Question

这是对这个问题的答案的后续问题：

pandas performance issue - need help to optimize

以下建议有效：

df = DataFrame(np.arange(20).reshape(5,4))
df2 = df.set_index(keys=[0,1,2])
df2.ix[(4,5,6)]

使用MultiIndex

所以我创建了一个文件sample_data.csv，如下所示：

col1,col2,year,amount 
111111,3.5,2012,700 
111112,3.5,2011,600 
222221,4.0,2012,222 
...

然后我运行了以下内容：

import numpy as np 
import pandas as pd 
sd=pd.read_csv('sample_data.csv') 
sd2=sd.set_index(keys=['col2','year']) 
sd2.ix[(4.0,2012)]

但这会产生以下错误： IndexError：索引越界

为什么它适用于前一种情况而不是后一种情况的任何想法？这就是错误的样子：

IndexError                                Traceback (most recent call last)
<ipython-input-19-1d72a961db95> in <module>()
----> 1 sd2.ix[(4.0,2012)]

/Library/Python/2.7/site-packages/pandas-0.8.1-py2.7-macosx-10.7-intel.egg/pandas/core/indexing.pyc in __getitem__(self, key)
     31                 pass
     32 
---> 33             return self._getitem_tuple(key)
     34         else:
     35             return self._getitem_axis(key, axis=0)

Answer 1

显示它适合我（pandas 0.10.1）：

In [1]: from StringIO import StringIO
In [2]: import numpy as np 
In [3]: import pandas as pd 
In [4]: s = StringIO("""col1,col2,year,amount 
   ...: 111111,3.5,2012,700 
   ...: 111112,3.5,2011,600 
   ...: 222221,4.0,2012,222""")

In [5]: sd=pd.read_csv(s) 
In [6]: sd2=sd.set_index(keys=['col2','year']) 
In [7]: sd2.ix[(4.0,2012)] 
Out[7]: 
col1       222221
amount        222
Name: (4.0, 2012)

但是，如果我添加一行重复索引，我也会收到同样的错误：

In [8]: s = StringIO("""col1,col2,year,amount 
   ...: 111111,3.5,2012,700 
   ...: 111112,3.5,2011,600 
   ...: 222221,4.0,2012,222
   ...: 222221,4.0,2012,223""")

In [9]: sd=pd.read_csv(s) 
In [10]: sd2=sd.set_index(keys=['col2','year']) 
In [11]: sd2.ix[(4.0,2012)] 
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-7-1b787a1d99df> in <module>()
----> 1 sd2.ix[(4.0,2012)]

C:\Python27\lib\site-packages\pandas\core\indexing.pyc in __getitem__(self, key)
     32                 pass
     33 
---> 34             return self._getitem_tuple(key)
     35         else:
     36             return self._getitem_axis(key, axis=0)

...

IndexError: index out of bounds

您是否可能在（'col1'，'year'）中有重复值？

我不知道它是一个bug还是只是对MultiIndex的约束（但在这种情况下，我认为错误信息可能更清晰）。但您可以在设置索引之前删除重复值，如下所示：

In [21]: sd=pd.read_csv(s) 

In [22]: sd = sd.drop_duplicates(['col2', 'year'])

In [23]: sd2=sd.set_index(keys=['col2','year']) 

In [24]: sd2.ix[(4.0,2012)] 
Out[24]: 
col1       222221
amount        222
Name: (4.0, 2012)

有关详情，请参阅：http://pandas.pydata.org/pandas-docs/stable/indexing.html#duplicate-data和http://pandas.pydata.org/pandas-docs/dev/generated/pandas.DataFrame.drop_duplicates.html。

在DataFrame上使用MultiIndex

1 个答案: