aggregate.formula: formula from string

-----Original Message-----
From: arun [mailto:smartpink111 at yahoo.com]
Sent: Mittwoch, 31. Oktober 2012 13:15
To: Thaler,Thorn,LAUSANNE,Applied Mathematics
Cc: R help
Subject: Re: [R] aggregate.formula: formula from string

Hi,
Try this:
res<- aggregate(eval(mF),d,mean)

res

#? a b????????? NA????????? NA
#1 1 A -1.48354978 -0.37141485
#2 2 A -0.08862713? 0.35359250
#3 3 A? 1.17519518 -0.47595290
#4 1 B? 0.10214686 -0.70005131
#5 2 B? 0.41185154? 0.03707291
#6 3 B? 0.20507062 -0.67946389


res1<-aggregate(cbind(y, z) ~ a + b, d, mean)
colnames(res)[3:4]<-colnames(res1)[3:4]
?identical(res,res1)
#[1] TRUE
A.K.




----- Original Message -----
From: "Thaler,Thorn,LAUSANNE,Applied Mathematics"
<Thorn.Thaler at rdls.nestle.com>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc:
Sent: Wednesday, October 31, 2012 5:46 AM
Subject: [R] aggregate.formula: formula from string

Dear all,

I want to use aggregate.formula to conveniently summarize a data.frame. I have
quiet some variables in the data.frame and thus I don't want to write all
these names by hand, but instead create them on the fly. This approach has the
advantage that if there will be even more columns in the data.frame I don't
have to change the code.

I've hence tried to construct a formula object and to pass that to aggregate:

d <- expand.grid(a = factor(1:3), b = factor(LETTERS[1:2]))
d <- rbind(d,d,d)
d$y <- rnorm(18)
d$z <- rnorm(18)
mF <- as.formula(paste("cbind(", paste(names(d)[-(1:2)], collapse = ","), ") ~
a + b", sep = ""))

But if I try to pass that formula to aggregate

aggregate(mF, d, mean)

I get the following error:

Error in m[[2L]][[2L]] : object of type 'symbol' is not subsettable

But if I pass the formula directly:

aggregate(cbind(y, z) ~ a + b, d, mean)

Everything is working as expected.

So I was wondering what went wrong? I know I could use a formula like . ~ a +
b instead and this would work fine, but I'm just interested in why the
outlined approach does not work as expected, and where my mistake lies? (that
means in particular I am not asking for a solution of how to get the thing
done - there are plenty of alternatives - but instead to understand why this
very approach does not work)

Thanks for your help!

Kind Regards,

Thorn Thaler
Mathematician

Applied Mathematics
Nestec Ltd,
Nestl? Research Center
PO Box 44
CH-1000 Lausanne 26
Phone: +41 21 785 8220
Fax: +41 21 785 9486