Python二分查找详解

这篇文章主要给大家汇总介绍了Python二分查找的几种实现的方法，有需要的小伙伴可以参考下。
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!')