[R] subsetting an ordered factor

[RenE J.V. Bertin]

[I hope all of you had some good holidays/vacation!]

My current flabbergaster:

## a dummy factor
> size<- rep( c('short', 'long'), 10 )
## take a subset:
> subset( size, size!='short')
 [1] "long" "long" "long" "long" "long" "long" "long" "long" "long" "long"
## take another subset, all elements that are not some value not actually found:
> subset( size, size!='small')
 [1] "short" "long"  "short" "long"  "short" "long"  "short" "long"  "short" "long"  "short" "long"  "short" "long"  "short" "long" 
[17] "short" "long"  "short" "long" 
## Now order the factor:
> size<-ordered( size, levels=c('short', 'long' ) )
## repeat that subset:
> subset( size, size!='small')
ordered(0)
Levels:  short < long 

So it appears to be invalid/illegal/undefined to ask for all elements in an *ordered* factor that are *not* some value that does *not* occur in that factor. Whereas it is perfectly legal to do so for the same, unordered factor?!

## however:
> size<-ordered( size, levels=c('short', 'long', 'small' ) )
> subset( size, size!='small')
 [1] short long  short long  short long  short long  short long  short long  short long  short long  short long  short long 
Levels:  short < long < small 

I have not seen anything in the manpage for subset nor ordered that suggests this behaviour.

Can someone explain the use of this peculiarity to me, or is this a bug that should be (has been) reported?

RenE Bertin
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