R devel - Sep 2001 - RFC: d/p/q/rgamma

dgamma and friends in S are documented as

dgamma(x, shape, rate=1)
pgamma(q, shape, rate=1)
qgamma(p, shape, rate=1)
rgamma(n, shape, rate=1)


whereas R has

dgamma(x, shape, scale=1, log = FALSE)
pgamma(q, shape, scale=1, lower.tail = TRUE, log.p = FALSE)
qgamma(p, shape, scale=1, lower.tail = TRUE, log.p = FALSE)
rgamma(n, shape, scale=1)

Note the use of rate vs scale.  Indeed, as both S and R use `rate' for
exponential this seems somewhat strange, and the only such inconsistency I
can find.

I propose that we add a `rate' argument to d/p/q/rgamma as an alternative
to `scale', which is easily done.   The question is where?

I think we have a trap here for most users, who will happily copy

dgamma(x, 17, 0.3)

from their S-PLUS example sheet, and get the wrong answer. My preference
would be to have

dgamma(x, shape, rate, scale=1, log = FALSE)

etc.  That would break existing R code using positional matching.
Question: is there any?

The only really safe alternative is add `rate' as the last parameter.

-- 
Brian D. Ripley,                  ripley@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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>>>>> Prof Brian Ripley writes:
> dgamma and friends in S are documented as
> dgamma(x, shape, rate=1)
> pgamma(q, shape, rate=1)
> qgamma(p, shape, rate=1)
> rgamma(n, shape, rate=1)
> whereas R has
> dgamma(x, shape, scale=1, log = FALSE)
> pgamma(q, shape, scale=1, lower.tail = TRUE, log.p = FALSE)
> qgamma(p, shape, scale=1, lower.tail = TRUE, log.p = FALSE)
> rgamma(n, shape, scale=1)
> Note the use of rate vs scale.  Indeed, as both S and R use `rate' for
> exponential this seems somewhat strange, and the only such
> inconsistency I can find.
> I propose that we add a `rate' argument to d/p/q/rgamma as an
> alternative to `scale', which is easily done.  The question is where?
> I think we have a trap here for most users, who will happily copy
> dgamma(x, 17, 0.3)
> from their S-PLUS example sheet, and get the wrong answer. My preference
> would be to have
> dgamma(x, shape, rate, scale=1, log = FALSE)
> etc.  That would break existing R code using positional matching.
> Question: is there any?
One could find out by unpackaging all CRAN packages and grepping for
usages of *gamma in the code.

-k
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