Re: [R] Skip loop combination when one factor's level is missing

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

[R] Skip loop combination when one factor's level is missing

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.