[R] RE: variable number of variables in data frames

[Henrik Bengtsson]

Kevin, you can treat data frames as matrices, i.e. you can do

  dat <- matrix(c(2,1,1,1, 1,1,1,1), 2, 4)
  D <- as.data.frame(dat)
  for (i in 1:ncol(D)) 
    D[,i] <- factor(D[,i], 1:2)


Henrik

> -----Original Message-----
> From: murphyk at relay.eecs.berkeley.edu
> [mailto:murphyk at relay.eecs.berkeley.edu]On Behalf Of Kevin Murphy
> Sent: Monday, July 16, 2001 6:19 PM
> To: r-help at hypatia.math.ethz.ch; Henrik Bengtsson
> Cc: murphyk at cs.berkeley.edu
> Subject: variable number of variables in data frames
> 
> 
> I would like to specify that certain columns in a data frame should be
> treated as ordered factors. I know what numbers these columns are, but
> not their names.
> How do I do this?
> 
> For example, if I know columns 1:4 are to be treated as factors, I can
> write
> 
> dat <- matrix(c(2,1,1,1, 1,1,1,1), 2, 4)
> D <- as.data.frame(dat)
> # force all variables to be treated as binary
> # regardless of the small data set
> D$V1 <- factor(D$V1, 1:2)
> D$V2 <- factor(D$V2, 1:2)
> D$V3 <- factor(D$V3, 1:2)
> D$V4 <- factor(D$V4, 1:2)
> 
> But how do I do this in general? What I would like to say is something
> like
> 
> for (i in my.factor.columns) {
>   D$Vi <- factor(D$Vi, 1:my.nlevels[i])
> }
> 
> Presumably I could do something tricky using eval, but I don't know how.
> Besides, I'd prefer to avoid eval, since it is slow.
> (Also, I don't want to rely on the fact that the columns are named "Vnn"
> by default.)
> 
> 
> Kevin
> 
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