[R] aggregate.formula: formula from string

[Thaler,Thorn,LAUSANNE,Applied Mathematics]

Dear Arun,

Thanks for your suggestion, that does the trick. Just because I'm curious, where does the problem come from? I figured that wrapping the formula object in brackets would work as well:

aggregate((mF), d, mean)

So I guess it has something to do with the scope of mF, or what is the root cause?

Thanks for your help!


KR,

-Thorn

> -----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,
> Nestlé Research Center
> PO Box 44
> CH-1000 Lausanne 26
> Phone: +41 21 785 8220
> Fax: +41 21 785 9486
> 
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