Dots in models formulae
On Tue, 16 Aug 2005, Sundar Dorai-Raj wrote:
Uwe Ligges wrote:
Laurent Valdes wrote:
I have seen, several times, dots (like this: "y ~." ) in formula descriptions, noticeably in R help. I am unable to see what it does correspond to. Any ideas ?
All other variables (except y) from the given data.frame... Uwe Ligges
Hi, Uwe, Doesn't this depend on the context? For example, z <- data.frame(y = rnorm(10), x = rnorm(10)) fit <- lm(y ~ ., z) update(fit, y ~ . + I(x^2)) The original poster did not say where he saw this formula. However, I think the reference in ?formula has the most authorative explanation.
(It does not cover this, as it is part of the interpretation of a formula.) Yes, it must depend on context, as an R function can do anything it likes with a formula (including making y ~ x mean the regression of x on y). If terms.formula() is used, y ~ . means what Uwe said, _if_ there is a 'data' argument. However, if not it has its literal meaning (a variable named '.'), at least until 2.2.0.
Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595