R中的循环内的ODE

Question

I'm new to programming and specially R. I find sometimes things easier to get yet more complex to give than Matlab. I'm trying to recreate a loop that I already have in a "script.m" but I haven't been able to get anything on R. Could anybody please explain me how to run the following for three times this?...

Thanks in advance for any help provided

pars  <- c(...)

times <- seq(ti, tf, delta)

state <- c(S = 50, X = 20, P = 0.5)   

SOLVE <- function(pars) { 

    derivs <- function(time, state, pars) {

        with(as.list(c(state, pars)), {
         .
         .
         .
            return(list(c(St, Xt, Pt)))

        })

    }

    return(as.data.frame(ode(y = state, times = times, func = derivs, parms = pars)))  
}


for(i in seq(ti, tf-span, span)) {

     times <- seq(i, i+span, delta)  

     out   <- SOLVE(pars) 

     state <- c(state[1], tail(out[3], 1), tail(out[4], 1))

 }

The error i get is:

Error in checkInput(y, times, func, rtol, atol, jacfunc, tcrit, hmin, : `y' must be numeric

Answer 1

Too long for a comment. Try:

state <- c(state[1], tail(out[[3]], 1), tail(out[[4]], 1))

The way you have it set up, out is a data.frame. out[3] returns a data.frame with one column. tail(out[3],1) returns a data.frame with one row and one column (but still a data.frame). When you combine these using, eg,

c(state[1],tail(out[3],1),...)

you generate a list , not a numeric, which is causing the error.

If you reference, eg, out[[3]] you return a vector which elements are the third column of out . This is because, in R, a data.frame is by definition a list of vectors, so out[[3]] returns the third element (vector) in that list.