Compare two different dataframes and add new values to a column

Question

I have 2 DataFrames, where a column of df1 must be compared to another column, in df2, that have different dimensions, and if true, modify de df1[3].

if df1.loc[df1[1]] == df2.loc[df2[3]]:  
    df1[3] = 4

df1:

0	1	2	3
yy	123	x
xx	445	r
za	336	f
gs	775	e
fr	447	w

df2:

0	1	2	3
T	0	x	336
S	4	r	447
R	3	f	445

Expected result should be:

df1:

0	1	2	3
yy	123	x
xx	445	r	4
za	336	f	4
gs	775	e
fr	447	w	4

 KeyError:None of [Index ... type='object', length=...)] are in the [index]"

Can someone help me with this? I know that probably is a simple thing, but i've tried a lot of ways and none of works.

Answer 1

Use boolean indexing with isin :

df1.loc[df1[1].isin(df2[3]), 3] = 4

output:

    0    1  2    3
0  yy  123  x  NaN
1  xx  445  r  4.0
2  za  336  f  4.0
3  gs  775  e  NaN
4  fr  447  w  4.0