>>>>> Philipp A <flying-sh...@web.de>
>>>>> on Wed, 14 Jan 2015 14:02:40 +0000 writes:
> Hi,
> creating a matrix from two vectors a, b by multiplying each combination
can
> be done e.g. via
> a %*% t(b)
> or via
> outer(a, b) # default for third argument is '*'
really the best (most efficient) way would be
tcrossprod(a, b)
> But this yields a normal matrix.
of course.
Please always use small self-contained example code,
here, e.g.,
a <- numeric(17); a[3*(1:5)] <- 10*(5:1)
b <- numeric(12); b[c(2,3,7,11)] <- 1:3
> Is there an efficient way to create sparse matrices (from the Matrix
> package) like that?
> Right now i’m doing
> a.sparse = as(a, 'sparseVector')
> b.sparse = as(t(b), 'sparseMatrix')
> a.sparse %*% b.sparse
> but this strikes me as wasteful.
not really wasteful I think. But there is a nicer and more efficient way :
require(Matrix)
tcrossprod(as(a, "sparseVector"),
as(b, "sparseVector"))
now also gives
17 x 12 sparse Matrix of class "dgCMatrix"
[1,] . . . . . . . . . . . .
[2,] . . . . . . . . . . . .
[3,] . 50 100 . . . 150 . . . 50 .
[4,] . . . . . . . . . . . .
[5,] . . . . . . . . . . . .
[6,] . 40 80 . . . 120 . . . 40 .
[7,] . . . . . . . . . . . .
[8,] . . . . . . . . . . . .
[9,] . 30 60 . . . 90 . . . 30 .
[10,] . . . . . . . . . . . .
[11,] . . . . . . . . . . . .
[12,] . 20 40 . . . 60 . . . 20 .
[13,] . . . . . . . . . . . .
[14,] . . . . . . . . . . . .
[15,] . 10 20 . . . 30 . . . 10 .
[16,] . . . . . . . . . . . .
[17,] . . . . . . . . . . . .
>
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