add values to data frame for only some specific elements - python

Question

With these two data frames

df1 = pd.DataFrame({'c1':['a','b','c','d'],'c2':[10,20,10,22]})
df2 = pd.DataFrame({'c3':['e','f','a','g','b','c','r','j','d'],'c4':[1,2,3,4,5,6,7,8,9]})

I'm trying to add the values of c4 to df1 for only the elements in c3 that are also present in c1:

Is there a simple way of doing this in pandas?

UPDATE:

If

df2 = pd.DataFrame({'c3':['e','f','a','g','b','c','r','j','d'],'c4':[1,2,3,4,5,6,7,8,9]},'c5':[10,20,30,40,50,60,70,80,90])

how can I achieve this result?

>>> df1
  c1  c2  c4  c5
  a   10  3   30  
  b   20  5   50
  c   10  6   60
  d   22  9   90

Doing:

>>> df1['c1'].map(df2.set_index('c3')['c4','c5'])

gives me a KeyError

You should not modify your question and change your requirements, if you have a new question then post a new question, anyway df1.merge(df2, left_on='c1', right_on='c3').drop('c3', axis=1) is what you want — EdChum
– EdChum, Commented Jul 31, 2015 at 15:33
@EdChum: Ok, I'll do it next time. Many thanks again for your help! — HappyPy
– HappyPy, Commented Jul 31, 2015 at 15:42

EdChum · Accepted Answer · 2015-07-31 14:43:38Z

2

You can call map on df2['c4'] after setting the index on df2['c3'], this will perform a lookup:

In [239]:
df1 = pd.DataFrame({'c1':['a','b','c','d'],'c2':[10,20,10,22]})
df2 = pd.DataFrame({'c3':['e','f','a','g','b','c','r','j','d'],'c4':[1,2,3,4,5,6,7,8,9]})
df1['c4'] = df1['c1'].map(df2.set_index('c3')['c4'])
df1

Out[239]:
  c1  c2  c4
0  a  10   3
1  b  20   5
2  c  10   6
3  d  22   9

answered Jul 31, 2015 at 14:43

EdChum

397k204 gold badges836 silver badges583 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

HappyPy Over a year ago

Is it actually possible to call "map" on more than one column? I have updated my example.

EdChum Over a year ago

To answer your question, no it won't work that way, for your updated example use merge

Collectives™ on Stack Overflow

add values to data frame for only some specific elements - python

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related