Homework 2: Conditionals and Boolean Expressions

Overview

This assignment practices reading, testing, and writing programs that use functions and conditionals. We also use it to build an understanding of what a “model” is, using the context of computing home energy usage. This familiar domain is designed to prepare us to explore the costs of generative AI a bit later in the semester.

Learning objectives

After this assignment, you should be able to:

• Read and write code with conditional and boolean expressions
• Compare different ways to do the same conditional computation
• Develop a good set of tests for a program that uses conditionals
• Explain what a model is, and apply one in the context of energy usage

Setup

• Open hw2-code.arr and save a copy for yourself. This file loads in another file of supporting functions and definitions (called energy-lib.arr). You do not need to download energy-lib.arr (the hw2-code file loads it automatically), but you'll want to look at it for reference.
• Open conditionals.arr and save a copy for yourself.
• Make a copy of the written answer worksheet and submit the link to it in the worksheet submission form (you can do this before filling anything in).

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Part 1Variations on Programs that Ask Questions

The file conditionals.arr contains three programming problems, each of which comes with 3 or 4 proposed solutions. This exercise has you explore different ways to write programs with conditionals, as well as learn to think about mistakes that can arise when working with conditionals. For this part, we want you to do two things:

Part 2Developing a Model of Home Energy Usage

One of your roommates is concerned about the energy usage in your apartment. They complain about lights being left on and phone chargers being plugged into the wall. At the same time, they also drive to campus, own three game consoles, and replace their laptop every two years. Which behavior is worse? In this part, we’ll learn what it means to build a model of energy usage and apply it to comparing scenarios of home energy usage. This sets the foundation for us to look at the energy usage of LLMs in upcoming assignments.

Energy terminology

Discussions of energy consumption often reference kilowatt-hours (units of energy consumed) and CO2e (carbon-dioxide equivalent, a shorthand for greenhouse gas emissions). Many gasses other than carbon dioxide trap heat in the atmosphere; in practice, all emissions get converted into units of CO2 to ease comparisons.

• Watts and watt-hours. Watts give the rate at which something uses energy. Watt-hours (Wh) give an amount of usage. For example, a 10-watt bulb does not “use 10 watts” over an evening. It draws 10 watts for as long as it’s on, the way a tap runs at a certain rate. Leave it on for one hour and you’ve used 10 watt-hours. Leave it for four and you’ve used 40. Watts × hours = watt-hours.
• Kilowatt-hours (kWh) are the units on a home energy bill. 1 kWh = 1000 Wh. Most of the interesting comparisons happen in kWh, so you’ll convert.
• CO2 is measured in kilograms. Unlike watt-hours, which are standard across locations, the amount of CO2 that gets released varies with the energy infrastructure in a given location. The same laptop, used identically, has wildly different footprints depending on which wall it’s plugged into.

The energy-lib.arr file mentioned in the Setup section has values for the wattage of various appliances and the CO2 emissions from various regions of the world. In practice, wattage is printed on each appliance. Emissions vary during the year, so these figures are annual averages. A more refined analysis would use a finer-grained model, but this suffices for our purposes; just don’t quote the CO2 figures as static facts.

Examples and Tests

As you write functions for the rest of this assignment, include a where: block on each one showing three examples of what the program does. These examples don't need to be comprehensive, but they should illustrate different input characteristics or scenarios.

Running one appliance

For someone with limited experience with energy, hearing that their usage is “114 grams of CO2e” isn’t relatable or meaningful. In practice, energy messaging often reports usage relative to other usages that people understand, such as “less than driving a mile”.

Comparing choices

In practice, we compare the (potential) energy usage of different device and lifestyle choices.

Settle the argument

Part 3Reflection

Grading

For the first two tasks, we'll check whether your tests are adequate and your identified fixes accurate. For the remaining programming tasks, we will run a collection of our own tests against your functions to check their behavior.

Code Conversations

For the remaining problems, you will sign up to have a conversation with the TAs about your solutions. These conversations are designed to help you practice talking about programs, while also being a good way for us to give you feedback about what you're learning from the assignments. They're also a way for us to make sure that you understand the work that you are turning in.

To help everyone get comfortable with code conversations, on this assignment you'll hold them in pairs: two students and two TAs meeting together (this form of grading is new for our TAs as well). As the semester progresses, you'll do these one-on-one with TAs. Don't worry about doing well on this first one -- we know you don't know how to do this, so this will give you a sense of what to expect for later in the course when these start to count much more towards your grade.