Say you want to convert a matrix to a list, where each element of the list contains one column. list() or as.list() obviously won't work, and until now I use a hack using the behaviour of tapply :
x I'm not completely happy with this. Anybody knows a cleaner method I'm overlooking?
(for making a list filled with the rows, the code can obviously be changed to :
tapply(x,rep(1:nrow(x),ncol(x)),function(i)i) )
In the interests of skinning the cat, treat the array as a vector as if it had no dim attribute:
split(x, rep(1:ncol(x), each = nrow(x))) Gavin's answer is simple and elegant. But if there are many columns, a much faster solution would be:
lapply(seq_len(ncol(x)), function(i) x[,i]) The speed difference is 6x in the example below:
> x system.time( as.list(data.frame(x)) ) user system elapsed 1.24 0.00 1.22 > system.time( lapply(seq_len(ncol(x)), function(i) x[,i]) ) user system elapsed 0.2 0.0 0.2 data.frames are stored as lists, I believe. Therefore coercion seems best:
as.list(as.data.frame(x)) > as.list(as.data.frame(x)) $V1 [1] 1 2 3 4 5 $V2 [1] 6 7 8 9 10 Benchmarking results are interesting. as.data.frame is faster than data.frame, either because data.frame has to create a whole new object, or because keeping track of the column names is somehow costly (witness the c(unname()) vs c() comparison)? The lapply solution provided by @Tommy is faster by an order of magnitude. The as.data.frame() results can be somewhat improved by coercing manually.
manual.coerce Converting to a data frame thence to a list seems to work:
> as.list(data.frame(x)) $X1 [1] 1 2 3 4 5 $X2 [1] 6 7 8 9 10 > str(as.list(data.frame(x))) List of 2 $ X1: int [1:5] 1 2 3 4 5 $ X2: int [1:5] 6 7 8 9 10 Using plyrcan be really useful for things like this:
library("plyr") alply(x,2) $`1` [1] 1 2 3 4 5 $`2` [1] 6 7 8 9 10 attr(,"class") [1] "split" "list" I know this is anathema in R, and I don't really have a lot of reputation to back this up, but I'm finding a for loop to be rather more efficient. I'm using the following function to convert matrix mat to a list of its columns:
mat2list Quick benchmark comparing with mdsummer's and the original solution:
x Under Some R Help site accessible via nabble.com I find:
c(unname(as.data.frame(x))) as a valid solution and in my R v2.13.0 install this looks ok:
> y y [[1]] [1] 1 2 3 4 5 [[2]] [1] 6 7 8 9 10 Can't say anythng about performance comparisons or how clean it is ;-)
You could use apply and then c with do.call
x And it looks like it will preserve the column names, when added to the matrix.
colnames(x) In the trivial case where the number of columns is small and constant, then I've found that the fastest option is to simply hard-code the conversion:
mat2list convertRowsToList {BBmisc}
Convert rows (columns) of data.frame or matrix to lists.
BBmisc::convertColsToList(x) ref: http://berndbischl.github.io/BBmisc/man/convertRowsToList.html