ffdfindexget: Reading and writing ffdf data.frame using ff subscripts

In kindlychung/ff: memory-efficient storage of large data on disk and fast access functions

Description

Function ffdfindexget allows to extract rows from an ffdf data.frame according to positive integer suscripts stored in an ff vector.
Function ffdfindexset allows the inverse operation: assigning to rows of an ffdf data.frame according to positive integer suscripts stored in an ff vector. These functions allow more control than the method dispatch of [ and [<- if an ff integer subscript is used.

Usage

  ffdfindexget(x, index, indexorder = NULL, autoindexorder = 3, FF_RETURN = NULL
  , BATCHSIZE = NULL, BATCHBYTES = getOption("ffmaxbytes"), VERBOSE = FALSE)
  ffdfindexset(x, index, value, indexorder = NULL, autoindexorder = 3
  , BATCHSIZE = NULL, BATCHBYTES = getOption("ffmaxbytes"), VERBOSE = FALSE)

Arguments

x	A ffdf data.frame containing the elements
index	A ff integer vector with integer subscripts in the range from 1 to length(x).
value	A ffdf data.frame like x with the rows to be assigned
indexorder	Optionally the return value of ffindexorder, see details
autoindexorder	The minimum number of columns (which need chunked indexordering) for which we switch from on-the-fly ordering to stored ffindexorder
FF_RETURN	Optionally an ffdf data.frame of the same type as x in which the returned values shall be stored, see details.
BATCHSIZE	Optinal limit for the batchsize (see details)
BATCHBYTES	Limit for the number of bytes per batch
VERBOSE	Logical scalar for verbosing

Details

Accessing rows of an ffdf data.frame identified by integer positions in an ff vector is a non-trivial task, because it could easily lead to random-access to disk files. We avoid random access by loading batches of the subscript values into RAM, order them ascending, and only then access the ff values on disk. Such ordering is don on-thy-fly for upto autoindexorder-1 columns that need ordering. For autoindexorder o more columns we do the batched ordering upfront with ffindexorder and then re-use it in each call to ffindexget resp. ffindexset.

Value

Function ffdfindexget returns a ffdf data.frame with those rows selected by the ff index vector.
Function ffdfindexset returns x with those rows replaced that had been requested by index and value.

Author(s)

Jens Oehlschl<c3><a4>gel

See Also

Extract.ff, ffindexget, ffindexorder

Examples

message("ff integer subscripts with ffdf return/assign values")
x <- ff(factor(letters))
y <- ff(1:26)
d <- ffdf(x,y)
i <- ff(2:9)
di <- d[i,]
di
d[i,] <- di
message("ff integer subscripts: more control with ffindexget/ffindexset")
di <- ffdfindexget(d, i, FF_RETURN=di)
d <- ffdfindexset(d, i, di)
rm(x, y, d, i, di)
gc()

kindlychung/ff documentation built on May 20, 2019, 9:58 a.m.