On 10/26/2009 11:05 AM, hadley wickham wrote: >> What are the values of >> >> length((Ht_cm[type=='SD'][from_treeline=='above'])[1]) > > I suspect the error is in the subsetting - the following seems more plausible: > > Ht_cm[type=='SD' && from_treeline=='above'] But don't double that ampersand: use Ht_cm[type=='SD' & from_treeline=='above'] Duncan