Re: [R] Automatic formula creation

[Alex van der Spek]

Thanks much to both Peter and Rui,

I am afraid that after 5 years with R I am still not able to understand 
your method Peter. Will stick with Rui's method for now...

Alex
On 08/09/2013 04:00 PM, peter dalgaard wrote:
On Aug 9, 2013, at 13:26 , Rui Barradas wrote:

Hello,

Maybe the following gives you some idea on how to vary the terms.

idx <- 1:5  # or any other indexes
ftext <- paste(terms[idx], collapse = ' * ')

You're not the first to use this sort of technique - it is happening in various parts of 
R's own internals too, but handling R expressions via their textual representation is 
really not a good principle (see fortune("rethink")) and it _does_ give rise to 
problems.

I much prefer techniques like this:

nm <- lapply(letters[1:6], as.name)
Reduce(function(a,b) bquote(.(a)*.(b)), nm)
a * b * c * d * e * f


Similarly, use

trm <- Reduce(function(a,b) bquote(.(a)*.(b)), nm)
formula(bquote(I(1 - Pass149) ~  .(trm) - 1))
I(1 - Pass149) ~ a * b * c * d * e * f - 1



Hope this helps,

Rui Barradas


Em 09-08-2013 11:40, Alex van der Spek escreveu:
Say I want to compare all 5 term models from a choice of 28 different
predictors and one known. All possible combinations of 5 out of 28 is
easy to form by combn(). With some string manipulation it is also easy
to make a text representation of a formula which is easy to convert by
as.formula() for use in lm().

The primitive part however is pasting together the terms which I do
explicitly for 5 terms, like so:


     ftext <- paste(terms[1], terms[2], terms[3], terms[4], terms[5],
sep = ' * ')


Works but is not great as I now need to edit this formula when the
number of terms changes. There ought to be a better way but I can't find
it.

Any help much appreciated! The full block of relevant code follows:
Alex van der Spek

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++


#Try all 3 band models
nbands <- 5
freqs <- c('4', '5', '6_3', '8', '10', '12_7', '16', '20', '25', '32',
'40', '51', '64', '81', '102', '128',
            '161', '203', '256', '323', '406', '512', '645', '813',
'1024', '1290', '1625', '2048')
bands <- paste(rep('kHz', 28), freqs, rep('_ave', 28), sep = '')
nc <- choose(28, nbands)
combs <- t(combn(bands, nbands))

models <- vector("list", nc)
for (ic in 1:nc) {
     terms <- c()
     for (jc in 1:nbands) {
         t <- paste('log10(', combs[ic, jc], ')', sep = '')
         terms <- append(terms, t)
     }

     ftext <- paste(terms[1], terms[2], terms[3], terms[4], terms[5],
sep = ' * ')

     ftext <- paste('I(1 - Pass149) ~ ', ftext, ' - 1', sep = '')
     forml <- as.formula(ftext)

     plus100.lm <- lm(forml, data = sd, subset = Use == 'Cal')
     plus100.sm <- step(plus100.lm, trace = 0)
}

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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.