smfsb (version 1.5)

simpleEuler: Simulate a sample path from an ODE model

Description

This function integrates an Ordinary Differential Equation (ODE) model using a simple first order Euler method. The function is pedagogic and not intended for serious use. See the deSolve package for better, more robust ODE solvers.

Usage

simpleEuler(t=50, dt=0.001, fun, ic, ...)

Value

An R ts object containing the sampled path of the model.

Arguments

t

The length of the time interval over which the ODE model is to be integrated. Defaults to 50 time units.

dt

The step size to be used both for the time step of the Euler integration method and the recording interval for the output. It would probably be better to have separate parameters for these two things (see StepEuler and simTs). Defaults to 0.01 time units.

fun

A vector-valued function representing the right hand side of the ODE model. The first argument is a vector representing the current state of the model, x. The second argument of fun is the current simulation time, t. In the case of a homogeneous ODE model, this argument will be unused within the function. The function may have additional arguments, representing model parameters. The output of fun should be a vector of the same dimension as x.

ic

The initial conditions for the ODE model. This should be a vector of the same dimensions as the output from fun, and the second argument of fun.

...

Additional arguments will be passed into fun.

See Also

rdiff, ts, StepEuler, simTs

Examples

# simple Lotka-Volterra example
lv <- function(x,t,k=c(k1=1,k2=0.1,k3=0.1))
{
        with(as.list(c(x,k)),{
                c( k1*x1 - k2*x1*x2 ,
                      k2*x1*x2 - k3*x2 )
        })
}
plot(simpleEuler(t=100,fun=lv,ic=c(x1=4,x2=10)),plot.type="single",lty=1:2)

# now an example which instead uses deSolve...
require(deSolve)
times = seq(0,50,by=0.01)
k = c(k1=1,k2=0.1,k3=0.1)
lvlist = function(t,x,k)
        list(lv(x,t,k))
plot(ode(y=c(x1=4,x2=10),times=times,func=lvlist,parms=k))