用Python实现的二分查找算法

先来看个用Python实现的二分查找算法实例

 
#!/usr/bin/env python 

import sys  

   

def search2(a,m): 

  low = 0 

  high = len(a) - 1 

  while(low <= high): 

    mid = (low + high)/2

    midval = a[mid] 

     

    if midval < m: 

      low = mid + 1 

    elif midval > m: 

      high = mid - 1 

    else: 

      print mid  

      return mid  

  print -1

  return -1

   

if __name__ == "__main__": 

  a = [int(i) for i in list(sys.argv[1])] 

  m = int(sys.argv[2]) 

  search2(a,m)

 

运行:

 
[email protected]:~/Python$ python test_search2.py 123456789 4

注:

1.’__’：由于python的类成员都是公有、公开的被存取public，缺少像正统面向对象语言的私有private属性。

于是就用__来将就一下，模拟私有属性。这些__属性往往是内部使用，通常情况下不用改写。也不用读取。

加上2个下划线的目的，一是不和普通公有属性重名冲突，二是不让对象的使用者(非开发者)随意使用。

2.name == "main"表示程序脚本是直接被执行的.

如果不等于表示脚本是被其他程序用import引入的.则其__name__属性被设为模块名

Python采用二分查找找出数字的下标

要考虑有重复数字的情况

 
class Solution(object):

  def searchRange(self, nums, target):

    """

    :type nums: List[int]

    :type target: int

    :rtype: List[int]

    """

    def binary_search(start,end,value):

      while end>=start:

        mid = (start+end)//2

        print(mid)

        if nums[mid]>target:

          end = mid-1

        elif nums[mid]<target: start="mid+1" else:="" if="" value="=-1:" mid-1="">=start and nums[mid+value] == target:

              end = mid+value

            else:

              return mid

          else:

            if mid+1<=end and nums[mid+value] == target:

              start = mid+value

            else:

              return mid

   

      return -1

    a=binary_search(0,len(nums)-1,-1)

    b=binary_search(0,len(nums)-1,1)

    return [a,b]

a = Solution()

l = [2,2]

print(a.searchRange(l,2))

  

</target:>

 

二分算法的定义不在多说了

 
import sys 

source = [1,2,3,4,5,6,7,8,9,10] #must be in order 

des = int(sys.argv[1]) 

low = 0

high = len(source) - 1

targetIndex = -1

print "des=",des 

while low <= high: 

  middle = (low + high)/2

  if des == source[middle]: 

    targetIndex = middle 

    break

  elif des < source[middle]: 

    high = middle -1

    print "middle element[index=",middle,",value=",source[middle],"] is bigger than des, continue search from[",low,"to",high,"]"

  else: 

    low = middle + 1

    print "middle element[index=",middle,",value=",source[middle],"] is smaller than des, continue search from[",low,"to",high,"]"

print "search complete, target element's index in source list is ",targetIndex

 

最后在分享一个

'fileName--BinarySearch.py'


 
src = [] 

   

def BinarySearch(low, high, target, *src): 

  '二分查找'

  while low <= high: 

    mid = (low + high) // 2

    midVal = src[mid] 

    if target < midVal: 

      high = mid - 1

    elif target > midVal: 

      low = mid + 1

    else: 

      return mid 

    BinarySearch(low, high, target, *src) 

   

print('Please input 10 number:') 

for number in range(10): 

  src.append(int(input('Num %d:' % number))) 

   

sortList = tuple(src) 

key = int(input('Please input key:')) 

location = BinarySearch(0, len(src) - 1, key, *sortList) 

   

if location != None: 

  print('Find target at %d' % (location + 1)) 

else: 

  print('No target!')