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

Henrik Bengtsson henrikb at braju.com
Tue Jul 17 08:16:39 CEST 2001

```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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