Python中的Numbers,Strings,Lists

经过Java语法的学习和他人的经验，我决定使用官方文档来学习python，锻炼英文阅读能力的同时，也获取的是最准确的最有说服力的知识，在学习的过程中我会贴一些官方文档的原文便于查看理解，下面进入正题。

首先 + - * / 加减乘除没什么问题，使用除法时用‘/’时得到的是float类型的结果，使用‘//’是取整，使用‘%’是取余，此外使用‘**’是乘方，可以看代码：

>>> 2 ** 7  # 2 to the power of 7
128

>>> 17 / 3  # classic division returns a float
5.666666666666667
>>>
>>> 17 // 3  # floor division discards the fractional part
5
>>> 17 % 3  # the % operator returns the remainder of the division
2

变量没有赋值时使用会报错

>>> n  # try to access an undefined variable
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'n' is not defined

上一个输出的值可以用'_'来使用

>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06

对于字符串，用单引号和双引号都可以包围，在字符串中含有单引号或双引号的可以用\来escape，也可以换成另一种引号直接使用，看代码

>>> 'spam eggs'  # single quotes
'spam eggs'
>>> 'doesn\'t'  # use \' to escape the single quote...
"doesn't"
>>> "doesn't"  # ...or use double quotes instead
"doesn't"
>>> '"Yes," he said.'
'"Yes," he said.'
>>> "\"Yes,\" he said."
'"Yes," he said.'
>>> '"Isn\'t," she said.'
'"Isn\'t," she said.'

使用print()可以省略引号，\n是换行但必须在print()里使用才有效，直接输出会被当作普通字符

>>> print('"Isn\'t," she said.')
"Isn't," she said.
>>> s = 'First line.\nSecond line.'  # \n means newline
>>> s  # without print(), \n is included in the output
'First line.\nSecond line.'
>>> print(s)  # with print(), \n produces a new line
First line.
Second line.

如果不想让\被理解成特殊字符，可以在字符串第一个引号前加一个r

>>> print('C:\some\name')  # here \n means newline!
C:\some
ame
>>> print(r'C:\some\name')  # note the r before the quote
C:\some\name

字符串常量可以展示在多行里，使用三个引号"""..."""或'''...'''
字符串可以用 + 来运算，用 * 来重复叠加

>>> # 3 times 'un', followed by 'ium'
>>> 3 * 'un' + 'ium'
'unununium'

两个或多个字符串常量相邻输出就会自动连接在一起，和变量相邻输出会报错

>>> 'Py' 'thon'
'Python'

>>> prefix = 'Py'
>>> prefix 'thon'  # can't concatenate a variable and a string literal
  ...
SyntaxError: invalid syntax

想让变量和常量相加，使用 + 来完成

>>> prefix + 'thon'
'Python'

字符串可以被索引化

>>> word = 'Python'
>>> word[0]  # character in position 0
'P'
>>> word[5]  # character in position 5
'n'

下标也可以是负数，从字符串的右边开始数

>>> word[-1]  # last character
'n'
>>> word[-2]  # second-last character
'o'
>>> word[-6]
'P'

取字符串也很方便，只需记住开始的下标是包括在内的，结束的下标是不包括在内的

>>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
'tho'

>>> word[:2] + word[2:]
'Python'
>>> word[:4] + word[4:]
'Python'

>>> word[:2]   # character from the beginning to position 2 (excluded)
'Py'
>>> word[4:]   # characters from position 4 (included) to the end
'on'
>>> word[-2:]  # characters from the second-last (included) to the end
'on'

越界下标在截取子字符串时可以被优雅地使用

>>> word[4:42]
'on'
>>> word[42:]
''

Python字符串和Java中的String都是不可改变的，想要给字符串某一下标的字符赋值也是不允许的

>>> word[0] = 'J'
  ...
TypeError: 'str' object does not support item assignment
>>> word[2:] = 'py'
  ...
TypeError: 'str' object does not support item assignment

len()函数能返回字符串的长度

>>> s = 'supercalifragilisticexpialidocious'
>>> len(s)
34

对于Lists和Strings有很多相似的地方，贴一下代码看一下

>>> squares = [1, 4, 9, 16, 25]
>>> squares
[1, 4, 9, 16, 25]
>>> squares[0]  # indexing returns the item
1
>>> squares[-1]
25
>>> squares[-3:]  # slicing returns a new list
[9, 16, 25]

>>> squares + [36, 49, 64, 81, 100]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

和字符串不一样的是List是可改变的

>>> cubes = [1, 8, 27, 65, 125]  # something's wrong here
>>> cubes[3] = 64  # replace the wrong value
>>> cubes
[1, 8, 27, 64, 125]

可以直接使用append()函数在数列队尾加入新的元素

>>> cubes.append(216)  # add the cube of 6
>>> cubes.append(7 ** 3)  # and the cube of 7
>>> cubes
[1, 8, 27, 64, 125, 216, 343]

部分改变也可以，甚至可以改变List的长度或直接清除

>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> letters
['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> # replace some values
>>> letters[2:5] = ['C', 'D', 'E']
>>> letters
['a', 'b', 'C', 'D', 'E', 'f', 'g']
>>> # now remove them
>>> letters[2:5] = []
>>> letters
['a', 'b', 'f', 'g']
>>> # clear the list by replacing all the elements with an empty list
>>> letters[:] = []
>>> letters
[]

嵌套也是可以的

>>> a = ['a', 'b', 'c']
>>> n = [1, 2, 3]
>>> x = [a, n]
>>> x
[['a', 'b', 'c'], [1, 2, 3]]
>>> x[0]
['a', 'b', 'c']
>>> x[0][1]
'b'

这些是最基本的语法，在使用中就会慢慢熟悉