CS61A

约 803 字大约 3 分钟

week1

Lecture

Welcome

The first lecture just a simple explaination of CS61A and explnation of the Expression Tree which is composed of a operator and several operands, so I just paste the picture of the Tree.

Easy but interesting.
If you learned data structure, you may think this is n-ary tree.

Functions

name

built-in

Like c/c++, python also has its own way to name or rename a parameter, a function or a variable and so on. The lecture will introduce it at beginning.

Ctrl + L is used to flash the terminal in python session.

>>> pi  
Traceback (most recent call last):  
  File "<stdin>", line 1, in <module>  
NameError: name 'pi' is not defined

oops! No pi! This means the parameter is not a built-in parameter. We have to import it.

>>> from math import pi  
>>> pi  
3.141592653589793

The same goes for imported functions.

>>> from math import sin  
>>> sin(pi)  
1.2246467991473532e-16  
>>> sin(pi/2)  
1.0

You may find it not zero after sin(pi), because of the type of data storage in the computer or just a data cut. Not important.

name by ourselves

We can also name our own parameter:

>>> radius = 10
>>> radius
10

Even multiple variables named simultaneously is also supported:

>>> area, circ = pi * radius *radius, 2 * pi * radius
>>> area
314.1592653589793
>>> circ
62.83185307179586

The same goes for imported functions.

define

We can edit our functions by def:

>>> def square(x):
...     return mul(x, x)
... 
>>> square(11)
121

If we change the value of parameter, the value of expression named by us will not change, but if define a functions will solve the issue:

>>> def area():
...     return pi * radius * radius
... 
>>> area()
314.1592653589793
>>> radius = 20
>>> area()
1256.6370614359173

A function differs from a name in that its return expression here gets re-evaluated every time it's called.

defining functions

structure of defining:

>>> def <name>(<formal parameters>)
    return <return expression>

Fuctions will be evaluated when it is called.

The lecture also introduce the difference between local frames and global frames. I passed it for my c&c++ basics.

Environment Diagrams

Online Python Tutor:

https://pythontutor.com/visualize.html#

Print and None

Just talk about the difference of print and evaluate and Nonetype.

>>> None
>>> print(None)
None

Lab

Lab0:Getting Started

Download starter files from: https://cs61a.org/lab/lab00/lab00.zip

# we don't have edu email of berkeley, so we need to add --local
python ok -u --local

Esay to pass:

=====================================================================
Assignment: Lab 0
OK, version v1.18.1
=====================================================================

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Unlocking tests

At each "? ", type what you would expect the output to be.
Type exit() to quit

---------------------------------------------------------------------
Python Basics > Suite 1 > Case 1
(cases remaining: 1)

What would Python display? If you get stuck, try it out in the Python
interpreter!

>>> x = 20
>>> x + 2
? 22
-- OK! --

>>> x
? 20
-- OK! --

>>> y = 5
>>> y = y + 3
>>> y * 2
? 16
-- OK! --

>>> y + x
? 28
-- OK! --

---------------------------------------------------------------------

Implementing Functions

lazy to think:

def twenty_twenty_four():
    """Come up with the most creative expression that evaluates to 2024
    using only numbers and the +, *, and - operators.

    >>> twenty_twenty_four()
    2024
    """
    return 2024 * 1 + 1 - 1

Running Tests

use ok to check:

python ok --local

check passed:

=====================================================================
Assignment: Lab 0
OK, version v1.18.1
=====================================================================

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Running tests

---------------------------------------------------------------------
Test summary
    2 test cases passed! No cases failed.

Appendix: Useful Python Command Line Options

-i: opens an interactive session

python3 -i lab00.py
>>> 1
1

-m doctest: run the examples in the docstrings of functions

# doctest
>>> twenty_twenty_four()
    2024

If all doctest passed, no outputs.

python3 -m doctest lab00.py