浅谈Pandas 排序之后索引的问题

In [1]: import pandas as pd
 ...: df=pd.DataFrame({"a":[1,2,3,4,5],"b":[5,4,3,2,1]})
In [2]: df
Out[2]: 
 a b
0 1 5
1 2 4
2 3 3
3 4 2
4 5 1
In [3]: df=df.sort_values(by="b") # 按照b列排序
In [4]: df
Out[4]: 
 a b
4 5 1
3 4 2
2 3 3
1 2 4
0 1 5
In [5]: df.loc[0,:] # 按索引来索引所以得到了是排序末位
Out[5]: 
a 1
b 5
Name: 0, dtype: int64
In [6]: df.iloc[0,:] # 按照绝对的索引来索引，所以得到了第一位
Out[6]: 
a 5
b 1
Name: 4, dtype: int64
In [7]: df.iloc[0,"b"] # 因为是绝对位置，所以列的参数不能是列名
ValueError: Location based indexing can only have [integer, integer slice (START point is INCLUDED, END point is EXCLUDED), listlike of integers, boolean array] types
In [8]: df.iloc[0,1] # “b”列的绝对位置是1，所以这就是索引了“b”列
Out[8]: 1
In [9]: df.iloc[0,:]["b"] # 和上述方法是一样的，不过这个更加容易懂一些
Out[9]: 1