[R] creating a derived variable in a data frame

[Martin Henry H. Stevens]

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
>
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Dr. Martin Henry H. Stevens, Assistant Professor
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