Pandas overwrite based on column

Question

I have the following dataframes:

df1

index  | colA  | colB
0          a       1
1          b       2
2          a       9

df2

index  | colA  | colB
5          a       3
8          c       4

I want to update df1 based on df2, meaning for all rows "i" such that

df1['ColA'].iloc[i] == df2['colB'].iloc[i]

The index do not match so .update() does not work here.

Desired output:

df1

index  | colA  | colB
0          a       3
1          b       2
2          a       3

Seems like you want a mapping by colA? – ALollz Mar 27 '20 at 18:47 — ALollz, Mar 27 '20 at 18:47
@YOBEN_S I don't think this is `merge`. – Quang Hoang Mar 27 '20 at 18:52 — Quang Hoang, Mar 27 '20 at 18:52

score 0 · Answer 1 · answered Mar 27 '20 at 18:48

0

You can do with boolean indexing:

idx = df1['colA'].values == df2['colA'].values
df1.loc[idx, 'colB'] = df2.loc[idx,'colB'].values

answered Mar 27 '20 at 18:48

Quang Hoang

Getting KeyError: False using the above – Asddfg Mar 27 '20 at 18:56
works with the sample data on my computer. make sure you have the correct column name. – Quang Hoang Mar 27 '20 at 18:58

1 Answers1