Pandas Dataframe Merging

Question

I have a bit of a weird pandas question.

I have a master Dataframe:

   a   b   c
0  22  44  55
1  22  45  22
2  44  23  56
3  45  22  33

I then have a dataframe in a different dimension which has some over lapping index's and column names

index   col_name  new_value
0        a          111 
3        b          234

I'm trying to then say if you find a match on index and col_name in the master dataframe, then replace the value.

So the output would be

   a   b   c
0  111  44  55
1  22  45  22
2  44  23  56
3  45  234  33

I've found "Combine_first" but this doesn't work unless I pivot the second dataframe (which I can't do in this scenario)

BENY · Accepted Answer · 2018-12-27 16:36:28Z

5

This is update problem

df.update(updated.pivot(*updated.columns))
df
Out[479]: 
       a      b   c
0  111.0   44.0  55
1   22.0   45.0  22
2   44.0   23.0  56
3   45.0  234.0  33

Or

df.values[updated['index'].values,df.columns.get_indexer(updated.col_name)]=updated.new_value.values
df
Out[495]: 
     a    b   c
0  111   44  55
1   22   45  22
2   44   23  56
3   45  234  33

answered Dec 27, 2018 at 16:36

BENY

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4 Comments

fred.schwartz Over a year ago

Thank you. My fault for not labelling but in your example is df the master dataframe and updated the one to add on?

BENY Over a year ago

@fred.schwartz yes , it is

Scott Boston Over a year ago

And yes, using @W-B's method of pivoting dim.pivot(*dim.columns).combine_first(master) works.

BENY Over a year ago

@coldspeed thank you so much ! Without your support, this would not have been possible!

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