[R] Error in calling stepAIC() from within a function

[Siew Leng TENG]

Hi,
 
 I am experiencing a baffling behaviour of stepAIC(),
 and I hope to get any advice/help on what went wrong
or I'd missed. I greatly appreciate any advice given.
 
 I am using stepAIC() to, say, select a model via
 stepwise selection method.
 
 R Version : 1.7.1
 Windows ME
 
 Many thanks and best regards,
Siew-Leng
 
 
 ***Issue :
 
 When stepAIC() is placed within a function, it seems
 that stepAIC() cannot detect the data matrix, and
 the
 program is halted as a result. However, when the
 same
 codes are copied, paste and run in R workspace,
 stepAIC can execute and R is able to produce the
 desired output.
 
 ***Actions taken :
 I had tried to look into and manipulate environments
 and formals(f), but with not much luck.
 
 
 ***Code snippets (an example):
 
   library(MASS)
   library(nls)
 
   Data<-data.frame(matrix(rnorm(120),ncol=6))
   colnames(Data)<-c("Y",paste("X",1:5,sep=""))
 
   f<-function(A)
   {
   a <-glm(Y~X1+X2+X3, data=A)
   b

<-stepAIC(a,scope=list(upper=~X1+X2+X3+X4+X5,lower=~1),direction="both",trace=FALSE)
   b
   }
 

  f(Data) 
 
 
 R gives the following error :
 
 > f(Data)
 Error in model.frame.default(formula = Y ~ X1 + X3,
 data = A, drop.unused.levels = TRUE) : 
         Object "A" not found
 
 
 However, when the same codes are copied, paste and
 run
 in the workspace, stepAIC() can run, and R is able
 to
 produce the desired output :
 
 A <- Data
 a <-glm(Y~X1+X2+X3, data=A)
 b

<-stepAIC(a,scope=list(upper=~X1+X2+X3+X4+X5,lower=~1),direction="both",trace=FALSE)
 b
 
> b
 
> Call:  glm(formula = Y ~ X4, data = A) 
> 
> Coefficients:
> (Intercept)           X4  
>      0.1942      -0.3314  
> 
> Degrees of Freedom: 19 Total (i.e. Null);  18
> Residual
> Null Deviance:      20.51 
> Residual Deviance: 17.95        AIC: 60.6 
> > 


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