>>>>> "TobiasV" == Tobias Verbeke <[EMAIL PROTECTED]> >>>>> on Sun, 17 Dec 2006 22:34:50 +0100 writes:
TobiasV> Martin Maechler wrote: >> Yes, I think all your propositions would be improvements. >> OTOH, I don't think the improvements warrant a big >> increase in code (complexity), nor do I think the >> improvements are crucial for R's integrity. >> >> So... If you (our someone else) provides a patch >> {against R-devel, as always} which keeps the code simple, >> I'd strongly consider adding that to R. >> TobiasV> Dear Martin, TobiasV> Thank you for the encouragement. (maybe I shouldn't have done that, see below ...) TobiasV> Please find a patch in attachment. As you will TobiasV> see, I also added a warning for models with more TobiasV> than 2 parameters. the latter is a good idea [and I wonder if we should not even signal an error ..] but for the patch, see below. TobiasV> Kind regards, Tobias >>>>>>> "Tobias" == Tobias Verbeke >>>>>>> <[EMAIL PROTECTED]> on Sat, 09 Dec 2006 >>>>>>> 22:42:38 +0100 writes: >>>>>>> >> Tobias> The abline function can be used to draw the Tobias> regression line when one passes the lm object as an Tobias> argument. >> Tobias> However, if it's an intercept-only model, it appears Tobias> to use the intercept as the slope of the abline: >> Tobias> mod <- lm(dist ~ 1, data = cars) Tobias> plot(dist ~ speed, data = cars) Tobias> abline(reg = mod) # nothing appears Tobias> This behaves as documented, but might catch Tobias> someone. Would it be an improvement if this Tobias> situation was detected so as to plot the appropriate Tobias> horizontal line, i.e. Tobias> abline(a = coef(mod), b = 0) ? Tobias> Would it also be an improvement if for a model like Tobias> mod2 <- lm(dist ~ 1 + offset(speed), data = cars) Tobias> abline(reg = mod2) would be equivalent to Tobias> abline(a = coef(mod2), b = 1) ? Tobias> For models through the origin, the current function Tobias> works fine, but one might even consider models Tobias> through the origin and having the independent Tobias> variable in an offset() to be fully fool-proof, i.e. Tobias> abline(a = 0, b = 1) Tobias> Kind regards, Tobias Comments on your patch: 11,12c11,15 < temp <- as.vector(coef(a)) < if(length(temp) == 1) { --- > tempa <- as.character(terms(a)) > coefa <- coef(a) > if (length(coefa) > 2) > warning("The abline function is meant to only plot the regression line for simple linear regression") > if (length(grep("-[[:blank:]]?1", tempa))) { 14c17 < b <- temp --- > b <- ifelse(length(grep("offset", tempa)), 1, coefa[1]) yet another misuse of ifelse() .... {as mentioned recently}. Do note that if(Cond) A else B is quite better than ifelse(Cond, A, B) in these scalar situations. 16,17c19,22 < a <- temp[1] < b <- temp[2] --- > a <- coefa[1] > if (length(grep("offset", tempa))) b <- 1 > else if (length(coefa) >= 2) b <- coefa[2] > else b <- 0 -- -- Hmm, grep()ing in as.character(terms(.)) is not really elegant though we could well live with that if .... The real problems are 1. your offset dealing cannot work in generality {and I should really have *dis*couraged you earlier on this}, since a) lm() can have an 'offset' *argument* instead of or in addition to the formula offset() component. b) instead of offset(speed) one could have had offset(speed^2) or offset(sqrt(dist)) or "anything". Because of the above, it really doesn't make sense trying to figure out offset corrections, at least not in abline()'s code, I think. 2. To catch the "horizontal line" situation, it's also not sufficient to grep for "-1" (basically); since, e.g., "0+" has been available to achieve the same and has actually been preferred by some. OTOH, there is a correct way of asking if there was an intercept [attr(terms(.), "intercept")], and I plan to use that in abline. Regards, Martin Maechler, ETH Zurich ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel