How to convert a matrix to a list of column-vectors in R?

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 <- matrix(1:10,ncol=2)

tapply(x,rep(1:ncol(x),each=nrow(x)),function(i)i)

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)

)

Solution 1:

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 <- matrix(1:1e6, 10)
> 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 

Solution 2:

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)))