You can use "drop=TRUE" as argument in indexing (brackets) to drop
unused levels, see the following example:
a <- factor(1:2, levels=1:3)
a
# [1] 1 2
# Levels: 1 2 3
for(i in levels(a)) print(i)
#[1] "1"
#[1] "2"
#[1] "3"
for(i in levels(a[, drop=TRUE])) print(i)
#[1] "1"
#[1] "2"
Uwe Ligges
Paulo E. Cardoso wrote:
Hi,
In a symmetric design one may have a data.frame with, say, two factors (two
columns), each one with n levels.
In a nested loop, to run a expression combining levels of each factors, to
produce a new data.frame, if one level is absent, the loop is interrupted;
one gets a empty object.
Is there a way to avoid or skip combinations of factors in such nested
loops?
I think that the approach below is a very crude example:
for(i in unique(DF$ID1)) #! For each ID1 level
{
for (e in unique(DF$ID2)) #! For each ID2 level
{
DF.N <- DF[DF$ID1==i & DF$ID2==e,]
if(nrow(DF.N)==0) DF2 <- ??
else
Any idea will be welcome
Paulo
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