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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