[R] sparse vectors

[Robin Hankin]

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