Lecture 1: Introduction, Functions, Strings

• No prior CS or math background
• May feel slow at the beginning, but the pace does pick up, so don't get complacent

• No late work, or else cascading overlapping deadlines (very stressful for both of us)

• avoid internet sources besides the course website, Python's official documentation, and our textbook
• peer helping is ok, but…
  • peer should never touch your keyboard or mouse
  • (ideally help on whiteboard)
  • don't take away someone's a-ha moment
  • obvious thing: no screen
• TA expectations
  • TAs will not answer your homework questions
  • They will instead ask you their own questions
    • Or point you to examples
  • Don't ask them to work outside of mentor sessions
  • Piazza is a great alternative

• ask lots of questions, participate, expect to do work in class.
• Do show up to lecture and lab, or you may fall behind.

• focus on writing down confusing things or questions, not every detail about the course.

• I want you to come out of this class a person who knows how to program and who understands AI.
  • We'll talk more about AI in the coming weeks, but for now the focus is Python
• This course is a freight train
• If you feel like the train is leaving without you (you are a train car in this metaphor), tell me
  • Or a TA
  • Usually it just takes one insight to break the logjam (in this metaphor you are maybe water) and get you back on track (you're a train car again)

• Python programming language and interpreter
• Used mainly through PyCharm IDE ("Integrated Development Environment")
  • Text editor plus tools for running, debugging, navigating, and executing code in "projects"
  • For now, mostly using the "Python Console" and text editor
• Setup instructions available on the website for reference and in lab today
• You can customize the editor and rearrange it how you like. In fact, please do make it yours as much as possible.

• You give Python a statement (a command); the Python program interprets this statement, makes sense of it as Python code, and replies with a response
• A program is just a sequence of such statements
  • Some statements may take up multiple lines
• (Some programming languages don't give a response until the whole program is input, compiled, and then run.)

• Hit enter or return to make it go
• Hit up or down to go through the history of things you've input before

• Standard math operations: +, -, *, /
• Exponentiation: **
• Modulus/remainder: %

• Example:

  >>> 4+4
  8
  >>> 15*20
  300
  >>> 20/4
  5
  >>> 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
  

• Operator precedence/order of operations
  • Same as in regular math: (), **, */%, +-

expressions

>>> 2 + 3
5
>>> 10/2
5.0

• How are these different? And why?
• Every value (5, 5.0) has a type
• Every expression (e.g. 2+3) produces a value
  • Therefore every expression also has a type
• In Python we have two main numeric types: integers and floating point (decimal) numbers, aka floats.
• Earlier I said that Python works on statements, which is still true
  • An expression by itself can be a statement
    • And a value by itself can be an expression!
  • A statement in English can be something like "walk over there"
  • In Python, "compute this expression" is the default, but there are others
  • Statements are separated by newlines
• Types are essential to programming
  • This will show up intensely in 54 and later courses: a well-typed program will never "go wrong"

• 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)

• We can name things with variables

  >>> teran = 1
  >>> teran
  1
  

• assignment of a value (right hand side) to a variable (left hand side) is a statement
• A variable is a reference to some location in computer memory

  • Two variables could point to the same thing

    >>> dog_legs = 4
    >>> cat_legs = 4
    >>> dog_legs
    4
    >>> cat_legs
    4
    

  • We can change what a variable points to—it only changes that one reference!

    >>> veggie_burgers = 3
    >>> hot_dogs = veggie_burgers
    >>> hot_dogs
    3
    >>> hot_dogs = 1
    >>> hot_dogs
    1
    >>> veggie_burgers
    3
    

  • This is not mathematical equality, it's assignment

• Using variables, write your bbq calculator again with one variable per person and a total_hotdogs variable

  >>> 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 12.5? brenda/2!
• We can fix this
  • Integral ("truncating") division operator x // y
  • How? (brenda+1)//2

• We generally want to give descriptive names for variables
• Avoid x or y unless it's a Cartesian coordinate!
• You will be graded on using good variable names!

• It is super tedious to type everything into the console over and over and over again
• In PyCharm, we can run a whole file at once
• open up the bbq.py example (I like to make an "examples" project) or copy its contents and paste it into a new file
• PyCharm treats any file ending in .py as a Python code file
• What differences do you notice about this code?
  • whitespace
  • comments (everything after the # is just for humans to read)
  • You'll want to be sure to comment your code to explain the "why" and maybe "how", rather than the "what"
• right click anywhere in the buffer and choose Run in Python Console
  • More about this in today's lab
  • What's different? No line-by-line feedback!
    • If you want to output text to the console, you need to print() it—more about that next week or a bit later

  • but! we do have all the variables from this file available in the console now

    >>> total_hotdogs
    13
    

• sometimes we have useful packets of code we want to use in lots of places
• there's other built in math functions as well: abs, round, …
• we can call functions with more than one argument: min(10, 5)
• these functions take inputs and produce an output

• we can also define our own functions using def
• our hot dog calculator is pretty great
• but what if jasmin or teran want different amounts of hot dogs for different cookouts?

• the general way to write a function that takes inputs ("arguments" or "parameters") is:

  def my_function(argument1, argument2, argument3):
      statement1
      statement2
      statement3
      return expression # not every function evaluates to a value, but this one does
  

• Python knows what's "in" and "out" based on indentation
  • Statements which line up on their left sides are in the same block
• do this for our bbq code: wrap it in a function with the right number of arguments (we want teran and jasmin as inputs) and a return value
• See some more examples of functions in simple_functions.py

• type(5) : <class 'int'>
• type(5.0) : <class 'float'>
• type('hello') : <class 'str'>

• 'hello ' + 'world' : "hello world"

• Must convert ints to strings using str(), eg. str(5.0)

• format can be useful if you have lots of variables or calculated values to push into a string, and you won't forget to call str()