On 08-Sep-09 13:06:28, 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
> $val
> [1] 0.1 2.2 3.4 4.4
>
> See how the value for index=30 (being common to both) is 3.4
> (=3.3+0.1). What's the best R idiom to achieve this?
I don't know about "the best", Robin, but how about something like:
indices <- sort(unique(c(a$index,b$index)))
N <- length(indices)
values <- NULL
for(i in indices){
if(i %in% a$index){A <- a$val[a$index==i]} else A <- 0
if(i %in% b$index){B <- b$val[b$index==i]} else B <- 0
values <- c(values,A+B)
}
AplusB <- list(index=indices,val=values)
## Test:
a<-list(index=c(20,30,100000000),val=c(2.2,3.3,4.4))
b<-list(index=c(3,30),val=c(0.1, 0.1))
indices <- sort(unique(c(a$index,b$index)))
N <- length(indices)
values <- NULL
for(i in indices){
if(i %in% a$index){A <- a$val[a$index==i]} else A <- 0
if(i %in% b$index){B <- b$val[b$index==i]} else B <- 0
values <- c(values,A+B)
}
AplusB <- list(index=indices,val=values)
AplusB
# $index
# [1] 3e+00 2e+01 3e+01 1e+08
# $val
# [1] 0.1 2.2 3.4 4.4
Ted.
--------------------------------------------------------------------
E-Mail: (Ted Harding) <ted.hard...@manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 08-Sep-09 Time: 14:42:53
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