[R] creating a derived variable in a data frame

[ronggui]

I suggest you use the recode function in car package to do your job.
	

======= 2005-10-20 08:09:08 ÄúÔÚÀ´ÐÅÖÐÐ´µÀ£º=======

>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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2005-10-20

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Deparment of Sociology
Fudan University

My new mail addres is ronggui.huang at gmail.com
Blog:http://sociology.yculblog.com