[R] Automatic formula creation

[Rui Barradas]

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 = ' * ')


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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