Question

In a loop, I am trying to get a column vector of class factor into numeric.

If there were not a loop, the code would look like

c1$values <- as.numeric(as.character(c1$values))

But how do I reference c1$values with a loop? I have tried two approaches:

get(paste('c',i,"$values", sep="")) 

just does not work even outside the loop, while

get(paste('c',"1", sep=""))[[1]]

works in itself (returns the column vector), but when trying to perform the operation:

assign(get(paste("c","1", sep=""))[[1]], as.numeric(as.character(get(paste("c","1", sep=""))[[1]])))

returns an error of “invalid first argument”.

Any ideas?

Thanks,

Roberto

Answer 1

Internally the $ operator is a function that can be explicitly called as "$" for getting and "$<-" for setting. assign is the opposite of get. So breaking things up into discrete steps we have:

varname<-paste("c",1,sep="")
obj<-get(varname)
x<-"$"(obj,"values")
x<-as.numeric(as.character(x))
obj<-"$<-"(obj,"values",x)
assign(varname,obj)

But having data (such as an index) encoded into a variable name is not good practice. It might be a better idea to turn the c1,c2 etc. into a list or a data frame and then iterate over them. The conversion can be done by something like this:

lapply(1:2,function(i) get(paste('c',i,sep='')))