[Rd] Potential bug in fitted.nls

[Bill Dunlap]

Doesn't nls() expect that the lengths of vectors on both sides of the
formula match (if both are supplied)?  Perhaps it should check for that.

-Bill

On Thu, Jan 26, 2023 at 12:17 AM Dave Armstrong <darmst46 using uwo.ca> wrote:

> Dear Colleagues,
>
> I recently answered [this question]() on StackOverflow that identified
> what seems to be unusual behaviour with `stats:::nls.fitted()`. In
> particular, a null model returns a single fitted value rather than a
> vector of the same fitted value of `length(y)`.  The documentation
> doesn’t make it seem like this is the intended behaviour, so I’m not
> sure if it’s a bug, a “Wishlist” item or something that is working
> as intended even though it seems unusual to me.  I looked through the
> bug reporting page on the R project website and it suggested contacting
> the R-devel list in cases where the behaviour is not obviously a bug to
> see whether others find the behaviour equally unusual and I should
> submit a Wishlist item through Bugzilla.
>
> Below is a reprex that shows how the fitted values of a model with just
> a single parameter is length 1, but if I multiply that constant by a
> vector of ones, then the fitted values are of `length(y)`.  Is this
> something that should be reported?
>
> ``` r
> dat <-
> data.frame(y=c(80,251,304,482,401,141,242,221,304,243,544,669,638),
>                    ones = rep(1, 13))
> mNull1 <- nls(y ~ a, data=dat, start=c(a=mean(dat$y)))
> fitted(mNull1)
> #> [1] 347.6923
> #> attr(,"label")
> #> [1] "Fitted values"
>
> mNull2 <- nls(y ~ a*ones, data=dat, start=c(a=mean(dat$y)))
> fitted(mNull2)
> #>  [1] 347.6923 347.6923 347.6923 347.6923 347.6923 347.6923 347.6923
> 347.6923
> #>  [9] 347.6923 347.6923 347.6923 347.6923 347.6923
> #> attr(,"label")
> #> [1] "Fitted values"
> ```
>
> Created on 2023-01-25 by the [reprex
> package](https://reprex.tidyverse.org) (v2.0.1)
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