[R] counting run lengths

[Mario Lavezzi]

Hello,
I have the following problem.

I am running simulations on possible states of a set of agents 
(1=employed, 0=unemployed).

I store these simulated time series in a matrix like the following, 
where rows indicates time periods, columns the number of agents (4 
agents and 8 periods in this case):

Atr=[
1    1    1    1
1    1    0    1
1    1    0    1
1    1    0    1
0    1    0    1
0    1    0    1
0    1    0    1
0    1    0    1]

At this point, I need to update a vector ("unSpells") which contains the 
lenghts of unemployment spells, and is initialized with ones. 
Practically, in the case represented I need to store the value "4" at 
position 1 of unSpells and "7" at position 3 of unSpells (that is, I 
care only of those who, in the last row, are zeros).

I am doing this in the following way (tt+1 indicates the time period 
reached by the simulation, n the number of agents):

    unSpells = matrix(1,nrow=1,ncol=n)   
    ppp=apply(Atr[1:(tt+1),],2,rle)
    for(i in (1:n)[Atr[tt+1,]==0]){
        unSpells[i]=tail(ppp[[i]]$lengths,1)
    }

It works, but the for (i in ...) loop slows down the simulation a lot.

Any suggestion on how to avoid this loop? (or in general, to speed up 
this part of the simulation)

Thanks!!
Mario

-- 
Andrea Mario Lavezzi
Dipartimento di Studi su Politica, Diritto e Società
Università di Palermo
Piazza Bologni 8
90134 Palermo, Italy
tel. ++39 091 6625600
fax ++39 091 6112023
skype: lavezzimario
email: lavezzi (at) unipa.it
web: http://www.unipa.it/~lavezzi