Reading specific rows of large matrix data file

Question

Suppose I have a gigantic m*n matrix X (that is too big to read into memory) and binary numeric vector V with length m. My objective is to

Answer 1

ffdfindexget from ff package is what you are looking for:

Function ffdfindexget allows to extract rows from an ffdf data.frame according to positive integer suscripts stored in an ff vector.

So in your example:

write.csv(X,"target.csv")
d <- read.csv.ffdf(file="target.csv")
i <- ff(which(V==1))
di <- ffdfindexget(d, i)