[R] limited formula length in tsls

[Prof Brian D Ripley]

On Wed, 22 Aug 2001, John Fox wrote:

> Dear Zsombor,
>
> I believe that tsls.formula is from my sem package. What you list as the
> "original source" and your modified version appear to be reversed.

The `tsls package' had me confused ...

> The reason that the original version manipulates (somewhat awkwardly) the
> model and instrumental-variables formulas in character form to produce a
> combined formula is to insure that missing data get treated consistently;
> this will not be the case, I believe, in your modified version. (I note, by
> the way, that you seem to have added a function for weighted two-stage
> least-squares estimation.)
>
> The source of the problem appears to be that when model formulas are
> translated into character, the right-hand-side of the model gets truncated
> at (I think) 68 characters. Consider the following example:
>
>  > mod <- this ~ is + a + very + long + formula + with + a + very + large +
> number + of + characters
>  > mod
> this ~ is + a + very + long + formula + with + a + very + large +
>      number + of + characters
>  > as.character(mod)
> [1] "~"
> [2] "this"
> [3] "is + a + very + long + formula + with + a + very + large + number + "
>  >
>
> Clearly, the current situation is unsatisfactory, but it's unclear to me
> what the best course of action is. A more elegant approach to treating the
> model formula and instruments consistently would solve the problem, but
> as.character should probably not truncate formulas.

as.character is not the best tool for formulae.  Use deparse:

> deparse(mod)
[1] "this ~ is + a + very + long + formula + with + a + very + large + "
[2] "    number + of + characters"

say via

paste(deparse(mod, 500), collapse="")

which will cope with arbitrary long formulae.

The cutoff limit is 500, and a trivial change to the internals of
as.character (which I have just made for 1.3.1) increases the cutoff from
60 to 500. Nevertheless. paste(deparse(mod, 500), collapse="") remains
better.

[...]

Brian

-- 
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 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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