R - How to apply different functions to certain rows in a column

Question

I am trying to apply different functions to different rows based on the value of a string in an adjacent column. My dataframe looks like this:

type    size
A            


        

Answer 1

This might be a might more R-ish (and I called my dataframe 'dat' instead of 'df' since df is a commonly used function.

> facs <- c(3,1,2)
> dat$size2= dat$size* facs[ match( dat$type, c("A","B","C") ) ]
> dat
  type size size2
1    A    1     3
2    B    3     3
3    A    4    12
4    C    2     4
5    C    5    10
6    A    4    12
7    B   32    32
8    C    3     6

The match function is used to construct indexes to supply to the extract function [.