CS61A笔记

lecture 01 Course About

• managing complexity

  • tool: mastering abstraction
  • abstraction is giving sth a name and talking about it without worrying about details
  • tool: Programming paradigms: about how to organize programs

• try a personal project if you have time

lecture 02 Functions

术语

• domain: the set of all inputs
• range: the set of all outputs
• behavior: relationship between input and output

expression evaluate

• all expressions can use function call notation
• evaluate procedure

• 上图称为表达式树。add(2,mul(4,6))称为操作数子表达式；其结果称为子表达式的值，同时它也是mul的第一个参数
• 赋值是抽象（abstraction）的最简单的一种方法：把名字绑定到值上
• 函数定义是抽象的一种更强大的手段：把名字绑定到表达式上

Environment Diagrams

• environment 是 memory， 它可以跟踪名称和值之间的对应关系。
• 所有的表达式都是在environment的情境下计算的。
• envrionment是frames的序列
• 它可以是全局frame，也可以是局部frame，后面跟着全局frame

可视化解释器的执行过程：

函数是如何执行的？

1. add a local frame, forming a new environment
2. bind the params to its arguments in that frame
3. execute the body of the function in that new environment

名称的值是如何查找的?

当前环境下，在最早的frame中找到的名称所对应的值。如在函数中，我们会先在local frame中查找名字，如果没找到，再去全局frame中查找。

why does it work?

from operator import mul
def square(square):
    return mul(square,square);

函数sqaure在全局frame中，参数square在局部frame中，互不干扰。

lecture 03 Control

print & None

• what happens when evalutate print(print(1),print(2))?

answer:

1
2
None None

• use the expression tree:

What is None?

• None 表示没有任何东西被返回

• None is not displayed by the interpreter as the value of an expression

纯函数&非纯函数

• 纯函数：单纯地返回值
• 非纯函数：有些side effects。例如 print，它返回None，但同时会输出值

Multiple Environments in One Diagram

• As you can see, there are three environments as the program executes.
• 失去了环境，名称将不再有意义。名称在不同的环境中有着不同的含义

lecture 04 High Order Functions

A guide to design functions

• Give each function exactly one job
• Don’t repeat yourself. Implement a process just once
• Define functions generally (share implementation)

Generalizing patterns with arguments

Generalizing Over Computational Processes

• all summing from 1 to 5
• how to generalize?

• term is a formal parameter which will be bound to a function
• so the summation is a high order function that take another function as an argument

Functions as return values

• function can be manipulated as values in programming
• high order function
  • take another function as an argument
  • return a function