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

[Uwe Ligges]

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
> 
> 
> 	[[alternative HTML version deleted]]
> 
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