Lecture 02 - CS61A fa2022

To-do

• How to publish slides?

"Zoom" rules - Learning English Time

Expressions and Values

Expressions: 'a' + 'hoy'
Value: 'ahoy'

Expressions

1. Two ways and combination: 1+2(expression with operation) pow(2,3) (call expression)
2. pow() is built-in, while others (like add()) must be imported by from operator import add. More opreators.
   • Why need add()?

3. How Python evaluates a call expression:

   Evaluate the operator Evaluate the operands Apply the operator (a function) to the evaluated operands (arguments)

   Operators and operands are also expressions, so they must be evaluated to discover their values.

   • What does it mean? functions and argument are not values?
     Answer: See the following example.

4. Nested Expressions: Expression Tree

   • Recursively or iteratively?
5. Name: just the name of the var.
   Binding a name to a value: (English!)
   1. var_a(name) =(assigned by) 1(value)
   2. var_a(name) =(assigned by) 1 + 1(expression)
6. Variable: A name that is bound to a value.
7. More of operators.

Environment Diagrams

Just a GUI simple debuger.

Functions

1. function, args and return value
2. def
   • Syntax:

     def <name>(<parameters>):        # ← Function signature
         return <return expression>   # ← Function body
     

   • A def statement creates a function object with certain parameters and a body and binds it to a name in the current environment.

3. Function is Object, which is very wierd for me with only little background of C.
   See more here.
   Example:

   f = min
   f = max
   g = min
   h = max
   max = g
   max(f(2, g(h(1, 5), 3)), 4)  # 
   

4. Note the parameters and return values of functions.
5. Calling a function is creating a stack frame.
6. Environment: Assembly consisting of all frames of stacks of a specific scope.
7. Name lookup rules: local -> global -> NameError

Terms

Vocabs and Phrases