Subtract columns from two data frames based on a common column

Question

df1:

1. Asia 34
2. America 74
3. Australia 92
4. Africa 44

df2:

1. Asia 24
2. Australia 90
3. Africa 30

I want the output of df1 - df2 to be

1. Asia 10
2. America 74
3. Australia 2
4. Africa 14

I am getting troubled by this, I am newbie into pandas. Please help out.

Answer 1

Use Series.sub with mapped second Series by Series.map :

df1['B'] = df1['B'].sub(df1['A'].map(df2.set_index('A')['B']), fill_value=0)
print (df1)
           A     B
0       Asia  10.0
1    America  74.0
2  Australia   2.0
3     Africa  14.0

If possible changed ordering of first column convert both first columns to index by DataFrame.set_index and subtract:

df2 = df1.set_index('A')['B'].sub(df2.set_index('A')['B'], fill_value=0).reset_index()
print (df2)
           A     B
0     Africa  14.0
1    America  74.0
2       Asia  10.0
3  Australia   2.0