[R-sig-teaching] pros/cons of teaching attach()

[Murray Jorgensen]

When will I ever learn to proof-read my emails?!
=============
I think what Greg sayS here is the "official wisdom" and it leads to 
more future-proofED code as the structure of various objects can change 
in new versions of R.

OTOH the structure of a kind of object can always be explored with 
str(), but it may not be easy to find out what extractor functions are 
available for the object.

Murray

On 25/09/2010 8:16 a.m., Greg Snow wrote:
> I think that the reference to not using $ or [[ was meant for cases where there is a proper extraction function, residuals being the example used.
>
> If "fit" is an lm object then I could plot the fitted vs. residuals plot like this:
>
>> plot( fit$fitted, fit$residuals )
>
> Or like this:
>
>> plot( fitted(fit), resid(fit) )
>
> The second one is preferred as it properly extracts the information without needing to know the exact contents of fit (and the axis limits look a little nicer).
>
> With an lm object the 2 plots will be essentially the same, but what if fit is a glm object?  Then fit$residuals does give something that fits the definition of residuals, but of the different types of residuals available for glm fits, this gives the one that is least interesting/interpretable to humans (they were useful to the program for getting the fit).  Here the resid (or residuals) function defaults to a more meaningful set of residuals and gives options for other types.
>
> If we have arbitrary objects without extractor functions then we need to use $ or [[ to extract the individual elements, but when working with fitted objects it is much better to teach students to use the proper extractor functions rather than directly working with elements of the object itself.
>


-- 
Dr Murray Jorgensen      http://www.stats.waikato.ac.nz/Staff/maj.html
Department of Statistics, University of Waikato, Hamilton, New Zealand
Email: maj at waikato.ac.nz    majorgensen at ihug.co.nz      Fax 7 838 4155
Phone  +64 7 838 4773 wk    Home +64 7 825 0441   Mobile 021 0200 8350

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