仅为某些特定元素向数据框添加值 - 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 :

>>> df1
  c1  c2  c4
  a   10  3  
  b   20  5
  c   10  6
  d   22  9

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

Answer 1

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