How to DROP columns from ffdf object ? (R)

Question

Could I easily drop column of ffdf object ?

library(ff);library(ffbase)    
irisdf=as.ffdf(iris)

How to contain only Sepal.length and Species columns ?

Answer 1

library(ff)  
irisdf <- as.ffdf(iris)
filename(irisdf)
## Referencing the same data on disk (just making an new virtual ffdf)
example1 <- irisdf[setdiff(colnames(irisdf), c("Sepal.Length", "Species"))]
filename(example1)
## Referencing new data (copying)
example2 <- clone(irisdf[setdiff(colnames(irisdf), c("Sepal.Length", "Species"))])
filename(example2)