## argmax, argmin argsort and quick sort | 快速排序

This Saturday class we went over indexing and ordering a group of items by their sorted indices. For those who are more advanced, please go over the section on quick sort.

For example,

>> import numpy as np
>>> packpack =np.array([‘snack’,’book’,’pen’,’eraser’,’apple’])
# Position of the biggest word (alphabetically)
`>>> np.argmax(packpack)`

[out]: 0
# Position of the smallest word (alphabetically)
`>>> np.argmin(packpack)`

[out]: 4

# Position of the words if we are to sort them alphabetically
`>>> np.argsort(packpack)`

[out]: array([4, 1, 3, 2, 0], dtype=int64)

Now let us sort them:
`>>> packpack[np.argsort(packpack)]`

[out]: array([‘apple’, ‘book’, ‘eraser’, ‘pen’, ‘snack’], dtype='<U6′)

Then we tried sorting numbers:

numbers = np.array([2,3,5,7,1,4,6,15,5,2,7,9,10,15,9,17,12])
`>>> numbers[np.argsort(numbers)]`

[out]: array([ 1, 2, 2, 3, 4, 5, 5, 6, 7, 7, 9, 9, 10, 12, 15, 15, 17])

Finally we dig deeper: how do you really sort things fast systematically? Using quick sort!

```def quick_sort(data):
"""快速排序"""
if len(data) >= 2:  # 递归入口及出口
mid = data[len(data)//2]  # 选取基准值，也可以选取第一个或最后一个元素
left, right = [], []  # 定义基准值左右两侧的列表
data.remove(mid)  # 从原始数组中移除基准值
for num in data:
if num >= mid:
right.append(num)
else:
left.append(num)
return quick_sort(left) + [mid] + quick_sort(right)
else:
return data
numbers= [2,3,5,7,1,4,6,15,5,2,7,9,10,15,9,17,12]
Backpack = ['snack','book','pen','eraser','apple']
print(quick_sort(Backpack))
print(quick_sort(numbers))```

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