[R] listing nodes in paths

[Federico Calboli]

Hi All,

I have the following adjacency matrix for a directed graph:

      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,]    0    0    0    0    0    0    0    0
[2,]    0    0    0    0    0    0    0    0
[3,]    1    0    0    0    0    0    0    0
[4,]    0    0    1    0    0    0    0    0
[5,]    0    0    1    0    0    0    0    0
[6,]    1    1    0    0    0    0    0    0
[7,]    0    0    0    1    1    0    0    0
[8,]    0    0    0    0    0    1    1    0

My interest is the numberof path between (8) and (1). Using a  
standard matrix moltiplication I can see I have one pathe of length 2  
and two paths of length 4.

(paths of length 2)

      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,]    0    0    0    0    0    0    0    0
[2,]    0    0    0    0    0    0    0    0
[3,]    0    0    0    0    0    0    0    0
[4,]    1    0    0    0    0    0    0    0
[5,]    1    0    0    0    0    0    0    0
[6,]    0    0    0    0    0    0    0    0
[7,]    0    0    2    0    0    0    0    0
[8,]    1    1    0    1    1    0    0    0

(paths of length 4)

      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,]    0    0    0    0    0    0    0    0
[2,]    0    0    0    0    0    0    0    0
[3,]    0    0    0    0    0    0    0    0
[4,]    0    0    0    0    0    0    0    0
[5,]    0    0    0    0    0    0    0    0
[6,]    0    0    0    0    0    0    0    0
[7,]    0    0    0    0    0    0    0    0
[8,]    2    0    0    0    0    0    0    0

All in all I have 3 paths (not all of the same length) between (8)  
and (1).

Is there already a function in R (whatever the library) that will  
list the nodes touched in all those three paths (i.e. 8 -> 6 -> 1; 8 - 
 > 7 -> 4 -> 3 -> 1; 8 -> 7 -> 5 -> 3 -> 1)?

Regards,

Federico Calboli

--
Federico C. F. Calboli
Department of Epidemiology and Public Health
Imperial College, St. Mary's Campus
Norfolk Place, London W2 1PG

Tel +44 (0)20 75941602   Fax +44 (0)20 75943193

f.calboli [.a.t] imperial.ac.uk
f.calboli [.a.t] gmail.com