sparse.extract.index: Sparse indexing of matrices
In xrobin/xavamess: Useful Miscellaneous Functions

Description Usage Arguments Details Value Examples

Computes a matrix index that extracts individual (i, j) elements. This differ from the standard extraction where all columns in rows i and all rows in columns j are extracted.

sparse.extract.index(
  i,
  j,
  nrow,
  ncol,
  byrow = FALSE,
  na.rm = FALSE,
  quiet = TRUE
)

`i, j`	the indices of the elements to extract. `i` and `j` must have the same length.
`nrow, ncol`	the number of rows and colums of the index. currently, `ncol` is ignored with `byrow = FALSE`, `nrow` is ignored with `byrow = TRUE`
`byrow`	only `FALSE` is supported for now
`na.rm`	remove elements where i or j are `NA`
`quiet`	if `FALSE`, prints a `message` before removing the missing values. If `TRUE`, missing values are silently removed.

Note that this is likely to NOT work on data frames.

the vector index for the matrix. Typically one would want to use this index to extract from a matrix, and a vector will be returned, regardless of the status of drop.

m <- matrix(1:25, 5, 5)
m[1:5, 1:5] # Returns the whole matrix
m[sparse.extract.index(1:5, 1:5, nrow = 5)] # Returns the diagonal only
m[c(1, 3, 5), c(2, 4, 4)]
m[sparse.extract.index(c(1, 3, 5), c(2, 4, 4), nrow = 5)]