Python：索引总结

#字符串可以直接使用索引，不需要专门的变量引用
>>> 'Hello World!'[0]
'H'
>>> 'Hello World!'[11]
'!'
>>> 'Hello World!'[-1]
'!'
>>> 'Hello World!'[-11]
'e'
#注意以下情况会出现越界
>>> 'Hello World!'[12]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range
>>> 'Hello World!'[-12]
'H'
>>> 'Hello World!'[-13]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range

# Hello World!：总共有12个元素，含空格和标点符号
print('Hello World!'[8]) # r
print('Hello World!'[-4])# r

#注意不包括右边索引值对应的元素，索引下标从0开始
>>> num = [1,2,3,4,5,6,7,8,9,10]
>>> num[3]
4
>>> num[6]
7
>>> num[3:6]
[4, 5, 6]
#注意右边界，左闭右开区间
>>> num[3:9]
[4, 5, 6, 7, 8, 9]
#右边越界，刚好是10对应的下一个索引值
>>> num[3:10]
[4, 5, 6, 7, 8, 9, 10]
#右边越界但只取到最后一个元素
>>> num[3:11]
[4, 5, 6, 7, 8, 9, 10]
#num = [1,2,3,4,5,6,7,8,9,10]
>>> num[-3:-1]#注意是不包含最后一个元素的
[8, 9]
>>> num[-4:-1]
[7, 8, 9]
>>> num[-10:-1]#同样满足左闭右开区间
[1, 2, 3, 4, 5, 6, 7, 8, 9]

#注意右边界
>>> num[-10:0]
[]
>>> num[-5:-1]
[6, 7, 8, 9]

#注意右边界
>>> num[-5:]
[6, 7, 8, 9, 10]
>>> num[-5:1]
[]>>> num[3:1]
[]
>>> num[3:-1]
[4, 5, 6, 7, 8, 9]

#取最右3个元素
>>> num[-3:]
[8, 9, 10]

#取最左3个元素
>>> num[:3]
[1, 2, 3]

#可以加入step length
>>> num[1:10:3]
[2, 5, 8]
>>> num[0:10:3]
[1, 4, 7, 10]

#起始位置默认为最左和最右，但不包括最右的元素
>>> num[::2]
[1, 3, 5, 7, 9]
>>> num[0:9:1]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> num[0:10:1]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

#如果step length为0，会报错，也就是说步长不可以为0
>>> num[1:5:0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: slice step cannot be zero

#如果步长为负数，表示从右向左提取元素
#注意不包括6对应的元素
num[10:6:-1]
[10, 9, 8]

#倒序输出
>>> num[10::-1]#或者 num[::-1]
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

#从最左边开始
>>> num[:-3:1] 
[1, 2, 3, 4, 5, 6, 7]
>>> num[:3:1]
[1, 2, 3]
#从最右边开始
>>> num[:3:-1]
[10, 9, 8, 7, 6, 5]

num = [1,2,3,4,5,6,7,8,9,10]
num1 = num[-1] # 输出：10  type:int
num2 = num[-1:]# 输出：[10] type:list

#列表
>>> [1,2,3] + [3,2,1]
[1, 2, 3, 3, 2, 1]
#字符串
>>> 'Hi' + ',' + 'Baby'
'Hi,Baby'
#元组(tuple)
>>> ('This','is','a') + ('test','case')
('This', 'is', 'a', 'test', 'case')
#如果序列类型不一致，则会报错
>>> 'Hi' + [1,2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'list' objects

var1 = (1,2,3)
var2 = var1 * 2 # (1, 2, 3, 1, 2, 3)
var3 = var2 + "4,5,6" #报错：  File "C:/Users/Dell/test1.py", line 22, in <module> var3 = var2 + "4,5,6" TypeError: can only concatenate tuple (not "str") to tuple
var4 = var1 + (4,5,6) # (1, 2, 3, 4, 5, 6)

>>>print('hi '*5)
'hi hi hi hi hi '
>>> print("hello" * 3)
'hellohellohello'
>>> (1,2,3) * 4 # tuple类型
(1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3)
>>> [1,2,3] * 4 # list类型
[1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3]
>>> [None]*5 # none
[None, None, None, None, None]
>>> 4 * [1,2,3] # 支持交换律
[1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3]