Assign a matrix to a subset of a data.table
dt1[,c("a1","a2")] <- as.data.table(m1)
gives a simple solution but does make a copy.
@Simon O'Hanlon provides a solution in the data.table
way:
dt1[ , `:=`( a1 = m1[,1] , a2 = m1[,2] ) ]
and in my opinion an even better data.table
solution is provided by @Frank:
dt1[,c("a1","a2") := as.data.table(m1)]
A data.frame
is not a matrix
, nor is a data.table
a matrix
. Both data.frame
and data.table
objects are lists
. These are stored very differently, although the indexing can be similar, this is processed under the hood.
Within [<-.data.frame
splits a matrix-valued value
into a list with an element for each column.
(The line is value <- split(value, col(value))
)).
Note also that [<-.data.frame
will copy the entire data.frame in the process of assigning something to a subset of columns.
data.table
attempts to avoid this copying, as such [<-.data.table
should be avoided, as all <-
methods in R
make copies.
Within [<-.data.table
, [<-.data.frame
will be called if i
is a matrix, but not if only value
is.
data.table
usually likes you to be explicit in ensuring that the types of data match when assigning. This helps avoid any coercion and related copying.
You could, perhaps put in a feature request here to ensure compatibility, but given your usage is far outside what is recommended, then perhaps the package authors might request you simply use the data.table
conventions and approaches.