constrOptim and further arguments

Dear List,

I have the code below, where I am using the constrained optimisation
package, 'constrOptim.nl' to find the values of two values, b0 and b1.

I have no problems when I enter further variable information DIRECTLY into
the functions, fn, and heq. In this instance I require fn to have -0.0075
appended to it, and in the case of heq, h[1] has -0.2.


library(alabama)

fn<-function(x)
(((1/(1+exp(-x[1]+x[2]))+(1/(1+exp(-x[1])))+(1/(1+exp(-x[1]-x[2])))))/3)-0.0075

heq<- function(x) {
h<- rep(NA,2)
h[1]<- (1-(1/(1+exp(-x[1]))/(1/(1+exp(-x[1]-x[2])))))-0.2    #Constraint
1: diff = 20%
h[2]<- -log((1-0.0075)/0.0075)-x[1]                #Constraint 2: set
b0
h
}

p0<-c(-4,0.3)
ans<-constrOptim.nl(par=p0,fn=fn,heq=heq)

This code does indeed work as expected and the results have been verified
elsewhere.

However, I do have problems when I wish to enter this information
indirectly via the constrOptim function, e.g.

fn<-function(x,RR)
(((1/(1+exp(-x[1]+x[2]))+(1/(1+exp(-x[1])))+(1/(1+exp(-x[1]-x[2])))))/3)-RR

heq<- function(x,Diff) {
h<- rep(NA,2)
h[1]<- (1-(1/(1+exp(-x[1]))/(1/(1+exp(-x[1]-x[2])))))-Diff    #Constraint
1: diff = 20%
h[2]<- -log((1-RR)/RR)-x[1]
#Constraint 2: set b0
h
}

p0<-c(-4,0.3)
ans<-constrOptim.nl(par=p0,fn=fn,heq=heq, RR=0.0075, Diff=0.2)


In this instance I get the following error,

'Error in heq(theta, ...) : unused argument(s) (RR = 0.0075)'

I do not appear to be able to input further arguments in this manner, any
help from the list would be gratefully received.

Regards

Mike Griffiths



On Tue, Dec 20, 2011 at 11:53 AM, Michael Griffiths<
griffiths at upstreamsystems.com>  wrote:

Dear List,

I am using constrOptim to solve the following

fr1<- function(x) {
     b0<- x[1]
     b1<- x[2]
     ((1/(1+exp(-b0+b1))+(1/(1+exp(-b0)))+(1/(1+exp(-b0-b1)))))/3
}

As you can see, my objective function is
((1/(1+exp(-b0+b1))+(1/(1+exp(-b0)))+(1/(1+exp(-b0-b1)))))/3 and I would
like to solve for both b0 and b1.

If I were to use optim then I would derive the gradient of the function
(grr) as follows:

fr2<-
expression(((1/(1+exp(-b0+b1))+(1/(1+exp(-b0)))+(1/(1+exp(-b0-b1)))))/3)
grr<- deriv(fr2,c("b0","b1"), func=TRUE)

and then simply use optim via

optim(c(-5.2,0.22), fr1, grr)

My problem is that I wish to place constraints (b0>=-0.2 and b1>= 0.1)
upon the values of b0 and b1. I can set the constraints matrix and boundary
values to

ui=rbind(c(1,0),c(0,1)) and ci=c(-0.2,0.1), however, when I come to run
constrOptim function via


constrOptim(c(-0.1,0.2), fr1, grr, ui=rbind(c(1,0),c(0,1)), ci=c(-0.2,0.1))

I get the following error message:

"Error in .expr1 + b1 : 'b1' is missing"

So, it seems to me that I am doing something incorrectly in my
specification of grr in constrOptim.

If the list could help, I would be most grateful.

Regards

Mike Griffiths



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