henry R henry R - 1 year ago 124
R Question

Solving ODE with deSolve in R- number of derivatives error

Hey I am trying to use the deSolve package for a set of ODE's with equations as auxillary variables. I keep getting the error where the number of derivatives isn't the same length as the initial conditions vector. What should I change?


exponential=function(t,state,parameters){ with(as.list( c(state,parameters)), {

#Aux. Var.

fX2 = pmax(0,1-(1-(d2/r12)*(X2/K2)))
fX1 = X1/(X1+k1);

# equations (ODE)

dX1 = C-((d1)*(X1))-(r12)*(X2)*fX2*fX1 # differential equaion
dX2 = r12*(X2)*fX2*fX1-((d2)*(X2))

return(list(c(dX1, dX2)))



# Set Initial Values and Simulation Time
state = c(X1=2,X2=0.01,K2= 10)

# Assign Parameter Values
parameters = c(d1=0.001, d2=0.008, r12=0.3,C=0.5,k1= 0.001)

for (i in 1:length(times)){
out= ode(y=state,times=times,func=exponential,parms=parameters)

**> for (i in 1:length(times)){
+ out= ode(y=state,times=times,func=exponential,parms=parameters)
+ }
Error in checkFunc(Func2, times, y, rho) :
The number of derivatives returned by func() (2) must equal the length of the initial conditions vector (3)**

Answer Source

The error comes from the return in your defined function: Your input parameter y has length 3, but you only return 2 values back, that's the error. You can solve your problem with

return(list(c(X1, X2, K2)))

Another possibility is to take K2 to the parameters, then your old return was right. You have to decide if K2 is a variable or a parameter.

And BTW: Why a for loop with the time? In my opinion that is not necessary, because the ODEs are solved in the timeinterval you submitted to the odefunction.

out= ode(y=state,times=times,func=exponential,parms=parameters)
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