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Carter Zenke
carter@cs50.harvard.edu
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David J. Malan
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From Afterplace
Problem to Solve
You have tended to a vast library at the edge of the woods for many years, helping readers find the knowledge they need to continue on their journey.
In a folder named library, in files named log.R and answers.txt, help several readers find the books they are looking for.
Distribution Code
For this problem, you’ll need to download log.R, answers.txt, and two CSV files: books.csv and authors.csv.
Download the distribution code
Open RStudio per the linked steps and navigate to the R console:
>Next execute
getwd()to print your working directory. Ensure your current working directory is where you’d like to download this problem’s distribution code. If using RStudio through cs50.dev the recommended directory is /workspaces/NUMBER where NUMBER is a number unique to your codespace.
If you do not see the right working directory, use setwd to change it! Try typing setwd("..") if in the working directory of another problem, which will move you one directory higher.
Next execute
download.file("https://cdn.cs50.net/r/2024/x/psets/2/librarian.zip", "librarian.zip")in order to download a ZIP called librarian.zip into your codespace.
Then execute
unzip("librarian.zip")to create a folder called librarian. You no longer need the ZIP file, so you can execute
file.remove("librarian.zip")Now type
setwd("librarian")followed by Enter to move yourself into (i.e., open) that directory. Your working directory should now end with
librarian/If all was successful, you should execute
list.files()and see answers.txt, authors.csv, books.csv, and log.R. If not, retrace your steps and see if you can determine where you went wrong!
Schema
Before jumping in, it will be helpful to get a sense for the “schema” (i.e., organization!) of the data you’re given.
Learn about this data
Provided to you are two CSV files: books.csv and authors.csv.
In books.csv, you’ll find that each row is a book in your library. For each book, you have the following information:
- title, which is the title of the book
- author, which is the author of the book
- topic, which is the topic of the book
- year, which is the year the book was published
- pages, which is the number of pages in the book
In authors.csv, you’ll find that each row is an author of a book in your library. For each author, you have the following information:
- author, which is the name of the author
- hometown, which is the hometown of the author
Specification
For this problem, equally as important as finding the books is the process you use to do so. In log.R, keep a log of all R code you run to find each reader’s book. Label each section with a comment: in R, comments are any lines that begin with #, per the below:
# This is a comment in RThe comment should describe why you’ve written the code you’ve written, as well as what information you’re hoping to glean from it. Ultimately, log.R should serve as evidence of the process you used to find each book!
Upon finding a book, complete the corresponding line in answers.txt by filling in the book’s title. Be sure not to change any of the existing text in the file or to add any other lines to the file!
See a sample completed answers.txt file
The Writer is looking for: Navigating the Literary MarketThe Musician is looking for: The Art of Piano PlayingThe Traveler is looking for: Elemental ConjurationsThe Painter is looking for: The Abstract RevolutionThe Scientist is looking for: The Diversity of BirdsThe Teacher is looking for: Fostering a Sense of BelongingUltimately, you should submit both your log.R and answers.txt files.
The Writer
Your first reader of the day walks up to your desk and greets you:
Dearest librarian, curator, cataloger! I yearn, hunger, dream for the author Mia Morgan’s sole, only, exclusive book! Please uncover it, fetch it, retrieve it for me. I will owe you a great debt, a wonderful sum, an immortal obligation.
In log.R, write R code to find the title of the book that The Writer is referencing. Once you’ve found the title, write it in answers.txt.
The Musician
A reader with a ukulele walks up to your desk:
Hey, I’m on the lookout for a book on the topic of music, a real classic. I think it hit the shelves back in 1613. Music history is just so fascinating, don’t you think?
In log.R, write R code to find the title of the book that The Musician is referencing. Once you’ve found the title, write it in answers.txt.
The Traveler
A hunched, cloaked figure approaches your desk, handing you a sheet of paper without saying a word. On it, you see two possible author names: Lysandra Silverleaf or Elena Petrova. Below, you see the year in which the book was published: 1775.
In log.R, write R code to find the title of the book that The Traveler is referencing. Once you’ve found the title, write it in answers.txt.
The Painter
From behind, you hear a reader call to you:
Oh, I remember this wonderful book on the topic of art from my childhood! It was like a burst of colors—vivid reds, soothing blues, vibrant yellows. It was not too long, not too short, probably between 200 and 300 pages. And it was definitely published in either 1990 or 1992, but absolutely not 1991.
In log.R, write R code to find the title of the book that The Painter is referencing. Once you’ve found the title, write it in answers.txt.
The Scientist
You receive a phone call:
I need the book with “Quantum Mechanics” in the title.
In log.R, write R code to find the title of the book that The Scientist is referencing. Once you’ve found the title, write it in answers.txt.
Hint
The function grepl determines—TRUE or FALSE—whether a pattern of characters is present in another character string. For instance, consider the following:
grepl("Biology", "The Biology of Coral Reefs")which would return:
TRUEgrepl can also be used with vectors:
grepl("Biology", c("The Biology of Coral Reefs", "The Biology of Plants", "Marine Conservation"))The above would return a vector of length three:
TRUE TRUE FALSEThe Teacher
A small knock on the door reveals your next reader:
Apologies for the trouble, but I’m looking for a book on the topic of education published in the 1700s. Unfortunately, I can’t recall the author, but I do remember they hailed from the town of Zenthia.
In log.R, write R code to find the title of the book that The Teacher is referencing. Once you’ve found the title, write it in answers.txt.
Hint
The logical operator %in% returns whether a given value is in a vector—TRUE or FALSE. For instance, consider the following:
"Kenji Sato" %in% c("Jack Parker", "Kenji Sato")which would return:
TRUESimilar to other logical operators such as & and |, %in% is vectorized. Consider the following:
c("Alaric Runeweaver", "Kenji Sato", "Thalia Starbinder") %in% c("Jack Parker", "Kenji Sato")which would return a vector of length three:
FALSE TRUE FALSEUsage
Assuming log.R is in your working directory, you might find it most helpful to use Command + Enter (if on Mac) or Control + Enter (if on Windows) to run individual lines of code from log.R.
How to Test
check50
You can check your code using check50, a program that CS50 will use to test your code when you submit. But be sure to test it yourself as well!
Run the following command in the RStudio console:
check50("cs50/problems/2024/r/librarian")Green smilies mean your program has passed a test! Red frownies will indicate your program output something unexpected. Visit the URL that check50 outputs to see the input check50 handed to your program, what output it expected, and what output your program actually gave.
How to Submit
You can submit your code using submit50.
Keeping in mind the course’s policy on academic honesty, run the following command in the RStudio console:
submit50("cs50/problems/2024/r/librarian")