Python二分查找详解

先来看个实例

#!/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:

administrator@ubuntu:~/Python$ python test_search2.py 123456789 4

3

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:
           if 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))

二分算法的定义不在多说了，百度一下就知道（支持国产大笑）

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!')