选择排序
复杂度：O（n**2)
一、思路
index 0 1 2 3 4 ... n-1 n
value 3 1 2 5 4 ... 8
- 思考过程：
1、在整个数组[0,n)内寻找最小值1，将其与list[0]交换位置：1 3 2 5 4 8，此时index=0的位置已排序完成
2、在[1,n)内寻找最小值，将最小值与list[1]交换位置，此时index=1的位置排序完成
3、迭代下去，直到[i,n]长度为1，原地不动即可
二、实现
from random import randint
from timeit import Timer
from timeit import timeit
def selectionsort1(myarr):
for i in range(len(myarr)):
#求子数组[i,n)中的最小值对应的index
min_index = i
for j in range(i,len(myarr)):
if myarr[j] < myarr[min_index]:
min_index = j
#交换list[i]与list[min_index]
myarr[i], myarr[min_index] = myarr[min_index],myarr[i]
return myarr
if __name__ == "__main__":
array = [randint(0, 1000) for i in range(0,1000)]
timer = Timer('selectionsort1(array)','from __main__ import selectionsort1,array')
print(timer.timeit(number=10))
- 借助魔法函数，实现运算符重载，类似于c++模板
可以比较任意对象，只要实现了魔法函数
from random import randint,choice
from timeit import Timer
from timeit import timeit
class Student():
def __init__(self,name,age):
self.name = name
self.age = age
def __lt__(self,other):
return self.age < other.age
def __eq__(self,other):
return self.age == other.age
def selectionsort1(myarr):
for i in range(len(myarr)):
#求子数组[i,n)中的最小值对应的index
min_index = i
for j in range(i,len(myarr)):
if myarr[j] < myarr[min_index]:
min_index = j
#交换list[i]与list[min_index]
myarr[i], myarr[min_index] = myarr[min_index],myarr[i]
return myarr
if __name__ == "__main__":
name_bank = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
array = [Student(choice(name_bank),randint(1,100)) for _ in range(1000)]
timer = Timer('selectionsort1(array)','from __main__ import selectionsort1,array')
print(timer.timeit(number=10))
__lt__ __gt__ __eq__ ,三个里面实现两个即可，剩余操作符的定义取余即可；当仅实现一个时，无法补全剩余运算符（但也合法）
- 重载时的改进：两个对象的大小关系不仅取决于年龄，还取决于姓名首字母
from random import randint,choice
from timeit import Timer
from timeit import timeit
class Student():
def __init__(self,name,age):
self.name = name
self.age = age
def __lt__(self,other):
#小于的定义：年龄相同时还要比较首字母大小；年龄不同时以年龄为准
return self.name < other.name if self.age == other.age else self.age < other.age
def __eq__(self,other):
return self.age == other.age
def selectionsort1(myarr):
for i in range(len(myarr)):
#求子数组[i,n)中的最小值对应的index
min_index = i
for j in range(i,len(myarr)):
if myarr[j] < myarr[min_index]:
min_index = j
#交换list[i]与list[min_index]
myarr[i], myarr[min_index] = myarr[min_index],myarr[i]
print([(i.age,i.name) for i in myarr])
if __name__ == "__main__":
name_bank = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
array = [Student(choice(name_bank),randint(1,100)) for _ in range(1000)]
timer = Timer('selectionsort1(array)','from __main__ import selectionsort1,array')
print(timer.timeit(number=1))
结果符合预期，注意，必须在函数内print,因为timeit处于不同命名空间
三、排序算法练习常用辅助函数
from random import randint
from time import time
def random_array(n, left, rught):
return [randint(left,right) for _ in rnage(n)]
def is_sorted(array):
sorted = True
for i in range(len(array)-1):
if array[i] > array[i+1]:
sorted = False
break
return sorted
def test_sort(sort_name, sort,array):
start = time()
sort(array)
spend = time() - start
#不正确时会抛出异常
assert(is_sorted(array))
print(sort_name + ' : '+ str(spend) + ' seconds')