creating a derived variable in a data frame

"Martin Henry H. Stevens" <HStevens at MUOhio.edu> 10/20/05 08:47AM

Hi Avram-
How many countries do you have?
I would do it the following way because it is simple and I don't know 

any better, even if it is  absurdly painstaking.

#Step 1
mydata$continent <- factor(NA, levels=c("NoAm","Euro"))

#Steps 2 a-z
mydata$continent[mydata$country=="US" |
                                mydata$country=="CA" |
                               mydata$country=="MX" ]  <- "NoAm"

#Repeat for all countries and continents.

Hank


On Oct 19, 2005, at 8:09 PM, Avram Aelony wrote:

Hello,

I have read through the manuals and can't seem to find an answer.

I have a categorical, character variable that has hundreds of  
values.  I want to group the existing values of this variable into 

a new, derived (categorical) variable by applying conditions to the 

values in the data.

For example, suppose I have a data frame with variables: date,  
country, x, y, and z.

x,y,z are numeric and country is a 2-digit character string.  I  
want to create a new derived variable named "continent" that would 

also exist in the data frame. The Continent variable would have  
values of "Asia", "Europe", "North America", etc...

How would this best be done for a large dataset (>10MB) ?
I have tried many variations on following without success (note in 

a real example I would have a longer list of countries and  
continent values):

mydata$continent <- mydata[ mydata$country==list 
('US','CA','MX'), ] -> "North America"

I have read about factors, but I am not sure how they apply here.

Can anyone help me with the syntax?  I am sure it is trivial and a 

common thing to do.
The ultimate goal is to compute percentages of x by continent.

Thanks for any help in advance.

-Avram