On 10/24/2011 12:35 AM, Philipp Fischer wrote:
Hello,
I am just starting with R and I am having a (most probably) stupid problem by
creating a new variable in a data.frame based on a part of another character
variable.
I have a data frame like this one:
A B C
AWI-test1 1 i
AWI-test5 2 r
AWI-tes75 56 z
UFT-2 5 I
UFT56 f t
UFT356 9j t
etc. etc. 89 t
I now want to look in the variable A if the string AWI is present and then create a variable D and
putting "Arctic" inside. However, if the string UFT occurs in the variable A, then the
variable D shall be "Boreal" etc. etc.
The resulting data.frame file should look like
A B C D
AWI-test1 1 i Arctic
AWI-test5 2 r Arctic
AWI-tes75 56 z Arctic
UFT-2 5 I Boreal
UFT56 f t Boreal
UFT356 9j t Boreal
etc. etc. 89 t
Hi Philipp,
Since you mentioned that you were just starting with R, it might be a
little optimistic to throw you into the regular expression cage and
expect you to emerge unscathed. You can do this by constructing a 2
column matrix or data frame of replacement values:
replacements<-matrix(c("AWI","UFT","Arctic","Boreal"),ncol=2)
replacements
[,1] [,2]
[1,] "AWI" "Arctic"
[2,] "UFT" "Boreal"
Then write a function using grep to replace the values:
swapLabels<-function(x,y) {
for(swaprow in 1:dim(y)[1])
if(length(grep(y[swaprow,1],x))) return(y[swaprow,2])
return(NA)
}
Finally, apply the function to the first row of the data frame:
pf.df$D<-unlist(lapply(pf.df[,1],swapLabels,replacements))
pf.df$D
[1] "Arctic" "Arctic" "Arctic" "Boreal" "Boreal" "Boreal"
Jim
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