Pandas creating a new column by merging two dataframes and matching rows [duplicate]-CodePudding

I have two dataframes with one matching column [ID].

DF 1

ID     VAR
1      442
1      429
1      58
2      928
2      8493
3      093
3      809
3      913
4      133
4      490

DF2

ID       CODE
1        10foo
2        20bar
3        30foo
4        40bar

I'm trying to merge these dataframes so that I get something like this:

DF 3

ID     VAR      CODE
1      442      10foo
1      429      10foo
1      58       10foo
2      928      20bar
2      8493     20bar
3      093      30foo
3      809      30foo
3      913      30foo
4      133      40bar
4      490      40bar

I've tried this with DF3 = DF1.merge(DF2, on='ID', how='inner', right_index=True) This does actually work but ends up duplicating loads of values, more than doubling the amount of rows for some reason. Really not sure why that is happening. I need the number of rows in DF3 to be the same as DF1

Thanks for the help.

CodePudding user response：

To keep the same number of rows as DF1, you're going to want a left merge:

df1.merge(df2, on='ID', how='left')

CodePudding user response：

We can try this one :

>>> df = pd.merge(df1,
...               df2,
...               how='left',
...               left_on=['ID'],
...               right_on=['ID'])              
>>> df
   ID   VAR     CODE
0   1   442     10foo
1   1   429     10foo
2   1   58      10foo
3   2   928     20bar
4   2   8493    20bar
5   3   93      30foo
6   3   809     30foo
7   3   913     30foo
8   4   133     40bar
9   4   490     40bar