【375】COMP 9021 相关笔记

1. Python 中的逻辑否定用 not

2. 对于下面的代码直邮输入整数才能运行，无论字符串或者浮点型都会报错

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1	int(input('How many games should I simulate? '))

　　可以通过 try 来修改，同时注意 raise 的使用

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while True:     try:         nb_of_games = int(input('How many games should I simulate? '))         if nb_of_games <= 0:             raise ValueError         print('Ok, will do!')         break     except ValueError:         print('Your input is incorrect, try again.')

3. set 与 dict 都是大括号

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# Checking for membership in a list 'y' in ['yes', 'y', 'no', 'n'] 'Y' in ['yes', 'y', 'no', 'n'] # Checking for membership in a set 'y' in {'yes', 'y', 'no', 'n'} 'Y' in {'yes', 'y', 'no', 'n'}   ''' Curly braces are used for both literal dictionaries and literal sets. <br>There is no potential conflict, except for empty set versus empty dictionary; <br>{} denotes an empty dictionary, not an empty set: '''   # Singleton dictionary and set, respectively type({'one': 1}) type({'one'}) # Empty dictionary and set, respectively type({}) type(set())

4. random.choice() 可以随机选择列表里面的元素

　　random.randrange()，在 0 与 n 之间随机产生一个数

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from random import choice doors = ['A', 'B', 'C'] for i in range(12):     choice(doors)

5. list.pop() 默认删除最后一个，否则按照索引删除

6. format

　　To output information about the game as it is being played, it is convenient to use formatted strings; they are preceded with f and can contain pairs of curly braces that surround expressions meant to be replaced with their values. Also, though strings can be explicitly concatenated with the + operator, they can also be implicitly concatenated when they are separated with nothing but space characters, including possibly new lines:

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x = 10 u = 4.5 v = 10 print(f'x is equal to {x}.'       ' That is not all: '       f'{u} divided by {v} equals {u / v}.'      )

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x = 123 / 321 f'{x}' f'{x:.0f}' f'{x:.1f}' f'{x:.2f}' f'{x:.3f}' f'{x:.4f}' f'{x:.30f}'

7. 神奇的 * 号，乘号，可以扩展字符串，可以扩展列表

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1 2	[1, 2, 3]*3 "abc"*3

8. 输出格式，0补全

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# A field width of 3 at least, padding with spaces if needed f'{90:3}', f'{90:3b}', f'{90:3o}', f'{90:3x}', f'{90:3X}' # A field width of 3 at least, padding with 0's if needed f'{90:03}', f'{90:03b}', f'{90:03o}', f'{90:03x}', f'{90:03X}' # A field width of 8 at least, padding with spaces if needed f'{90:8}', f'{90:8b}', f'{90:8o}', f'{90:8x}', f'{90:8X}' # A field width of 8 at least, padding with 0's if needed f'{90:08}', f'{90:08b}', f'{90:08o}', f'{90:08x}', f'{90:08X}'

(' 90', '1011010', '132', ' 5a', ' 5A')

('090', '1011010', '132', '05a', '05A')

('      90', ' 1011010', '     132', '      5a', '      5A')

('00000090', '01011010', '00000132', '0000005a', '0000005A')

sorted

Let us still not "hardcode" the sequence of bits as (s[7], s[3], s[5], s[1], s[6], s[2], s[4], s[0]), but generate it. Let us first examine the sorted() function. By default, sorted() returns the list of members of its arguments in their default order:

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sorted([2, -2, 1, -1, 0]) # Lexicographic/lexical/dictionary/alphabetic order sorted({'a', 'b', 'ab', 'bb', 'abc', 'C'}) sorted(((2, 1, 0), (0, 1, 2), (1, 2, 0), (1, 0, 2)))

[-2, -1, 0, 1, 2]

['C', 'a', 'ab', 'abc', 'b', 'bb']

[(0, 1, 2), (1, 0, 2), (1, 2, 0), (2, 1, 0)]

sorted() accepts the reverse keyword argument:

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sorted([2, -2, 1, -1, 0], reverse = True) sorted({'a', 'b', 'ab', 'bb', 'abc', 'C'}, reverse = True) sorted(((2, 1, 0), (0, 1, 2), (1, 2, 0), (1, 0, 2)), reverse = True)

[2, 1, 0, -1, -2]

['bb', 'b', 'abc', 'ab', 'a', 'C']

[(2, 1, 0), (1, 2, 0), (1, 0, 2), (0, 1, 2)]

sorted() also accepts the key argument, which should evaluate to a callable, e.g., a function. The function is called on all elements of the sequence to sort, and elements are sorted in the natural order of the values returned by the function:

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sorted([2, -2, 1, -1, 0], key = abs) sorted({'a', 'b', 'ab', 'bb', 'abc', 'C'}, key = str.lower) sorted({'a', 'b', 'ab', 'bb', 'abc', 'C'}, key = len)

[0, 1, -1, 2, -2]

['a', 'ab', 'abc', 'b', 'bb', 'C']

['C', 'b', 'a', 'ab', 'bb', 'abc']

We can also set key to an own defined function: 按照自定义的顺序进行排序

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def _2_0_1(s):     return s[2], s[0], s[1]   def _2_1_0(s):     return s[2], s[1], s[0]   sorted(((2, 1, 0), (0, 1, 2), (1, 2, 0), (1, 0, 2)), key = _2_0_1) sorted(((2, 1, 0), (0, 1, 2), (1, 2, 0), (1, 0, 2)), key = _2_1_0)

[(1, 2, 0), (2, 1, 0), (0, 1, 2), (1, 0, 2)]

[(2, 1, 0), (1, 2, 0), (1, 0, 2), (0, 1, 2)]

So we could generate the sequence (0, 4, 2, 6, 1, 5, 3, 7) as follows: 

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def three_two_one(p):     return p % 2, p // 2 % 2, p % 4   for p in sorted(range(8), key = three_two_one):     p, f'{p:03b}'

(0, '000')

(4, '100')

(2, '010')

(6, '110')

(1, '001')

(5, '101')

(3, '011')

(7, '111')

lambda 表达式

There is a better way, using a lambda expression. Lambda expressions offer a concise way to define functions, that do not need to be named:

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# Functions taking no argument, so returning a constant f = lambda: 3; f() (lambda: (1, 2, 3))()

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(1, 2, 3)

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# Functions taking one argument, the first of which is identity f = lambda x: x; f(3) (lambda x: 2 * x + 1)(3)

3

7

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# Functions taking two arguments f = lambda x, y: 2 * (x + y); f(3, 7) (lambda x, y: x + y)([1, 2, 3], [4, 5, 6])

20

[1, 2, 3, 4, 5, 6]