Python 列表理解和切片
原文:https://www . geeksforgeeks . org/python-list-intelligence-and-slicing/
列表理解是 python 中定义和创建列表的一种优雅方式。我们可以创建列表，就像数学陈述一样，并且只在一行中。列表理解的语法更容易掌握。
列表理解通常包括以下几个部分:
- 输出表达式，
- 输入序列，
- 一个表示输入序列成员的变量，并且
- 可选谓词部分。
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.
例 1:
蟒蛇 3
# 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]
在获得列表之后，我们可以使用 python 的切片操作符获得它的一部分，该操作符具有以下语法:
[start : stop : steps] which means that slicing will start from index startwill go up to stop in step of steps. Default value of start is 0, stop is last index of listand for step it is 1
因此**【:stop】将从开始到停止索引对列表进行切片，【开始:】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的遍历，而不是从左到右的遍历，这就是为什么【::-1】**以相反的顺序打印列表。
例 2:
计算机编程语言
# 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]
我们可以使用过滤器功能，根据某个条件过滤列表，该条件以λ表达式作为第一个参数，以列表作为第二个参数，如下所示:
例 3:
计算机编程语言
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
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