Data in Trees

Structuring and Navigating Files

Concept Acquisition

1. Data is stored on a computer disk in files.
2. Files are stored in directories (or folders).
3. Files and directories can be structured using a tree.
4. Each file’s location is defined by its path.

Tool Acquisition

1. The structure of relative and absolute paths.
2. UNIX commands
   • pwd
   • ls
   • cd
   • cat

Concept Application

1. Given a file system tree,
   1. write down the absolute path to a file.
   2. given two file system items, write the relative path from one to the other.
2. Given a set of file paths, reconstruct the file system tree.
3. Navigate a file system at the command line using the provided UNIX commands.

Welcome to Stat 133 Lab

As you come in, please drop your index card from lecture on the front desk or fill out a new card using the same prompt:

On one side of your card write answers to the following questions (borrow a pen from a neighbor if needed):

1. What is your lab number? (integer)
2. How many years have you been at Cal? (integer)
3. Are you a Statistics major? (true / false)

On the other side of the card write answers to the following questions:

1. What was your “song of the summer”? (character string)
2. What is the farthest that you were from Cal this summer in miles? (numeric)

Gather all of the index cards that you have from students in your section for the Data as Trees activity. If you are missing a few cards, no worries.

Introductions

01:00

Go ahead and use the icebreaker of your choice! If you’d like to use the timer, click it to start/stop, and hit the +/- buttons to add or subtract time.

About the Course

Syllabus

05:00

Please take 5 minutes to read the syllabus and jot down at least two questions that you have.

Pass out printed copies of the syllabus and have students read it silently for ~ 5 min.

Afterwards ask for questions. Do your best to answer them. If you’re not sure of the answer, sign-in to Ed and demonstrate how to make a new post to ask a question. Then, while still projecting, ask that student to open their laptop and make that post. You should see it appear on your screen. You can respond with something like, “I’m not sure but I’ll find out!”.

The goals here are two-fold: answer logistical questions about the class and be sure people have a sense of how to ask questions on Ed.

Discussion forum: Ed

Course website

berkeley-stat133/fall-2025

Data on Trees

boardwork

Gather up the stack of index cards from your students with answers to the questions asked in lecture.

On the board sketch a very simple version of the Question > Data Product diagram seen in lecture: Question: what were the songs of summer for Stat 133 students? Then for data, draw a box for each individual index card. Then draw a big empty “computational” box, and then ask what sort of data products could answer this question. Students may suggest different types of visualizations (e.g.) barchart ordered in descending order by popularity), summary statistics (most popular song of Stat 133 students), estimates (most popular song of Berkeley students).

Today we’ll focus on how to store that data on a computer. Draw one box inside the bix “computational” box labelled “Storage”.

Terminology

File

A named container that stores data – such as text, images, or programs – on a computer disk. The file name usually ends in an extension (.txt, .png) that indicates the format.

Example

127.txt

Golden Klaxons
3000?

One way to store the data on the index card is as a text file. Read more about this particular format here: https://en.wikipedia.org/wiki/Text_file

Questions you could ask after showing the .txt:

1. Where did the number 127 come from? It was an arbitrary ID.
2. Does anyone see anything that might be a challenge as we try to analysis this txt file as data? We will likely have to remove the ? to treat 3000 as a number.
3. Does anyone know this song? Probably not, because it seems to be a mix of a song and an artist … see next slide.

I’m not sure what happened here! I can’t find a song called “Golden Klaxons”, only this one. The question for the students - how should we handle this? Should we fix it?

(we’ll leave it for now and fix it in a later class)

Directory (or folder)

A named container that stores a set of files or other folders.

How should we organize our cards into directories?

Boardwork.

Lead students into thinking about the information that we have on the other side of the card: Lab #, Years at Cal, and Stats Major (yes/no).

As they propose one form of sorting, say by years at Cal, ask them if we can do any sorting within each directory – in this case, by Stats Major.

Whichever sorting scheme they propose, draw it on the board as a tree with a root node (an empty circle), with lines leading to each of the top level directory (say, circles labelled “One”, “Two”, “Three”, and “Four”), and then for each, lines leading to circles of the subdirectories (“S” for stats major, “NS” for non-stats major, say), then with lines leading to the files. For those, tape the index cards in the appropriate place.

How can we specify the location of a file/directory in the tree?

Path

A description of a file or directory’s location in the file system, written as a sequence of folder names that leads to it, separated by /.¹

Absolute path: Starts from the root of the file system, indicated by /. Example: /Documents/Schoolwork/HW1.pdf

What absolute paths are found in our tree?

Return to the board and circle a file or two and ask for someone to provide you the absolute path. Write it on the board (be sure to start with /). Repeat by asking the path to a directory. Circle that directory and write the path next to it (it will start with a / and end with the name of that directory).

1. \ is used on older Windows computers but newer versions recognize the more standard /.

Path

A description of a file or directory’s location in the file system, written as a sequence of folder names that leads to it, separated by /.

Absolute path: Starts from the root of the file system, indicated by /. Example: /Documents/Schoolwork/HW1.pdf

Relative path: Starts from the current directory and shows how to reach a target. .. means “go up one level”. Example if you’re in Documents, Schoolwork/HW1.pdf

What relative paths are found in our tree?

On the board pick at least two relative paths for them to give you, one that goes from a directory to a file in a subdirectory, and another that requires going “up one level” to a file in a differet directory at the same level. You can indiated the “start” and “end” of the relative path using, say a square and a triangle.

Practicing on a computer

Don’t have students open their laptop, first have them just watch you.

Click on the link on the following slide to go to DataHub with our practice sample of index card files.

1. First use the GUI to show how the file system looks similar to Finder on Mac or Windows or Google Drive - it is structured in a tree.
2. Then click File > New > Terminal to introduce them to the command line / Terminal.
3. Describe it as a way to write and record the way that we navigate a file system. We’ll soon be using these paths in computer code to navigate and manipulate files much faster and more predictably than with a GUI.
4. Tell them now you’ll show them four very useful commands to use at the command line.

You can either show the commands using slides that follow or write them on the board.

UNIX¹ Commands

pwd

Returns your present working directory.

ls

Lists the contents of your present working directory.

1. UNIX is a family of operating systems that includes Linux and MacOS. Most of the commands work on windows too.

cd

Changes your directory to the path provided (after a space). Examples:

• cd Homework: (relative) only takes you into the Homework directory if you’re in Documents.
• cd /Document/Homework: (absolute) takes you into the Homework directory regardless of your pwd.

cat

Prints to the screen the contents of the file at the path provided. Example: cat 127.txt.

Return to DataHub and show them examples of each of the commands. At some point (either now or when reviewing answers to the practice problems), you can show them the tree command.

DataHub

We will practice on a sample of our index cards on a cloud-hosted machine that has a directory containing sample of our index card files.

Login Here:

https://datahub.berkeley.edu/hub/user-redirect/git-pull?repo=https%3A%2F%2Fgithub.com%2Fberkeley-stat133%2Fdata-in-trees&urlpath=tree%2Fdata-in-trees%2F&branch=master

Practice

10:00