Python的一道面试题

 

题目：
将下面的函数按照执行效率高低排序。它们都接受由0至1之间的数字构成的列表作为输入。这个列表可以很长。
一个输入列表的示例如下：[random.random() for i in range(100000)]。

def f1(lIn):
    l1 = sorted(lIn)
    l2 = [i for i in l1 if i<0.5]
    return [i*i for i in l2]

def f2(lIn):
    l1 = [i for i in lIn if i<0.5]
    l2 = sorted(l1)
    return [i*i for i in l2]

def f3(lIn):
    l1 = [i*i for i in lIn]
    l2 = sorted(l1)
    return [i for i in l1 if i<(0.5*0.5)]

你如何证明自己的答案是正确的。

可以通过python3自带的unittest来测试下

import unittest
import random

class PerfExp:
    case=[random.random() for i in range(100000)]
    @staticmethod
    def sort_by_list(self):
        self.case=sorted(PerfExp.case)
    @staticmethod
    def get_case(self):
        return PerfExp.case

class FuncPerfTests(unittest.TestCase):
    def test_f1(self):
     #   PerfExp.sort_by_list(self)
        l1 = sorted(PerfExp.case)
        l2 = [i for i in l1 if i < 0.5]
        return [i * i for i in l2]
    def test_f2(self):
        l1 = [i for i in PerfExp.get_case(self) if i < 0.5]
        l2 = sorted(l1)
        return [i * i for i in l2]
    def test_f3(self):
        l1 = [i * i for i in PerfExp.get_case(self)]
        l2 = sorted(l1)
        return [i for i in l1 if i < (0.5 * 0.5)]

if __name__ == '__main__':
    unittest.main()

 

结果：

当然我们可以用timeit，改写下：

from timeit import timeit

f1 = """
import unittest
import random
class PerfExp:
    case=[random.random() for i in range(100000)]
    @staticmethod
    def sort_by_list(self):
        self.case=sorted(PerfExp.case)
    @staticmethod
    def get_case(self):
        return PerfExp.case
class FuncPerfTests():

    def test_f1(self):
     #   PerfExp.sort_by_list(self)
        l1 = sorted(PerfExp.case)
        l2 = [i for i in l1 if i < 0.5]
        return [i * i for i in l2]
"""
#### 静态方法不用再次声明
f2= """
import unittest
import random

class FuncPerfTests():

 def test_f2(self):
    l1 = [i for i in PerfExp.get_case(self) if i < 0.5]
    l2 = sorted(l1)
    return [i * i for i in l2]
    
"""


f3= """
import unittest
import random

class FuncPerfTests():
 def test_f3(self):
    l1 = [i * i for i in PerfExp.get_case(self)]
    l2 = sorted(l1)
    return [i for i in l1 if i < (0.5 * 0.5)]
"""

print("FuncPerfTests.test_f1: "+ str(timeit("FuncPerfTests.test_f1", f1)))
print("FuncPerfTests.test_f2: "+ str(timeit("FuncPerfTests.test_f2", f2)))
print("FuncPerfTests.test_f3: "+ str(timeit("FuncPerfTests.test_f3", f3)))

#if __name__ == '__main__':
    #unittest.main()

结果：