Row-wise lookup from other data frame

Question

I have two data frames that I would like to combine based on certain conditions. This is the first data frame, each line represents one obversation (thus IDs occure multiple

Answer 1

I don't really know how efficient it is, but you can do that:

df1 = df1.set_index(['ID', 'Publication'])
df2 = df2.set_index('ID').stack()
df2.index.rename(['ID', 'Publication'], inplace=True)
df1['df2_value'] = df2
df1['result'] = df1['Count'] * df1['df2_value']

Answer 2

Using lookup

df2.set_index('ID').lookup(df1.ID,df1.Publication.astype(str))
Out[189]: array([1.1, 2.3, 1.1, 2.4, 1.2])

df1['Results']=df2.set_index('ID').lookup(df1.ID,df1.Publication.astype(str))*(df1.Count)
df1
Out[194]: 
  ID  Count  Publication  Results
0  A     10         1990     11.0
1  B     15         1990     34.5
2  A     17         1990     18.7
3  B     19         1991     45.6
4  A     13         1991     15.6