Homework 3: Tables

Overview

To help stay within your weekly budget, you want a program that determines which store(s) you should visit to minimize your grocery bill. Your shopping list is a table of the items and quantities you need, and you have pricing data from three nearby food stores. You'll develop a program that helps you decide where to shop this week, trading off lowest price against convenience.

Learning objectives

After this assignment, you should be able to:

• Practice writing tables programs, on both single and multiple tables
• Practice breaking down tables problems into plans
• Practice creating testing strategies for table-based programs
• Practice using LLMs for various software tasks

Setup

• Make a copy of hw3-code.arr.
• Use this Gemini Gem for the AI-permitted parts of this assignment.
• 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).
• NEW 9/30: Make a new Pyret file named hw3-tests-examples.arr and put all of your check tests for the required functions in that file. This way, the part of the autograder that inspects your tests can get them in isolation of your functions. That file will not run on its own (since the function defintions are missing), but that's okay -- the autograder combines your tests with our implementations behind the scenes.

The CS111 tables documentation lists the table operations that are in the dcic2024 context.

Part 1Understanding the Data and Developing a Testing Strategy

We're going to capture a shopping list as a table with two columns: the name of the item, and the number of that item you need to buy.

item      | quantity
avocado   | 4
olive oil | 1
salt      | 1

For this assignment, you have access to three stores: MegaMart, Farmstand, and Corner Market (these load automatically in the hwk3-code.arr starter file). For each one, you have a table of the current prices of the items it carries. Each table has two columns: the name of the item and the price per unit:

item      | price
corn      | .75
olive oil | 6.99
bread     | 3.49

Overall, we're going to write programs to determine the lowest cost of your list under two different strategies: one where you shop at the single store that yields the lowest total bill, and one where you visit up to three stores, buying each item at the store where it costs least. The key functions for these are total-costs and where-to-buy. Each takes a shopping table and the three stores' pricing tables. total-costs returns a table with one row per store showing the total price of the shopping list at that store. where-to-buy returns a table of the shopping items and which store to buy each one at.

Note: For a reminder of what testing and code plans look like, see the class notes from the airplane-cabin exercise

Part 2Developing the Code

Now we'll write the code for these functions. The tasks below name the functions our test cases will look for. You're welcome to create helper functions as part of any of these.

To avoid looking in all three store tables every time you compare prices, it may make more sense to add columns to your shopping list showing the price of each item at each store. We want a function build-price-columns that takes a shopping table and three store tables and returns an extended shopping table with a column of prices for each store. You may assume every store sells every item on your list — raise an error "item not found" if an item is missing from an individual store.

build-column ( t :: Table,         # the table to add the column to
              colname :: String,  # the name of the new column
              col-func :: (Row -> Value) ) # compute the new column value
            -> Table

# add col for exam average
fun exam-avg(r :: Row) -> Boolean:
  (r["exam1"] + r["exam2"]) / 2
end

build-column(gradebook, "avg", exam-avg)

Rather than build one wide shopping table with a column per store, we could instead make a tall table: all the rows from the three store tables, plus a column naming which store each row came from. Such a table might look like:

item      | price | store
corn      | .75   | corner-market
olive oil | 6.99  | corner-market
bread     | 3.49  | megamart
milk      | 3.69  | megamart
corn      | 1.00  | farmstand

Part 3Design Questions