false`.

**Task 15:** Now, it's time to put all the criteria together to determine whether an ad should be shown to a potential customer or not. Write a function called `show-ad`. The inputs are (in this specific order) a `Number` representing a customer's age, a `String` representing their town, a `String` representing their hobby, and a `Boolean` representing whether they have a car. The output is a `Boolean` which is `true` when their age is within five years of the target age (inclusive), their town is `TARGET-TOWN` or they are nearby and have a car,, and `TARGET-HOBBIES` contains their hobby. The output is `false` otherwise.

**Hint:** Try to reuse your work from earlier tasks on this assignment (that's part of the point of this sequence of tasks).

### A Different Representation for Ads

So far, we have represented an ad as three constants (`TARGET-AGE`, etc). What if we represented each ad as a single string with all of its text instead? What other ways of matching people and ads might we try?

**Task 16:** Write the function `show-ad2`, which takes in a `String` representing the text of an ad and a `Number` representing a person's age, and produces a `Boolean`. This `Boolean` will be `true` if **any** of the following conditions are met, and `false` otherwise:

- The customer's age is 35 or younger and the ad contains the word "active"
- The customer's age is 65 or older and the ad contains the word "healthy"
- The ad contains the word "sport"

**Task 17:** Task 11 includes a real world example that shows how functions can produce unexpected or incorrect outcomes for certain individuals/groups. However, even when a function itself is “perfectly accurate” and does exactly what it's supposed to do, it can be *used* in harmful ways that we will continue to explore throughout the course. The function `show-ad2` makes assumptions about the customers that these ads are targeting. In a multi-line comment under `show-ad2`, list out at least two of these assumptions (consider the relationship between age and health, and what the criteria might be leaving out).

### Comparing different targeting methods

The functions `show-ad` and `show-ad2` are our first attempt at writing functions to match ads to people. We will revisit this theme as we learn more programming concepts, and our functions will become more sophisticated.

**Task 18**: Write a multi-line comment (in your code file) responding to these questions about the approaches taken in these two functions:
- How might these functions differ from how real ads work?
- Think about the way we set up the code and the programming operations that we used. What are the limitations of our current code/operations for matching ads with people? How might the ways that you processed data in this homework differ from how real-world data is stored and processed? (You don't have to know or research how this is done, but you should make an educated guess.)

**Task 19:** In a `check` block (just like a `where` block but not attached to any particular function), write **one** example for each ad-placing method (`show-ad` and `show-ad2`). This means that you should have a total of **two** examples, one for `show-ad` and one for `show-ad2`.

Each example should show a situation in which the function would output ``'false'`` (because of a limitation of the function), even though the ad would otherwise seem a good match for the person. Leave a short comment under each example explaining why you chose it.

Your `check` block should look like:

check "show-ad tests":
  show-ad(...) is false
  # Explanation
  show-ad2(...) is false
  # Explanation
end

## Part 4: Cleaning Up Code

:::info
Learning Goals:
- To practice reading code
- To practice cleaning up code by adding local names and/or helper functions
:::

In this exercise, we want you to use three concepts we've learned---local names, helper functions, and examples---to make a piece of code easier to read without changing what it computes.

**Task 20:** Rewrite the following function, which computes the cost of publishing an ad based on the length of the ad text, into a new version called `ad-charge-clean` that meets the Design and Clarity requirements of the [CS0111 Style Guidelines](http://cs.brown.edu/courses/csci0111/fall2021/assets/docs/pyret-clarity-design-testing.html).

fun ad-charge-messy(ad-text):
  short-length = 10
  medium-length = 40
  long-length = 70
  if ((string-length(ad-text) >= short-length) and (string-length(ad-text) < medium-length)):
    (string-length(ad-text) * (short-length / 2)) + (string-length(ad-text) * 5)
  else if ((string-length(ad-text) >= medium-length) and (string-length(ad-text) < long-length)):
    (string-length(ad-text) * (medium-length / 2)) + (string-length(ad-text) * 5)
  else if (string-length(ad-text) >= long-length) :
    (string-length(ad-text) * (long-length / 2)) + (string-length(ad-text) * 5)
  else: 0
  end
where:
  ad-charge-messy("Go Bruno!") is 0
  ad-charge-messy("Apply to Brown") is 140
end

*Hint:* Part of the point of this exercise is to figure out what the code is doing. You can either copy the code and edit it to clean up or try writing parts from scratch: follow whichever makes the most sense to you. Try looking for repeated or similar expressions: repeated expressions turn into locally-named ones, while similar expressions get turned into helper functions.

**Task 21:** We want to make sure that the clean version produces the same output as the original messy one. Provide a `check` block of the following form to convince yourself that your two versions are consistent in their outputs:

check:
  ad-charge-messy("Go Bruno!") is ad-charge-clean("Go Bruno!")
  ...
end

## Check Block (Autograder Compatibility)
When you have completed all the tasks of this homework, please copy the following check block into the bottom of your code file! Then, hit "Run".

The check block will be checking that all required functions are included, correctly named, and that they handle inputs in the right order. Please note that it does **not** check or confirm if the body of your function is correct – you should write your own examples to convince yourself of this.

check "functions exist and have correct inputs":
  within-5(0)
  hobby-relates("")
  is-nearby("")
  in-range("", true)
  show-ad(0, "", "", true)
  show-ad2("", 0)
  ad-charge-messy("Go Bruno!") is ad-charge-clean("Go Bruno!")
end

If you see the block below appear in the interactions window after running it, then all tests have passed!
&NewLine;

If not, double-check your function names, input types, and input order. If you are stuck at any point, please feel free to come to hours or post on [Ed](https://edstem.org/us/courses/73794)!

Once the check block passes, please submit your code file to [Gradescope](https://www.gradescope.com/courses/953715). To help you, we have made some of our autograder tests on Gradescope visible to you – wait a while for the autograder to finish running, and check Gradescope to make sure you also pass those tests! The tests on Gradescope check for *correctness*, but all visible tests passing *may not* mean that your function is fully correct. See the post on [Ed](https://edstem.org/us/courses/73794) about the autograder for more information!

-------------

## Handin
- Make sure your code file is called `hw2-code.arr`. Make sure your responses for the SRC tasks are in a separate PDF file titled `hw2-src.pdf`. Hand both in on [Gradescope](https://www.gradescope.com/courses/953715). Because we only see the latest submission, **make sure that you submit both files in the same submission when you are done**.
- 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).

## Theme Song
[Space Cowboy](https://www.youtube.com/watch?v=Kw0ih4jPOBo) by Kacey Musgraves.

## Spotify Playlist
<iframe style="border-radius:12px" src="https://open.spotify.com/embed/playlist/0P94UJbqA1eHGzxgcnUZyX?utm_source=generator" width="100%" height="372" frameBorder="0" allowfullscreen="" allow="autoplay; clipboard-write; encrypted-media; fullscreen; picture-in-picture" loading="lazy"></iframe>
------

> Brown University CSCI 0111 (Spring 2025)

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