CS 61A: SICP - week2 - 1

一 Print and None

1. None Indicates that Nothing is Returned

    The special value None represents nothing in Python
    A function that does not explicitly return a value will return None
    Careful: None is not displayed by the interpreter as the value of an expression
    
   

2. Pure Functions & Non-Pure Functions

    Pure Functions: just return values
    Non-Pure Functions: have side effects
   

二 Multiple Environments

1. Life Cycle of a User-Defined Function

   Def statement:  A new function is created!
                   Name bound to that function in the current frame
                   
   Call expression: Operator & operands evaluated
                    Function (value of operator)called on arguments (values of operands)
   
   Calling/Applying: A new frame is created!
                     Parameters bound to arguments Body is executed in that new environment
                     
   

2. Names Have No Meaning Without Environments

   Every expression is evaluated in the context of an environment.
   A name evaluates to the value bound to that name in the earliest frame of the current environment in which that name is found.
   

3. write test in doctest

    """<description>
    >>> q,r = divide_exact(2013,10)
    >>> q
    201
    >>> r
    3
    """
    
   

   in cmd input

    python -m doctest -v **.py
    
   

4. Conditional Statements 4.1 Statements

   A statement is executed by the interpreter to perform an action
   

4.2 Compound Statements

4.3 Conditional Statements

False values in Python: False, 0, '', None
True values in Python: Anything else (True)

5. Iteration 5.1 While Statements

Execution Rule for While Statements:
1. Evaluate the header’s expression.
2. If it is a true value, execute the (whole) suite, then return to step 1.