Trying to revive, possibly conclude a forgotten thread ...
>>>>> Abby Spurdle
>>>>> on Mon, 20 May 2019 14:11:47 +1200 writes:
> Hi Pavel
> (Back On List)
> And my two cents...
>> At this time, the update.formula() method always performs a number of
>> transformations on the results, eliminating redundant variables and
>> reordering interactions to be after the main effects.
>> This the proposal is to add an option simplify= (defaulting to TRUE,
>> for backwards compatibility) that if FALSE will skip the simplification
>> step.
>> Any thoughts? One particular question that Martin raised is whether the
>> UI should be just a single logical argument, or something else.
> Firstly, note that the constructor for formula objects behaves differently
> to the update method, so I think any changes should be consistent between
> the two functions.
Not so easily: The ` ~ ` constructor does not sensibly (in my
opinion) get optional arguments,
whereas Pavel was suggesting a new *optional* argument to update.formula()
>> #constructor - doesn't simplify
>> y ~ x + x
> y ~ x + x
>> #update method - does simplify
>> update (y ~ x, ~. + x)
> y ~ x
> Interestingly, this doesn't simplify.
>> update (y ~ I (x), ~. + x)
> y ~ I(x) + x
well, I hope so:
The whole point of I(.) is to *not* be identical (but close) to its argument.
> I think that simplification could mean different things.
Good point, I tend to agree, with the above,
(whereas I'm less happy with this example : )
> So, there could be something like:
>> update (y ~ x, ~. + x, strip=FALSE)
> y ~ I (2 * x)
> I don't know how easy that would be to implement.
> (Symbolic computation on par with computer algebra systems is a discussion
> in itself...).
> And you could have one argument (say, method="simplify") rather than two
or
> more logical arguments.
Yes exactly; given what we've heard till now, I'd also go for a
new argument (possibly 'method') which should be a string
(and keep the current behavior as default), ideally here with a
match.arg() setup.
> It would also be possible to allow partial forms of simplification, by
> specifying which terms should be collapsed, however, I doubt any possible
> usefulness of this, would justify the complexity.
> However, feel free to disagree.
> You made an interesting comment.
>> This is not
>> always the desired behavior, because formulas are increasingly used
>> for purposes other than specifying linear models.
> Can I ask what these purposes are?
> kind regards
> Abs
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