Extracts one cell per row (column) from a matrix

Source: R/rowCollapse.R

Extracts one cell per row (column) from a matrix. The implementation is optimized for memory and speed.

rowCollapse(x, idxs, rows = NULL, dim. = dim(x), ..., useNames = TRUE)

colCollapse(x, idxs, cols = NULL, dim. = dim(x), ..., useNames = TRUE)

Arguments

x

An NxK matrix or, if dim. is specified, an N * K vector.

idxs

An index vector of (maximum) length N (K) specifying the columns (rows) to be extracted.

rows

A vector indicating subset of rows to operate over. If NULL, no subsetting is done.

dim.

An integer vector of length two specifying the dimension of x, also when not a matrix. Comment: The reason for this argument being named with a period at the end is purely technical (we get a run-time error if we try to name it dim).

...

Not used.

useNames

If TRUE (default), names attributes of the result are set, otherwise not.

cols

A vector indicating subset of columns to operate over. If NULL, no subsetting is done.

Value

Returns a vector of length N (K).

See also

Matrix indexing to index elements in matrices and arrays, cf. [().

Author

Henrik Bengtsson

Examples

x <- matrix(1:27, ncol = 3)

y <- rowCollapse(x, 1)
stopifnot(identical(y, x[, 1]))

y <- rowCollapse(x, 2)
stopifnot(identical(y, x[, 2]))

y <- rowCollapse(x, c(1, 1, 1, 1, 1, 3, 3, 3, 3))
stopifnot(identical(y, c(x[1:5, 1], x[6:9, 3])))

y <- rowCollapse(x, 1:3)
print(y)
#> [1]  1 11 21  4 14 24  7 17 27
y_truth <- c(x[1, 1], x[2, 2], x[3, 3], x[4, 1], x[5, 2],
             x[6, 3], x[7, 1], x[8, 2], x[9, 3])
stopifnot(identical(y, y_truth))