Pandas dataframe selecting with index and condition on a column-CodePudding

I am trying for a while to solve this problem:

I have a daraframe like this:

import pandas as pd
df=pd.DataFrame(np.array([['A', 2, 3], ['B', 5, 6], ['C', 8, 9]]),columns=['a', 'b', 'c'])
j=[0,2]

But then when i try to select just a part of it filtering by a list of index and a condition on a column I get error...

df[df.loc[j]['a']=='A']

There is somenting wrong, but i don't get what is the problem here. Can you help me?

This is the error message:

IndexingError: Unalignable boolean Series provided as indexer (index of the boolean Series and of the indexed object do not match).

CodePudding user response：

There is filtered DataFrame compared by original, so indices are different, so error is raised.

You need compare filtered DataFrame:

df1 = df.loc[j]
print (df1)
   a  b  c
0  A  2  3
2  C  8  9

out = df1[df1['a']=='A']
print(out)
   a  b  c
0  A  2  3

Your solution is possible use with convert ndices of filtered mask by original indices by Series.reindex:

out = df[(df.loc[j, 'a']=='A').reindex(df.index, fill_value=False)]
print(out)
   a  b  c
0  A  2  3

Or nicer solution:

out = df[(df['a'] == 'A') & (df.index.isin(j))]
print(out)
   a  b  c
0  A  2  3

CodePudding user response：

A boolean array and the dataframe should be the same length. here your df length is 3 but the boolean array df.loc[j]['a']=='A' length is 2

You should do:

>>> df.loc[j][df.loc[j]['a']=='A']
   a  b  c
0  A  2  3