cifti.read.rows: Read the requested matrix rows of a CIFTI-2 file.
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cifti.read.rows

R Documentation

Read the requested matrix rows of a CIFTI-2 file.

Description

Workhorse of read.cifti.rows: read some indices of matrix dimension 0 without reading the rest of the rows. If a column selection is given, only those columns are read (they are contiguous blocks of the file), otherwise the file is read once, in chunks, keeping the requested rows.

Usage

cifti.read.rows(cii, rows, columns = NULL, chunk_values = 4000000L)

Arguments

`cii`	an `fs.cifti` instance, see `read.cifti.header`.
`rows`	integer vector, the indices of matrix dimension 0 to read. At least one index has to be given; the indices are 1-based, see `read.cifti`.
`columns`	integer vector or `NULL`, the indices of matrix dimension 1 to read, i.e. the columns of the returned array. This is the matrix dimension that holds the brainordinates of a dense file (the grayordinates of a `.dscalar`, `.dtseries` or `.dlabel`), and it is also one of the two dimensions that hold them in a connectome file like a `.dconn`. Selecting columns avoids reading the rest of the file, which is the only practical way to read a small part of a huge file like a `.dconn` (9 to 38 GB): for a 2-dimensional matrix, one contiguous block of the file holds all values of one column. Note that `rows` does not have this property, selecting rows still reads all values of the file.
`chunk_values`	integer, the number of data values that are read from the file per chunk. This does not change the result, only the peak memory usage and the I/O granularity, so it is rarely needed: the default of 4 millions values corresponds to about 16 MB. The chunk size is rounded up to a whole number of matrix columns, and the result is the same for every chunk size.