Reading and writing ff vectors using ff subscripts
Function ffindexget allows to extract elements from an ff vector according to positive integer suscripts stored in an ff vector.
Function ffindexset allows the inverse operation: assigning to elements of an ff vector 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.
ffindexget(x, index, indexorder = NULL, FF_RETURN = NULL
, BATCHSIZE = NULL, BATCHBYTES = getOption("ffmaxbytes"), VERBOSE = FALSE)
ffindexset(x, index, value, indexorder = NULL
, BATCHSIZE = NULL, BATCHBYTES = getOption("ffmaxbytes"), VERBOSE = FALSE)x |
A |
index |
A |
value |
An |
indexorder |
Optionally the return value of |
FF_RETURN |
Optionally an |
BATCHSIZE |
Optinal limit for the batchsize (see details) |
BATCHBYTES |
Limit for the number of bytes per batch |
VERBOSE |
Logical scalar for verbosing |
Accessing integer positions in an ff vector is a non-trivial task, because it could easily lead to random-access to a disk file.
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.
Since ordering is expensive, it may pay to do the batched ordering once upfront and then re-use it with ffindexorder,
similar to storing and using hybrid index information with as.hi.
Function ffindexget returns an ff vector with the extracted elements.
Function ffindexset returns the ff vector in which we have updated values.
Jens Oehlschlägel
message("ff integer subscripts with ff return/assign values")
x <- ff(factor(letters))
i <- ff(2:9)
xi <- x[i]
xi
xi[] <- NA
xi
x[i] <- xi
x
message("ff integer subscripts: more control with ffindexget/ffindexset")
xi <- ffindexget(x, i, FF_RETURN=xi)
x <- ffindexset(x, i, xi)
rm(x, i, xi)
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