install.packages('ffbase')ff vectordroplevels is used to drop unused levels from a ff
factor or , more commonly, from factors in a ffdfff vectors as indexesff vectors.ff vector xff vector with draws from distributionff vectorsffdf to another ffdfff vectorffdf data.frameffdf from All Combinations of Factorsdroplevels is used to drop unused levels from factors
in a ffdfff vectors.ff vectors.ffdfff vector Elementsff vectors.