使用python进行线性搜索和二进制搜索之间经过的时间

Question

我需要一些关于python代码的帮助。在

我做了两个函数，一个用于顺序（线性）搜索，另一个用于二进制搜索。在

我想对给定列表中的每个大小值执行以下3项操作：

1）生成给定列表大小的随机整数值列表（介于1到10000000之间）

2）对列表中的-1运行顺序搜索，并记录顺序搜索所用的时间

3）在排序后的列表上运行-1的二进制搜索（在对列表排序后），并记录二进制搜索所用的时间

我所做的是：

def sequentialSearch(alist, item):
    pos = 0
    found = False

    while pos < len(alist) and not found:
        if alist[pos] == item:
            found = True
        else:
            pos = pos + 1

    return found

def binSearch(list, target):
    list.sort()
    return binSearchHelper(list, target, 0, len(list) - 1)

def binSearchHelper(list, target, left, right):
    if left > right:
        return False

    middle = (left + right)//2
    if list[middle] == target:
        return True
    elif list[middle] > target:
        return binSearchHelper(list, target, left, middle - 1)
    else:
        return binSearchHelper(list, target, middle + 1, right)

import random
import time
list_sizes = [10,100,1000,10000,100000,1000000]
for size in list_sizes:
    list = []
    for x in range(size):
        list.append(random.randint(1,10000000))

    sequential_search_start_time = time.time()
    sequentialSearch(list,-1)
    sequential_search_end_time = time.time()
    print("Time taken by linear search is = ",(sequential_search_end_time-sequential_search_start_time))

    binary_search_start_time = time.time()
    binSearch(list,-1)
    binary_search_end_time = time.time()
    print("Time taken by binary search is = ",(binary_search_end_time-binary_search_start_time))

    print("\n")

我得到的输出是：

我们知道二进制搜索比线性搜索快得多。 所以，我只想知道为什么二元搜索所花费的时间比线性搜索要多？在

Answer 1

1）您需要说明分拣时间。二进制搜索只对已排序的列表起作用，因此排序需要时间，这就需要时间复杂性来O(nlogn)。在你的例子中，你是在计时器开始后排序的，所以它会更高。在

2）您正在搜索列表中不存在的元素，即-1，这不是二进制搜索的平均情况。二元搜索最糟糕的情况就是要跳转很多次才找不到元素。在

3）请不要使用list作为变量，它是python的关键字，而且您显然在重写它。用别的东西。在

现在，如果你不定时地对列表进行排序。结果变化很大。这是我的。在

Time taken by linear search is =  9.059906005859375e-06
Time taken by binary search is =  8.58306884765625e-06


Time taken by linear search is =  1.2159347534179688e-05
Time taken by binary search is =  4.5299530029296875e-06


Time taken by linear search is =  0.00011110305786132812
Time taken by binary search is =  5.9604644775390625e-06


Time taken by linear search is =  0.0011129379272460938
Time taken by binary search is =  8.344650268554688e-06


Time taken by linear search is =  0.011270761489868164
Time taken by binary search is =  1.5497207641601562e-05


Time taken by linear search is =  0.11133551597595215
Time taken by binary search is =  1.7642974853515625e-05

我所做的只是在时间安排之前把名单整理一下。太好了，比你一次把它分类和搜索要好得多。在