r - Adding a column with character data to a ffdf -

i've tried add source column ffdf, can't seem work... if normal df write

mtcars$newcol <- "ab"  

if ffdf returns error

require(ff) require(ffbase)  mtcarsff <- as.ffdf(mtcars)  mtcars$newcol <- "testname"  error in `[[<-.ffdf`(`*tmp*`, i, value = "testname") :  assigned value must ff 

any ideas?

this should work:

mtcarsff$newcol <- as.ff(     rep(factor("ab"), times = nrow(mtcarsff)) ) 

note "ab" must considered factor, not character.