[R] Generating new variable based on values of an existing variable

[Christos Hatzis]

One way to do this is through transform, assuming that there is one-to-one
correspondence between regions and elements:

mydf <- data.frame(region=c(rep("North", 5), rep("East", 5), rep("South",
5), rep("West", 5)))
elements <- c("earth", "water", "air", "fire")
transform(mydf, element = factor(region, levels=c("North", "East", "South",
"West"), labels=elements)) 

-Christos

> -----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
> 
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