R help - Oct 2010 - All other variables in upper scope arg for stepAIC

Hi - I am trying to substitute for "the_other_y" in the code below.  I
want
y2 and y3 to be there when i is 1, y1 and y3 to be there when i is 2 and y1
and y2 to be there when i is 3.  I'm sure it's to do with what format
the
data should be in and I've tried alldata[,-i], but it fits all the columns
of alldata except i rather than each column one at a time.  I've tried
changing the data i.e. as.matrix or as.list but that's still not the right
format.  I'm sure it's a simple trick, but don't know how to search
it out.
Any help on how to search this as well as the solution would be greatly
appreciated.

y1<-rnorm(20);y2<-rnorm(20);y3<-rnorm(20);alldata<-cbind(y1,y2,y3)

library(MASS)
for (i in 1:3)
{mod<-lm(get(paste("y", i, sep=""))~1)
stepAIC(mod, scope=list(lower=~1, upper=~the_other_y))}

Thanks

Paul

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On Oct 21, 2010, at 4:21 AM, Paul Chatfield wrote:
> Hi - I am trying to substitute for "the_other_y" in the code
below.
> I want
> y2 and y3 to be there when i is 1, y1 and y3 to be there when i is 2  
> and y1
> and y2 to be there when i is 3.  I'm sure it's to do with what  
> format the
> data should be in
No. It's because you expected R to read your mind about creating the  
formula. Fortunately the stepAIC function was designed for taking a  
character value and turning it into a formula object, or we would also  
have needed to use as.formula to create a language object.
> and I've tried alldata[,-i],
Actually all you did was create that matrix but then you never used  
it. the lm() function can accept a data= argument but you never used  
it, so all the references are to the naked y's.
> but it fits all the columns
> of alldata except i rather than each column one at a time.  I've tried
> changing the data i.e. as.matrix or as.list but that's still not the  
> right
> format.  I'm sure it's a simple trick, but don't know how to
search
> it out.
> Any help on how to search this as well as the solution would be  
> greatly
> appreciated.
>
>
y1<-rnorm(20);y2<-rnorm(20);y3<-rnorm(20);alldata<-cbind(y1,y2,y3)
>
> library(MASS)
> for (i in 1:3)
> {mod<-lm(get(paste("y", i, sep=""))~1)
> stepAIC(mod, scope=list(lower=~1, upper=~the_other_y))}
 > library(MASS)
 > for (i in 1:3) { the_other_y <- paste("~",
paste("y",(1:3)[-i],
sep="", collapse="+"))
+   mod<-lm(get(paste("y", i, sep=""))~1)
+   stepAIC(mod, scope=list(lower=~1, upper= the_other_y))}
Start:  AIC=-5.21
get(paste("y", i, sep = "")) ~ 1

        Df Sum of Sq    RSS     AIC
<none>              13.948 -5.2075
+ y3    1  0.166746 13.782 -3.4480
+ y2    1  0.097947 13.850 -3.3484
Start:  AIC=3.24
get(paste("y", i, sep = "")) ~ 1

        Df Sum of Sq    RSS    AIC
<none>              21.276 3.2365
+ y1    1  0.149400 21.126 5.0956
+ y3    1  0.025849 21.250 5.2122
Start:  AIC=1
get(paste("y", i, sep = "")) ~ 1

        Df Sum of Sq    RSS    AIC
<none>              19.027 1.0023
+ y1    1  0.227457 18.799 2.7618
+ y2    1  0.023117 19.004 2.9780
>
-- 

David Winsemius, MD
West Hartford, CT