Python如何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

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)om/weixin.html#_labeldown

运行:

administrator@ubuntu:~/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
  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!')

实例补充

#!/usr/bin/python env
# -*- coding:utf-8 -*-

def half_search(array,target):
low = 0
high = len(array) - 1
while low < high:
  mid = (low + high)/2
  if array[mid] > target:
  high = mid - 1
  elif array[mid] < target:
  low = mid + 1
  elif array[mid] == target:
  print 'I find it! It is in the position of:',mid
  return mid
  else:
  print "please contact the coder!"
return -1

if __name__ == "__main__":
array = [1, 2, 2, 4, 4, 5]

运行结果如下：

I find it! It is in the position of: 4
4
-1
I find it! It is in the position of: 0
0
-1

来源：https://www.py.cn/jishu/jichu/10384.html