one simple way could be:
sparse.vec <- function (..., fun = sum) {
lis <- list(...)
values <- unlist(lapply(lis, "[[", "value"))
inds <- factor(unlist(lapply(lis, "[[", "index")))
out <- tapply(values, inds, FUN = fun)
list(index = as.numeric(levels(inds)), values = out)
}
a <- list(index = c(20, 30, 100000000), value = c(2.2, 3.3, 4.4))
b <- list(index = c(3, 30), value = c(0.1, 0.1))
sparse.vec(a, b)
sparse.vec(a, b, fun = prod)
sparse.vec(a, b, fun = function(x) Reduce("-", x))
I hope it helps.
Best,
Dimitris
Robin Hankin wrote:
Hi
I deal with long vectors almost all of whose elements are zero.
Typically, the length will be ~5e7 with ~100 nonzero elements.
I want to deal with these objects using a sort of sparse
vector.
The problem is that I want to be able to 'add' two such
vectors.
Toy problem follows. Suppose I have two such objects, 'a' and 'b':
> a
$index
[1] 20 30 100000000
$val
[1] 2.2 3.3 4.4
> b
$index
[1] 3 30
$val
[1] 0.1 0.1
>
What I want is the "sum" of these:
> AplusB
$index
[1] 3 20 30 100000000
$val
[1] 0.1 2.2 3.4 4.4
>
See how the value for index=30 (being common to both) is 3.4
(=3.3+0.1). What's the best R idiom to achieve this?
--
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus University Medical Center
Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
Tel: +31/(0)10/7043478
Fax: +31/(0)10/7043014
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