[R-sig-eco] Function in Genetic Algorithms

[Daniel Patón]

Dear colleagues:

I'm trying to fit a model by Genetic Algorithms. In the GAFIT package the expression I use for fitting is:

fitness <- expression((Z - (X^a + Y^b))^2)

Y a and b are coefficients of the variables X and Y. Z is the output variable.

When I try to use another package (rgenoud, GA, etc...) I must put the syntax in function form. I use this:

fitness <- function(X,Y,Z) { 
Z <- trees$BIOMASS
X <- trees$DIAMETER
Y <- trees$HEIGHT
observed <- (X + X^2 + Y + Y^2 + X*Y)
expected <- Z
error <- (observed-expected)^2
return(error)
}

I don't know what I'm doing wrong. 

I'd appreciate a little help.
Thank you

-- 
****************************************************************************
 Daniel Patón
 Numerical Ecology. Ecology Unit
 Department of Plant Biology, Ecology and Earth Sciences
 Faculty of Sciences. University of Extremadura
 Avda. Elvas s/n 06071 Badajoz (Spain)
 https://orcid.org/0000-0003-2500-3964
 https://cvn.fecyt.es/0000-0003-2500-3964
 http://unex.academia.edu/DanielPatonDominguez
 https://www.researchgate.net/profile/Daniel_Paton/
 http://sites.google.com/site/numericalecologyuex/home
 http://twitter.com/NumericalEcolo1
 https://www.youtube.com/channel/UCcDUrzoZQ5OID6uVxhcwBng