Re: [R] How to construct a formula

[Hugh Morgan]

Thank you very much, that is exactly what I needed.

Hugh

On 01/22/2012 03:38 PM, Duncan Murdoch wrote:

On 12-01-22 9:27 AM, Hugh Morgan wrote:

Hi,

I need to construct a formula programaticly, and pass it to a function
such as the linear mixed model lme.  The help says it requires "a
two-sided linear formula object describing the fixed-effects part of the
model" but I do not know how to create this formula.  I have tried
various things using formula(x, ...), as.formula(object, env =
parent.frame()) and as.Formula(x, ...) but I cannot get it to work.  Can
anyone give me any pointers?

What I want to do is pass particular columns to my mixed model depending
on the result of a previous test (anova) in a programatic way.  I could
use hard coded if statements as in the example below, but I want to be
able to do this in a more programatic way.  I hope the code below will
show what I am trying to do:

Current code:

data=read.csv("dataMini.csv", header=TRUE, sep=",", dec=".")
colnames(data)
library(nlme)

if(weight_significant) {
      if(gender_significant) {
          if(weight_gender_interaction_significant) {
              model=lme(test_variable~Genotype + Weight + Gender +
Weight*Gender, random=~1|Assay.Date, data, na.action="na.exclude",
method="REML")
          } else {
              model=lme(test_variable~Genotype + Weight + Gender,
random=~1|Assay.Date, data, na.action="na.exclude", method="REML")
          }
      } else {
.... etc

What I want to do:

formulaObject = test_variable~Genotype
if(weight_significant) {
      formulaObject = formulaObject + Weight;
}
if(gender_significant) {
      formulaObject = formulaObject +  Gender;
}
if(weight_gender_interaction_significant) {
      formulaObject = formulaObject +  Weight*Gender;
}

I think you don't really want that; you want to add those terms to the right hand side of the formula.

This function would do that:

addterm <- function(fla, term) {
  fla[[3]] <- call("+", fla[[3]], substitute(term))
  fla
}

For example,

> formulaObject
test_variable ~ Genotype
> addterm(formulaObject, Weight)
test_variable ~ Genotype + Weight

Duncan Murdoch

model=lme(formulaObject, random=~1|Assay.Date, data,
na.action="na.exclude", method="REML")

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