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?
--
Robin K. S. Hankin
Uncertainty Analyst
University of Cambridge
19 Silver Street
Cambridge CB3 9EP
01223-764877
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