Avoiding looping on vector with stochastic dependency

Thanks for your reply. In fact my code example was simplified from something
more general

for (i in 2:length(w[,1])) {
               w[i,] <- foo( w[i-1,] )
       }

and I was trying to write this without a loop. I will look at the method you
describe, but  I'm not sure if it can still be applied if foo() is quite
complex. (sorry for sending this to the wrong list, I thought this one was
about everything that can make R running faster!)