CS50 - Spring 2026 - Class 1

Example code in this lecture

   bbq.py

Lecture notes

  • What is Computer Science?

  • What previous experience do you have with programming/computer science?

  • Why are you taking this class?

  • About the class

  • Course Administrative (http://www.cs.pomona.edu/classes/cs50/administrivia.html)
       - Schedule
          - T/Th class
          - Roughly weekly assignments, released on Tuesday and due the following Monday
       - All material will be posted on the course web page
       - We will use slack for discussion
          - Use this to ask any content-based questions
          - Check it regularly
       - Late work policy
       - Grading
       - Academic honesty and collaboration (read the policy!)
       - AI tools are *NOT* to be used in any way on the assignments

  • Style of the course
       - interactive!
          - ask lots of questions
          - participate
          - be expected to do group work in class
       - You'll be expected to be here in class
       - I'll post my notes and examples online. You may still want to write some things down, but you don't have to write every word down

  • Experience
       - I assume no prior computer science, programming or science background
       - If you have previous programming experience or have take the AP CS class, come talk to me and we can figure out if this is the right place for you.

  • VS Code
       - We'll be using the VS Code IDE for this course as our interface into Python
       - What is an IDE?
          - Integrated Development Environment
          - Incorporates a text editor with other tools for running, debugging and navigating through the code
       - When it starts, a number of different parts of the window
          - For now (and most of this course), we're only going to be using two main components:
             - Main editor window
             - Terminal window on the right
             - (and also the Explorer, which allows you to navigate your files)
          - You can rearrange them however you'd like, but I prefer main editor on left and Terminal, etc. on right

  • Terminal
       - The terminal is another way to interact with your computer (specifically, the operating system) using typed commands rather than point-and-click
       - In VS Code, we can get the terminal window by selecting "View->Terminal"
       - We won't use the terminal extensively in this class beyond using it to start up Python

  • Python shell
       - Python is an interpreted language
          - you can type commands and get an immediate response back
          - many programming languages require you to compile the program first and then run it
       - To run Python, type "python3" (often just "python" works
       - Python makes a great calculator
          - the ">>>" is the shell prompt where you type statements
          - when you hit enter/return, Python executes the code and gives you the answer (if there is one)
          - Python has all of the standard mathematical operations
             - What math operations might you want?
                - +, -, *, /
                - ** (power or exponentiation)
                - % (mod aka remainder)

                >>> 4+4
                8
                >>> 15*20
                300
                >>> 20/4
                5.0
                >>> 10-20
                -10
                >>> 10+4*2
                18
                >>> (10+4)*2
                28
                >>> 2**10
                1024
                >>> 2**30
                1073741824
                >>> 2**-2
                0.25
                >>> 10%4
                2
                >>> 11 % 7
                4

             - What is operator precedence?
             - Python follows the normal operator precedence you're used to for math
                - things in parenthesis get evaluated first
                - ** is next
                - %, * and / next
                - + and - last

  • Expressions
       - Why are these different?
          >>> 2 + 3
          5
          >>> 10/2
          5.0

       - anything that represents a value (e.g. a number) is called an expression
       - contrast this with a statement, which tells the computer to do something
          - for example, "walk over there" is a statement in English
          - or, for computers, something like, "draw something on the screen"
       - All the things we've seen so far have been expressions
       - Even just typing a number, e.g. '5' is an expression
       - In Python (and many programming languages) all expressions have a type
       
  • types
       - the "type" of an expression determines how Python interprets and understands an expression
       - Python is a "strongly typed" language: every expression in Python has a type
       - what are the types of the two expressions above?
          - they are both numbers, but Python makes a distinction between integers and floating point numbers (i.e. numbers with decimals)
          
       - For now, just make sure that you understand:
          1) There are two different types of numbers in Python (integers and floating point numbers, aka floats)
          2) When you'll end up with an integer or a float
             - Division always ends up with a float
             - If any one number in an expression is a float, then the whole expression will be a float

             >>> 2+5*6
             32
             >>> 2.0 + 5 * 6
             32.0
             >>> 2**3
             8
             >>> (10/5)**3
             8.0
             >>> (10%3)*4
             4

  • bbq party
       - you're having a party and you're trying to figure out how many hot dogs to buy. Here's what you know:
          - teran isn't a big fan of hot dogs, so he'll only eat 1
          - jasmin generally eats 2
          - chris always eats twice as many as jasmin
          - brenda eats one less than chris
          - grace eats half as many as brenda at the party and also likes to take one extra one home
       - try and do this on paper
          - 13 (assuming that if someone eats half a hot dog, we still have to count the whole thing)
       - how did you do it?
       - how could you figure this using math operations in Python?
          - might be able to do it, but would require a lot of remembering (or writing things down)

  • variables
       - variables allow us to store things and then use them in other expressions
       - a variable is storage for a value
          - it holds a value
          - we can change its value
       - we change the value of a variable using '=' (also known as assigning to a variable)
          >>> teran = 1
          >>> teran
          1

          - changing the value of a variable is a statement. It tells the interpreter to do something, but does NOT represent a value         
          - Using variables, we can do our hot dog calculation as a sequence of assignments

          >>> teran = 1
          >>> jasmin = 2
          >>> chris = 2 * jasmin
          >>> brenda = chris - 1
          >>> grace = brenda/2 + 1
          >>> total_hotdogs = teran + jasmin + chris + brenda + grace
          >>> total_hotdogs
          12.5

       - Why did we get 12.5?
          - brenda/2 gave us 1.5
          
       - There are many ways to fix this so that we get an integer answer
          - One way: the // operator
             - "floor" division, rounds the answer towards negative infinity
                - for positive integers, drops the decimal portion
             >>> 11//2
             5
             >>> 10//3
             3
             >>> 11//3
             3
             >>> 11.0//2
             5.0

             Note that if you use // on integers, it gives you an integer answer and if use it on a float, it gives you a float, but still with the decimal part gets truncated
          - How does this help us?
             grace = (brenda + 1) // 2 + 1
             
             - we add one to force it to round up
                - if it's an odd number, it does what we want
                >>> 3 // 2
                1
                >>> (3 + 1) // 2
                2
                - if it's an even number, it doesn't change the answer
                >>> 4 // 2
                2
                >>> (4 + 1) // 2
                2

             >>> teran = 1
             >>> jasmin = 2
             >>> chris = 2 * jasmin
             >>> brenda = chris - 1
             >>> grace = (brenda + 1) // 2 + 1
             >>> total_hotdogs = teran + jasmin + chris + brenda + grace
             >>> total_hotdogs
             13

  • naming variables
          - generally you want to give good names to variables (x and y are not good names unless they represent x and y coordinates)
          - variables should be all lowercase
          - multiple words should be separated by an '_' (underscore)

  • variable assignment: when assigning to a variable, Python evaluates what is on the right hand side of the equals sign and then assigns this value to the variable
       - After typing the above, if I then type jasmin = 4, what happens to chris?
          - chris stays the same!
          - '=' is not mathematical equivalence

  • what if you get a text from jasmin and she now says she's planning on eating 4?
       - we'd have to re-enter each of the lines (well except the first one)
       - I already had to do this once to change for the // and it was annoying. I don't want to have to keep doing it!

  • programs in Python
       - besides interacting with the shell, we can write statements in a file and then run them
       - the file editing window in VS Code allows us to do this
       - on many levels, it behaves a lot like a text editor (e.g. Word). You can:
          - create new files
          - open files
          - save files
          - edit files
       
  • look at bbq.py code
       - I've typed in the same code we typed into the shell, but now in the text editing section
       - I've saved it in a file called bbq.py
          - we'll use the extension .py by convention to indicate a Python program
       - Anything different from what I'd typed before into the shell?
          - I've used whitespace
             - you can use spaces without affecting how the code executes
             - we use blank lines to make the code more understandable
          - I've included "comments"
          - These don't affect how the program runs, but are important for writing programs that you (and others) can understand
             - this is part of the "style" of the program

  • comments
       - comments in Python are designated using '#' (the pound sign)
       - python ignores everything from the # to the end of the line
          - you can put them on lines by themselves
          - or if you have short comments, you can add them at the end of a line
       - comments are VERY important
          - they allow you to communicate in plain English (to others and to yourself when you look at the code later)
          - you will be required to put them in your programs for this course

  • running the code
       - With an IDE, not only can you edit code, you can also run it
       - The easiest way is to click the "run" arrow
          - it executes your program a line at a time from the top
             - it's like you typed all of those commands in the shell
          - the one difference is that you don't get line-by-line feedback
             - for example, if you put 4+4 in a program on a line by itself, you won't see anything
          - if you want to see the value of a variable or an expression, you need to "print" it (more on this next time)
       - Often, it can be useful to run the program and then still be able to interact with python
       - You can run python "interactive" from the terminal, by typing "python3 -i your_program_name.py"
          - If you do this, just like running a program normally, it executes each statement a line at a time
          - However, when the program finishes, it leaves you in the python shell.
          - You can look at the value of any variables, etc. E.g.,
             >>> total_hotdogs
             13

  • What is the syntax of a language (say English)?
       - It describes what are valid things to say in that language
          - "I like dogs" is syntactically correct (it's a determiner followed by a verb followed by a noun)
          - "I dog like" is not syntactically correct (it's a determiner followed by a noun followed by a verb)
       - programming languages also have their own syntax
       - it describes what is valid in a language

  • when syntax fails...
       - have we seen any examples of syntax so far?
          - all of the math operations have implicit syntax
          
             >>>4 +
              File "<input>", line 1
              4 +
              ^
             SyntaxError: invalid syntax
       
             - python explicitly tells us that we had a syntax error
             - we also get some warning with the red squiggly line

          - assignment as well

          >>> dave =
           File "<input>", line 1
           dave =
           ^
          SyntaxError: invalid syntax
       
       - the nice thing about using the VS Code is that it checks the syntax and tries (but doesn't always succeed) at highlighting where the syntax error is
          - sometimes look at the line right before it if you can't find it on that line
    •