编写函数以求和列表中的任意两个数字-Python

如何解决编写函数以求和列表中的任意两个数字-Python

当“如果num内的任意两个数字加总到目标，则该函数应返回True，否则返回False”，请帮忙告知该函数哪一部分出错。

非常感谢您的帮助！

random.seed(a=13,version=2)
random_numbers = random.sample(range(1000000),k=10000) # generate 10,000 random numbers in [1,1,000,000)

def pair_exists(nums,target):   
    for i in nums:
        for j in nums:
            if i != j and i + j == target:
                print (i,j)
                return True
            elif i+j != target:
                print (i,j)
                return False

print(pair_exists(random_numbers,38109)) # SHOULD RETURN TRUE
print(pair_exists(random_numbers,13538)) # SHOULD RETURN FALSE

我得到的结果是

271590 271590
False
271590 271590
False

解决方法

如@AChampion的评论中所述，

         elif i+j != target:
            print (i,j)
            return False

将在第一个错误的结果上退出该函数。删除return False继续搜索，最后只删除return False。

---

您的算法与AChampions itertools.combination方法一样有效，但是使用一组所有数字，并检查满足条件的数字的一半

 (targetnumber - aktualnumber) in all_numbers 

快得多：

import random

random.seed(a=13,version=2)
random_numbers = random.sample(range(1,1000001),k=10000) # fix numbers

def pair_exists(nums,target):
    first_half = nums[:len(nums)//2] # even number of elements in nums
    all_nums = set(nums) # use a set for speed reasons

    for num in first_half:
        if target - num in all_nums and target - num != num:
            print(num,target - num)
            return True

    return False

print(pair_exists(random_numbers,38109)) # True for me
print(pair_exists(random_numbers,2))  # always False

取决于种子较高的数字，True可能是，也可能不是，而2永远不会为真，这是因为范围为1 ... 1000000，最小的两个数字加起来为3。 >

---

性能指标

With  sets
0.0125215
0.0157244

With  for loops
51.583881399999996
170.0219317

With  itertools
57.5434453
100.71193929999998

要复制的代码：

import random
from itertools import combinations

def pair_exists(nums,target):
    first_half = nums[:len(nums)//2] # even number of elements in nums
    all_nums = set(nums) # use a set for speed reasons

    for num in first_half:
        if target - num in all_nums and target - num != num:
            # print(num,target - num)
            return True
    return False

def pair_exists_itertools(nums,target):
    for i,j in combinations(nums,r=2):
        if i != j and i + j == target:
            # print (i,j)
            return True
    return False

def pair_exists_forloops(nums,target):
    for i in nums:
        for j in nums:
            if i != j and i + j == target:
                # print (i,j)
                return True
    return False


random.seed(a=13,k=10000) # fix numbers

from timeit import timeit

def test_em(w,f):
    n = 10    # repeat only for 10 times to average measurements
    print("With ",w)
    print(timeit(lambda: f(random_numbers,38109),number=n))
    print(timeit(lambda: f(random_numbers,2),number=n))
    
for what,func in ( ("sets",pair_exists),("for loops",pair_exists_forloops),("itertools",pair_exists_itertools)):
    test_em(what,func)

,

在您撰写本文的地方

random.sample（range（1000000），k = 10000）＃在[1，1,000,000）中生成10,000个随机数

尝试将代码更改为random.sample（range（1，1000000），k = 10000）