Translates matrix indices by rows into indices by columns

Translates matrix indices by rows into indices by columns.

indexByRow(dim, idxs = NULL, ...)

Arguments

dim

A numeric vector of length two specifying the length of the "template" matrix.

idxs

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

...

Not used.

Value

Returns an integer vector of indices.

Known limitations

The current implementation does not support long-vector indices, because both input and output indices are of type integers. This means that the indices in argument idxs can only be in range [1,2^31-1]. Using a greater value will be coerced to NA_integer_. Moreover, returned indices can only be in the same range [1,2^31-1].

Author

Henrik Bengtsson

Examples

dim <- c(5, 4)
X <- matrix(NA_integer_, nrow = dim[1], ncol = dim[2])
Y <- t(X)
idxs <- seq_along(X)

# Assign by columns
X[idxs] <- idxs
print(X)
#>      [,1] [,2] [,3] [,4]
#> [1,]    1    6   11   16
#> [2,]    2    7   12   17
#> [3,]    3    8   13   18
#> [4,]    4    9   14   19
#> [5,]    5   10   15   20

# Assign by rows
Y[indexByRow(dim(Y), idxs)] <- idxs
print(Y)
#>      [,1] [,2] [,3] [,4] [,5]
#> [1,]    1    2    3    4    5
#> [2,]    6    7    8    9   10
#> [3,]   11   12   13   14   15
#> [4,]   16   17   18   19   20

stopifnot(X == t(Y))