[R] Scoping rule problem?!? step does not work in function

[Winfried Theis]

Hello!

I would like to use step() in a function because I have a list of responses
(independent!) with equal influence and want to apply stepwise regression on
each of them. So the function I just tested is the following:
fitlms <- function(y,infl,formel=NULL){
       if(is.null(formel)){
         var.names <- names(infl)
         dcv <- length(var.names)
         formel <- paste("y~",paste(rep("I(",dcv), var.names,
rep("^2)",dcv),sep="",collapse="+"),"+",paste(var.names,collapse="*"),sep="")
         formel <- as.formula(formel)
       }
       erg <- lm(formel,infl)
       erg <- step(erg)
       return(erg)}

Using this I get the following:

> test <- fitlms(grob.erg[,1],grob.erg[,3:5])
Start:  AIC= -30.84 
 y ~ I(f^2) + I(vS^2) + I(o^2) + f + vS + o + f:vS + f:o + vS:o +  
    f:vS:o 

          Df Sum of Sq     RSS     AIC
- f:vS:o   1     0.060   2.052 -32.188
<none>                   1.992 -30.842
- I(o^2)   1     0.762   2.754 -25.717
- I(f^2)   1     2.581   4.573 -14.558
- I(vS^2)  1     5.341   7.333  -4.170
Error in model.frame.default(formula = y ~ I(f^2) + I(vS^2) + I(o^2) +  : 
        Object "infl" not found


So the first step carried out correctly and then it seems to look into the
wrong environment (presumably the top-level). I checked with the code of step,
but could not spot the cause of this behaviour. At some place it looks
at the parent.frame, which should be the environment created by function fitlms
if I do unserstand things correctly. I checked also whether I can give step an
environment in which to operate, but could not find any such option.

I presume I could just use a for-loop, which might not be much slower than
lapply in this case. I simply would like to understand things better to avoid
similar errors.

Data is given below.

Thanks for your attention and any help appreciated,

Winfried




> grob.erg
           gwRZ      gwRA     f      vS       o
WV 1   3.562667 0.5742667 0.185  90.000 300.000
WV 2   3.585733 0.6028333 0.185  90.000 300.000
WV 3   3.893233 0.7162333 0.185  90.000 300.000
WV 4   4.035933 0.7603667 0.139 111.213 370.711
WV 5   2.250800 0.3411000 0.120  90.000 300.000
WV 6   4.342833 0.8782667 0.231 111.213 370.711
WV 7   2.537000 0.4682333 0.231  68.787 370.711
WV 8   3.058767 0.4457333 0.185  90.000 400.000
WV 9   3.613333 0.6000000 0.139  68.787 370.711
WV 10  4.757700 1.0705333 0.231 111.213 229.289
WV 11  4.491800 0.9555000 0.185  60.000 300.000
WV 12  3.316133 0.5622333 0.139 111.213 229.289
WV 13  3.364567 0.5709333 0.139  68.787 229.289
WV 14  3.834867 0.6982333 0.185  90.000 200.000
WV 15  4.116233 0.6855000 0.231  68.787 229.289
WV 16  3.769800 0.7355333 0.185  90.000 300.000
WV 17  6.631733 1.6468333 0.185 120.000 300.000
WV 18  4.282933 0.7365333 0.185  90.000 300.000
WV 19  3.784633 0.7029000 0.185  90.000 300.000
WV 20  3.669567 0.6854000 0.250  90.000 300.000
WV 21  4.548433 0.7231000 0.185  90.000 300.000
WV 21a 3.387467 0.5558667 0.185  90.000 300.000
 

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E-Mail: Winfried Theis <theis at statistik.uni-dortmund.de>
Date: 29-Jan-03

Dipl.-Math. Winfried Theis
SFB 475, Fachbereich Statistik, Universit"at Dortmund, 44221 Dortmund
Tel.: +49-231-755-5903 FAX: +49-231-755-4387
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