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### Introduction

This post is a simple introduction to Rcpp for disease ecologists,
epidemiologists, or dynamical systems modelers – the sorts of folks who will
benefit from a simple but fully-working example. My intent is to provide a
complete, self-contained introduction to modeling with Rcpp. My hope is that
this model can be easily modified to run any dynamical simulation that has
dependence on the previous time step (and can therefore not be vectorized).

This post uses a classic Susceptible-Infected-Recovered (SIR)
epidemiological compartment
model
.
Compartment models are simple, commonly-used dynamical systems models. Here
I demonstrate the tau-leap
method
, where a discrete number
of individuals move probabilistically between compartments at fixed intervals
in time. In this model, the wait-times within class are exponentially
distributed, and the number of transitions between states in a fixed time
step are Poisson distributed.

This model is parameterized for the spread of measles in a closed population,
where the birth rate (nu) = death rate (mu). The transmission rate (beta)
describes how frequently susceptible (S) and infected (I) individuals come
into contact, and the recovery rate (gamma) describes the the average time an
individual spends infected before recovering.

### C++ Code

Note: C++ Functions must be marked with the following comment for use in
R: // [[Rcpp::export]].

When functions are exported in this way via sourceCpp(), RNG setup is
automatically handled to use R’s engine. For details on random number
generation with Rcpp, see the this Rcpp Gallery
post
.

#include
using namespace Rcpp;

// This function will be used in R! Evaluates the number of events
// and updates the states at each time step
//
// [[Rcpp::export]]
List tauleapCpp(List params) {

// chained operations are tricky in cpp
// pull out list w/in list into its own object
List init = params["init"];

// use Rcpp as() function to "cast" R vector to cpp scalar
int nsteps = as(params["nsteps"]);

// initialize each state vector in its own vector
// set all vals to initial vals
//
// I use doubles (NumericVector) rather than
// ints (IntegerVector), since rpois returns double,
// and the domain of double is a superset of int
NumericVector SS(nsteps, init["S"]);
NumericVector II(nsteps, init["I"]);
NumericVector RR(nsteps, init["R"]);
NumericVector NN(nsteps, init["pop"]);

// fill time w/zeros
NumericVector time(nsteps);

// pull out params for easy reading
double nu = params["nu"];
double mu = params["mu"];
double beta = params["beta"];
double gamma = params["gamma"];
double tau = params["tau"];

// Calculate the number of events for each step, update state vectors
for (int istep = 0; istep < (nsteps-1); istep++) {

// pull out this step's scalars for easier reading
// and to avoid compiler headaches
double iS = SS[istep];
double iI = II[istep];
double iR = RR[istep];
double iN = NN[istep];

/////////////////////////
// State Equations
/////////////////////////

// R::rpois always returns a single value
// to return multiple (e.g. Integer/NumericVector,
// use Rcpp::rpois(int ndraw, param) and friends
double births = R::rpois(nu*iN*tau);

// Prevent negative states
double Sdeaths = std::min(iS, R::rpois(mu*iS*tau));
double maxtrans = R::rpois(beta*(iI/iN)*iS*tau);
double transmission = std::min(iS-Sdeaths, maxtrans);
double Ideaths = std::min(iI, R::rpois(mu*iI*tau));
double recovery = std::min(iI-Ideaths, R::rpois(gamma*iI*tau));
double Rdeaths = std::min(iR, R::rpois(mu*iR*tau));

// Calculate the change in each state variable
double dS = births-Sdeaths-transmission;
double dI = transmission-Ideaths-recovery;
double dR = recovery-Rdeaths;

// Update next timestep
SS[istep+1] = iS + dS;
II[istep+1] = iI + dI;
RR[istep+1] = iR + dR;

// Sum population
NN[istep+1] = iS + iI + iR + dS + dI + dR;

// time in fractional years (ie units parameters are given in)
time[istep+1] = (istep+1)*tau;
}

// Return results as data.frame
DataFrame sim = DataFrame::create(
Named("time") = time,
Named("S") = SS,
Named("I") = II,
Named("R") = RR,
Named("N") = NN
);
return sim;
};

### R Code

Next we need to parameterize the model. Modelers often deal with many named
parameters, some of which are dependent on each other. My goal here is to
specify parameters in R once (and only once), and then pass all of them
together to the main cpp function.

## Specify model parameters use within() to make assignments *inside* an
## empty (or existing) list. Yhis is a handy R trick that allows you to
## refer to existing list elements on right hand side (RHS)
##
## Note the braces, <-, and and no commas here:  everything in braces is a
## regular code block, except that assignments happen *inside* the list
params <- list()
params <- within(params, {

## set rng state
seed <- 0
tau <- 0.001 # in years
nyears <- 10

## total number of steps
nsteps <- nyears/tau

mu <- 1/70 #death rate
gamma <- 365/10 #recovery rate
R0 <- 10
## refers to R0 above
beta <- R0*(gamma+mu) #transmission rate
nu <- mu #birth rate

## initial conditions, list within list
## use within() to modify empty list, as above
init <- within(list(), {
pop <- 1e6
S <- round(pop/R0)
I <- round(pop*mu*(1-1/R0)/(gamma+mu))
## refers to S,I above
R <- pop-S-I
})
})

set.seed(params$seed) ## run the model once result.df <- tauleapCpp(params) library(plyr) nsim <- 12 ## run many sims, combine all results into one data.frame ## plyr will combine results for us result.rep <- ldply(1:nsim, function(.nn) { set.seed(.nn) ## run the model result <- tauleapCpp(params) ## this wastes space, but is very simple and aids plotting result$nsim <- .nn
return(result)
})

### Plot Results

Note that the model contains no seasonality. Rather, the system experiences
stochastic resonance,
where the “noise” of stochastic state transitions stimulates a resonant
here.

Sometimes epidemics die out. In fact, for this model, they will die out with
probability = 1 as time goes to infinity!

library(lattice)

## lattice plot of results
plot(
xyplot(I ~ time | sprintf("Simulation %02d",nsim),
data=result.rep, type=c('l','g'), as.table=T,
ylab='Infected', xlab='Year',
scales=list(y=list(alternating=F))
)
)