How can I join a dataframe and invert the joining column?

Question

I currently have two dataframes which I wish to join:

df1 = {'col1': [1, 2, 3, 4, 5]}

   col1 
0     1     
1     2     
2     3     
3     4    
4     5     

df2 = {'col2': ['nan', 'nan', 3, 4]}

   col2 
0     nan     
1     nan     
2     3     
3     4

Currently I would like to join the two dataframes. However, I would like for the second dataframe (df2) to be added from the bottom as shown below:

   col1  col2
0     1     nan
1     2     nan
2     3     nan
3     4     3
4     5     4

How might I do this? I currently have tried using pandas.join() but have had no luck with this.

Essentially I want to append from the bottom, as shown by the output, there is another null value that is created in the first row of the output.

That doesn't answer my question. Im not sure why you had to close the question... @jezrael — plebman952, Jun 29 '20 at 05:07
hmmm, because it seems there is match by index of df2 and column `col1` in df1, I am wrong? — jezrael, Jun 29 '20 at 05:07
My bad, my goal was to join the two dataframes but I would like to have the second column added from the bottom rather than the top — plebman952, Jun 29 '20 at 05:09

score 1 · Answer 1 · answered Jun 29 '20 at 05:10

1

Idea is set index by DataFrame.set_index in df2 by index of df1 from bottom by indexing for same length and then is used DataFrame.join:

df1 = pd.DataFrame({'col1': [1, 2, 3, 4, 5]})
df2 = pd.DataFrame({'col2': [np.nan, np.nan, 3, 4]})

idx = df1.index[-len(df2):]

df = df1.join(df2.set_index(idx))
print (df)
   col1  col2
0     1   NaN
1     2   NaN
2     3   NaN
3     4   3.0
4     5   4.0

answered Jun 29 '20 at 05:10

jezrael

629,482
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918
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@plebman952 - Super! If my answer was helpful, don't forget [accept](https://meta.stackexchange.com/a/5235) it. Thanks. – jezrael Jun 29 '20 at 05:20

score 1 · Answer 2 · answered Jun 29 '20 at 05:19

1

df1 = pd.DataFrame({'col1': [1, 2, 3, 4, 5]})
df2 = pd.DataFrame({'col2': ['nan', 'nan', 3, 4]})
pd.concat([df1,df2],axis=1)

output

    col1    col2
0   1   nan
1   2   nan
2   3   3
3   4   4
4   5   NaN

answered Jun 29 '20 at 05:19

NAGA RAJ S

409
4
10

How can I join a dataframe and invert the joining column?

2 Answers2