on 02/09/2009 01:30 PM Josip Dasovic wrote:
> 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$region=="North", etc. In STATA, this was relatively easy; is there a
> simple way to do this in R?
>
> This is what the final result should look like:
>
>> mydf
> region element
> 1 North earth
> 2 North earth
> 3 North earth
> 4 North earth
> 5 North earth
> 6 East water
> 7 East water
> 8 East water
> 9 East water
> 10 East water
> 11 South air
> 12 South air
> 13 South air
> 14 South air
> 15 South air
> 16 West fire
> 17 West fire
> 18 West fire
> 19 West fire
> 20 West fire
>
> Thanks in advance,
> Josip
I am going to presume that unlike your example data above, the real data
may not be sequenced in unique sequential runs. Thus, a more general
approach would be to set mydf$region as a factor, with the factor levels
set to match 1:1 the sequence in 'elements':
mydf$region <- factor(mydf$region,
levels = c("North", "East", "South", "West"))
element <- c("earth", "water", "air", "fire")
# Set mydf$element to the value in 'element' which corresponds to the
# underlying factor integer code for mydf$region
mydf$element <- element[as.numeric(mydf$region)]
> 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
HTH,
Marc Schwartz
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