List seems to drop empty levels of factors when containing them

2 messages · Andrew Robinson, Sundar Dorai-Raj

Tue, Nov 9, 2004 7:43 PM #

Greetings R community,

I am curious about the following behaviour: if I define a factor, and
then store a subset of it in a list, the stored version seems to drop
levels that were not included in the subset.  E.g. ..

[[1]]
[1] 1 2
Levels: 1 2

[[1]]
[1] 1
Levels: 1

It is as though the list were redefining the factor and dropping
empty levels.  I would like to keep them (I am using the list for a
two-dimensional tapply). Is there any way to avoid this?

I'm using R 2.0.0 on FreeBSD 5.2.1.

Thanks,

Andrew

Andrew Robinson                      Ph: 208 885 7115
Department of Forest Resources       Fa: 208 885 6226
University of Idaho                  E : andrewr at uidaho.edu
PO Box 441133                        W : http://www.uidaho.edu/~andrewr
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Sundar Dorai-Raj

Tue, Nov 9, 2004 8:18 PM #

Andrew Robinson wrote:

Andrew,

I cannot replicate this with R-2.0.0 patched on Win2000Pro:


 > mask <- c(T, F)
 > grp.1 <- factor(c(1,2))
 > list(grp.1)
[[1]]
[1] 1 2
Levels: 1 2

 > grp.1[mask]
[1] 1
Levels: 1 2
 > list(grp.1[mask])
[[1]]
[1] 1
Levels: 1 2

--sundar