[R] Efficient matrix slices
Gabriel Valiente
valiente at lsi.upc.edu
Sun Jul 8 19:30:40 CEST 2007
Indexing matrices by subsets of rows and columns is quite convenient,
but it seems to take time linear in the size of the matrix (even for
a small slice of the matrix):
> dim(y)
[1] 732 1332
> length(which(a[1,]==1))
[1] 4
> length(which(b[1,]==1))
[1] 12
> proc.time(y[which(a[1,]==1),which(b[1,]==1)])
[1] 32.596 1.809 510.928 0.000 0.000
> proc.time(sum(y))
[1] 33.082 1.914 547.469 0.000 0.000
Does anybody know how matrix slices are actually implemented in R?
Thanks a lot,
Gabriel
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