Generating new variable based on values of an existing variable

-----Original Message-----
From: r-help-bounces at r-project.org 
[mailto:r-help-bounces at r-project.org] On Behalf Of Josip Dasovic
Sent: Monday, February 09, 2009 2:30 PM
To: r-help at r-project.org
Subject: [R] Generating new variable based on values of an 
existing variable

Dear R Help-Listers:

I have a problem that seems like it should have a simple 
solution, but I've spent hours on it (and searching the 
r-help archives) to no avail. What I'd like to do is to 
generate a new variable within a data frame, the values of 
which are dependent upon the values of an existing variable 
within that data frame. 

Assume that I have the following data:

mydf<-data.frame(region=c(rep("North", 5), rep("East", 5), 
rep("South", 5), rep("West", 5)))

Assume, in addition, that I have a factor vector with four 
values (I actually have a factor with almost two-hundred values):

element<-c("earth", "water", "air", "fire")

I would like to add a new variable to the data frame (called 
"element") such that the value of "element" is "earth" in 
each observation for which mydf$region=="North", etc. In 
STATA, this was relatively easy; is there a simple way to do 
this in R? 

This is what the final result should look like:

mydf

   region element
1   North   earth
2   North   earth
3   North   earth
4   North   earth
5   North   earth
6    East   water
7    East   water
8    East   water
9    East   water
10   East   water
11  South     air
12  South     air
13  South     air
14  South     air
15  South     air
16   West    fire
17   West    fire
18   West    fire
19   West    fire
20   West    fire

Thanks in advance,
Josip