Announcements

• Hog
  • Released on Monday
  • Please start early! OH is usually far less busy at the start, so the more you wait to start, the more likely you'll have to wait longer in the queue
  • Getting started videos
  • Phase 1 due Feb 7 - worth 1 point
  • Submit full project 1 day early (Feb 9) for 1 bonus point (must have a full submission if you want full credit + the extra credit point)
• CSM Sections have opened
  • Small group tutoring sections (5-6 people), get to see more of the content
  • I joined a CSM section and found it incredibly useful! Would highly recommend

Questions and Comments from last section

• People like seeing stuff executed in the terminal
  • Neat, will do this more (often)
• Write bigger on the board!
  • whoops, ok!
• More examples
  • depends on whether i have the time to
• Write more specific on the slides so it's better for review
  • i prefer not having too much on slides just cause it makes me feel like i'm reading off them

Questions and Comments from last section

• BOTW
  • haven't played it, but i've played music from it in an orchestra lmao
• Marvel Snap
  • haven't gotten around to it, but it looks really interesting
  • card games/board games are fun
    • dominion

Questions and Comments from last section

• Midterm
  • don't have too much time to go over it, but i have resources you can look at
  • regardless, focus on environment diagrams
  • cs61a.rouxl.es midterm advice (links.rouxl.es/mt1-advice)
    • there are video walkthroughs of past midterm quetions that i like
    • i spent minutes of my life making it please use it
  • i'll go through the process of how i annotate a question after environment diagrams
• today is going to be me talking a lot

Temperature Check

• Environment Diagrams
• lambda functions
• Higher-order Functions

Environment Diagrams

• Environment diagrams are a great way to learn how coding languages work under the hood
• Keeps track of all the variables that have been defined, and the values that they hold
  • Done with the use of frames
• Expressions evaluate to values:
  • 1 + 1 → 2
• Statements do not evaluate to values:
  • def statements, assignments, etc.
• Statements change our environment

Frames

• The Global Frame exists by default
• Frames list bindings between variables and their values
• Frames also tell us how to look up values

Assignment

• Assignment statements bind a value to a name
  • The right side is evaluated before being bounded to the name on the left
  • = is not the same in Python and mathematics
• These are then put in the correct frame in the environment diagram

Assignment

def statements

• Creates function (objects), and binds them to a variable name
• The function is not executed until called!
• Name of the variable is the name of the function
• Parent of the function is the frame where the function is defined
• Keep track of:
  • Name
  • Parameters
  • Parent

Example

• Keep track of the name, parameters, and parent!
• Uses pointers (unlike for primitive values)

Example

• Keep track of the name, parameters, and parent!
• Uses pointers (unlike for primitive values)

Call Expressions

(Order of operations for nested call expressions)

Example 1

Example 2

Variable Lookup

• Look in your current frame to find your variable
• If it doesn't exist, repeat the same process in the parent frame (including the lookup if you don't find anything)
• If you reach the global frame and still can't find anything, the program errors
  • This is because the variable doesn't exist

Variable Lookup

Example

(Assume that we're looking for variables inside f2)

Variable Lookup

Example

• If we start off in f2, we already see z in f2, so there is no need to look at the frame above.
• However, for the case of y, we do need to look up to its parent frame, and for x, we need to lookup 2 levels

New Frames

• New frames are made when a function is called
• Label your frame with a unique index (convention is f1, f2, etc.)
• Write down the name of the function object
  • Not necessarily the name of the variable!
• Write down the parent that the function you're calling has
• Separately, all frames (other than the global frame) have a return value
  • This can be None if nothing is specified

Example

Example

Question 2

lambda Syntax

• lambda <args>: <body>
• What goes in <body> must be a single expression

lambda Example

lambda Example 2

lambda functions can also be used as the operator for a function!

Higher Order Functions (HOF)

• HOFs are functions that can do the following things (can be both):
  1. Take in other functions as inputs
  2. Return a function as an output
• You can treat a function as just an object or a value (there's nothing special about them)
• function and function() mean different things!
  • function refers to the object itself (in the environment diagram, it refers to what the arrow is pointing to)
  • function() actually calls and executes the body of the function

HOF Example 1 (Functions as input)

HOF Example 2 (Functions as output)

HOF Example 2

The two above are equivalent statements!

(Notice how the lambda one takes up far less space!)

lambda Functions and Higher-Order Functions

• A lambda expression evaluates to a lambda function
  • Can be used as the operator for a function!
• These functions work the same way as a normal function
  • Can be written in 1 line - faster way to make functions
  • Similar to def in usage, but different syntax
• lambdas are especially useful when you want to use a function once and then never use it again (will see examples of this)

Currying

Currying is one application of the HOFs from earlier.

Instead of just any expression on the inside (for example x + y), we use a function!

Currying

• Currying is the process of turning a function that takes in multiple arguments to one that takes in one argument.
• What's the point?
  • Sometimes functions with 1 argument are far easier to deal with
  • Can create a bunch of functions that have slightly different starting values which saves on repeating code
  • Useful for the map function (it requires functions that have only 1 argument)
• Kind of hard to see the benefits until you write production code

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