Question

I have a dataframe a, with A, B, C are separate entries

Source Target N
A B 100
A D 200

I have another dataframe b for entries’ attributes

Name Rate1 Rate2
A  0.1 0.2
B  0.2 0.3

I want to calculate a new column Flow in a, as it is calculated row based by Flow = a$N * b[Name == a$Source]$Rate1. I tried to use apply by row, but I felt it’s slow. Is there a faster way?

Answer 1

Here’s a fairly expressive solution, fairly similar to the code you tried:

>  a$Flow <- a$N*b$Rate1[ match(a$Source, b$Name) ]
> a
  Source Target   N Flow
1      A      B 100   10
2      A      D 200   20

The match function is the basis for merge and %in%. It is particularly useful for constructing index vectors to pick from alternatives.