MIT麻省理工学院公开课：计算机科学及编程导论 Python 笔记1-3

Lecture1：Goals of the course; what is computation; introduction to data types, operators, and variables

Python

• High （√） VS. low
• General （√） VS. targetted

• Interpreted （√） VS. compile

  1. Syntax语法：what are legal expressions
     “cat dog boy “
  2. Static semantics 静态语义：which programs are meaningful
     “ My desk is Suson“
  3. Full semantics 完整语义：what does program mean
     what will happen when i run it

Operation
+ - * /

>>>‘a‘*3
‘aaa‘
>>>3/5
0
>>>3**3
27

Variables

>>> a = 3
>>> print a
3

Lecture2：Operators and operands; statements; branching, conditionals, and iteration

Operators and operands

>>> ‘ab‘ + ‘c‘
‘abc‘
>>> 3 + ‘c‘
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: ‘int‘ and ‘str‘
>>> str(3) + ‘c‘
‘3c‘

• type conversion 类型转换 str(3)
• type checking 类型检查 weak VS. strong typing
  TypeError: unsupported operand type(s) for +: ‘int’ and ‘str’
• type discipline
• operation precedence 算符优先
  * >> / >> + - when is doubt， use ()

>>> ‘a‘ < 3
False
>>> 4 + ‘3‘
True
>>> 9／5
1
>>> 9％5
4
>>> 3＋4＊5
23

• Variables has a value－－Assignment x = 3

• type of Variables－－get from value
  Dynamic types 动态类型
  x = ‘abc’

  don’t change types arbitrarily 不要反复无常的改变变量类型

• Variables used any place it’s legal to use value

  ---

statements

• statements = legal commands that Python can interpret
• print, assignment
  

  ---

branching 分支

• change the order of instructions based on some test (usually a value of a variable)
• Syntax
  冒号colon : begin a sequence of instructions
  identifies a block of instructions.

冒号: start
carriage 回车 is end

x = 15
if（x/2）* 2 == x:
    print ‘Even‘
else: print ‘Odd‘

conditionals 条件

＃ if语句可嵌套
if <some test> :
    Block of instructions.
else: 
    Block of instructions.

• Boolean combination：AND, OR, or NOT

iteration 迭代 or loops 循环

# y ＝ x的平方
y = 0
x = 3
itersLeft = x
while(itersLeft>0) :
    y = y + x
    itersLeft = itersLeft -1
print y

Lecture3：Common code patterns：iterative programs

iterative programs

• choose a variable that’s going to “count“
• initialize it outside of the loop 循环外初始化
• set up the right end test （variable）正确结束循环
• construct the block of code
  
  • changing the variable
• what to do when done 循环结束后做什么
  

  ---

flow chart 流程图

# x开方
# find the square root of a perfect square
x = 16
ans = 0
while ans*ans <= x:
    ans = ans + 1
print ans

x = 15
if（x/2）* 2 == x:
    print ‘Even‘
else: print ‘Odd‘

Simulate 模拟

Defensive programming

• A, if you’re getting inputs from a user, they won’t necessarily give you the input you’ve asked for 使用者可能没有按照你指定的输入。
• B, if you’re using a piece of a program written by a programmer who is not perfect, perhaps yourself, there could be mistakes in that program, and so you write your program under the assumption that, not only might the user make a mistake, other parts of your program might make a mistake, and you just put in lots of different tests under the assumption that you’d rather catch that something has gone wrong, then have it go wrong and not know it. 程序中可能有错误。

# x开方进化版
# find the square root of a perfect square
# x = 1515361
ans = 0
if x > 0:
    while ans*ans < x:
        ans = ans + 1
        #print ‘ans =‘, ans
    if ans*ans != x:
        print x, ‘is not a perfect square‘
    else: print ans
else: print x, ‘is a negative number‘

Exhaustive enumeration 穷尽／枚举

• try all “reasonable” values until you find the solution

# find x‘s divisor
# x = 10
# i = 1 
    while i < x:
        if x%i == 0:
            print ‘divisor‘, i
        i = i+1

# find x‘s divisor
x = 10
for i in range(1, x):
    if x%i == 0:
        print ‘divisor‘, i

Tuple 元组
w3school Python元组
- ordered sequence of elements 有序的元素序列（immutable 不可变的）

>>>  foo = (1, 2, 3)
>>>> foo
(1, 2, 3)
>>> foo[0]
1
>>> foo[1]
2
>>> foo[10]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: tuple index out of range
>>> foo[-1]
3
>>> foo[-2]
2
>>> foo[1:3]
(2, 3)
>>> foo[1:2]
(2,)
>>> foo[:2]
(1, 2)
>>> foo[1:]
(2, 3)

# find x‘s divisor
x = 100
divisors = ()
for i in range(1, x):
    if x%i == 0:
        divisors = divisors + (i,)
print divisors

String
w3school Python字符串
- support selection, slicing, and a set of other parameters, other properties

sumDigits = 100
for c in str(1952):
    sumDigits += int(c) 
print sumDigits