filter

python內建的filter()用于过滤序列

和map()类似,filter()也接收一个函数和一个序列.

filter()把传入的函数依次作用于每个元素,然后根据返回值是True还是False决定保留还是丢弃该元素.

例如,在一个list中,删掉偶数,保留奇数

>>> L = [1,2,3,4,5,6,7,8,9,10]
>>> list(filter(lambda x:x % 2 == 1,L))
[1, 3, 5, 7, 9]

把一个序列中的空字符串删掉

>>> list(filter(lambda x:x and x.strip() ,["A"," ","",None,"C",1]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <lambda>
AttributeError: 'int' object has no attribute 'strip'
>>> list(filter(lambda x:x and x.strip() ,["A"," ","",None,"C","1"]))
['A', 'C', '1']
>>> list(filter(lambda x:x and str(x).strip() ,["A"," ","",None,"C",1]))
['A', 'C', 1]

filter() 函数返回的是一个 Iterator,也就是一个惰性序列,所以要强迫filter()完成计算结果,需要用list()函数获取所有结果并返回list

用filter求素数

>>> list(filter(lambda x:str(x) == str(x)[::-1],range(1,1000)))
[1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 22, 33, 44, 55, 66, 77, 88, 99, 101, 111, 121, 131, 141, 151, 161, 171, 181, 191, 202, 212, 222, 232, 242, 252, 262, 272, 282, 292, 303, 313, 323, 333, 343, 353, 363, 373, 383, 393, 404, 414, 424, 434, 444, 454, 464, 474, 484, 494, 505, 515, 525, 535, 545, 555, 565, 575, 585, 595, 606, 616, 626, 636, 646, 656, 666, 676, 686, 696, 707, 717, 727, 737, 747, 757, 767, 777, 787, 797, 808, 818, 828, 838, 848, 858, 868, 878, 888, 898, 909, 919, 929, 939, 949, 959, 969, 979, 989, 999]
>>>