Python列表理解和切片

列表推导式是一种在Python中定义和创建列表的优雅方式。我们可以像数学语句一样创建列表，并且仅在一行中。列表推导的语法更容易掌握。

列表理解通常由以下部分组成：

1. 输出表达式，
2. 输入序列，
3. 表示输入序列成员的变量和
4. 一个可选的谓词部分。

For example :

lst  =  [x ** 2  for x in range (1, 11)   if  x % 2 == 1] 

here, x ** 2 is output expression, 
      range (1, 11)  is input sequence, 
      x is variable and   
      if x % 2 == 1 is predicate part.

示例 1：

Python3

# Python program to demonstrate list comprehension in Python 
   
# below list contains square of all odd numbers from 
# range 1 to 10 
odd_square = [x ** 2 for x in range(1, 11) if x % 2 == 1] 
print (odd_square) 
   
# for understanding, above generation is same as, 
odd_square = [] 
for x in range(1, 11): 
    if x % 2 == 1: 
        odd_square.append(x**2) 
print (odd_square) 
   
# below list contains power of 2 from 1 to 8 
power_of_2 = [2 ** x for x in range(1, 9)] 
print (power_of_2) 
   
# below list contains prime and non-prime in range 1 to 50 
noprimes = [j for i in range(2, 8) for j in range(i*2, 50, i)] 
primes = [x for x in range(2, 50) if x not in noprimes] 
print (primes) 
   
# list for lowering the characters 
print ([x.lower() for x in ["A","B","C"]] )
   
# list which extracts number 
string = "my phone number is : 11122 !!"
   
print("\nExtracted digits") 
numbers = [x for x in string if x.isdigit()] 
print (numbers) 
   
# A list of list for multiplication table 
a = 5
table = [[a, b, a * b] for b in range(1, 11)] 
   
print("\nMultiplication Table") 
for i in table: 
    print (i)

输出：

[1, 9, 25, 49, 81]
[1, 9, 25, 49, 81]
[2, 4, 8, 16, 32, 64, 128, 256]
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]
['a', 'b', 'c']

Extracted digits
['1', '1', '1', '2', '2']

Multiplication Table
[5, 1, 5]
[5, 2, 10]
[5, 3, 15]
[5, 4, 20]
[5, 5, 25]
[5, 6, 30]
[5, 7, 35]
[5, 8, 40]
[5, 9, 45]
[5, 10, 50]

获取列表后，我们可以使用 python 的切片运算符获取其中的一部分，其语法如下：

[start : stop : steps]  

which means that slicing will start from index start
 will go up to stop in step of steps. 
 Default value of start is 0, stop is last index of list
 and for step it is 1 

所以[:stop]将从开始到停止索引对列表进行切片，并且[start : ]将从开始索引到结束对列表进行切片步骤的负值显示从右到左遍历而不是从左到右遍历这就是为什么[: : -1]以相反的顺序打印列表。

示例 2：

Python3

# Let us first create a list to demonstrate slicing
# lst contains all number from 1 to 10
lst =list(range(1, 11))
print (lst)
    
#  below list has numbers from 2 to 5
lst1_5 = lst[1 : 5]
print (lst1_5)
    
#  below list has numbers from 6 to 8
lst5_8 = lst[5 : 8]
print (lst5_8)
    
#  below list has numbers from 2 to 10
lst1_ = lst[1 : ]
print (lst1_)
    
#  below list has numbers from 1 to 5
lst_5 = lst[: 5]
print (lst_5)
    
#  below list has numbers from 2 to 8 in step 2
lst1_8_2 = lst[1 : 8 : 2]
print (lst1_8_2)
    
#  below list has numbers from 10 to 1
lst_rev = lst[ : : -1]
print (lst_rev)
    
#  below list has numbers from 10 to 6 in step 2
lst_rev_9_5_2 = lst[9 : 4 : -2]
print (lst_rev_9_5_2)

输出：

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[2, 3, 4, 5]
[6, 7, 8]
[2, 3, 4, 5, 6, 7, 8, 9, 10]
[1, 2, 3, 4, 5]
[2, 4, 6, 8]
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
[10, 8, 6]

我们可以使用filter函数根据作为第一个参数的lambda 表达式和作为第二个参数的列表提供的某些条件过滤列表，示例如下所示：

示例 3：

Python3

import functools
   
#  filtering odd numbers
lst = filter(lambda x : x % 2 == 1, range(1, 20))
print (list(lst))
    
#  filtering odd square which are divisible by 5
lst = filter(lambda x : x % 5 == 0, 
      [x ** 2 for x in range(1, 11) if x % 2 == 1])
print (list(lst))
    
#   filtering negative numbers
lst = filter((lambda x: x < 0), range(-5,5))
print (list(lst))
    
#  implementing max() function, using
print (functools.reduce(lambda a,b: a if (a > b) else b, [7, 12, 45, 100, 15]))

输出：

[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
[25]
[-5, -4, -3, -2, -1]
100