[Rd] qbinom returns NaN
Spencer Graves
spencer.graves at pdf.com
Wed Nov 23 23:01:29 CET 2005
Hi, All:
For most but not all cases, qbinom is the inverse of pbinom.
Consider the following example, which generates an exception:
> (pb01 <- pbinom(0:1, 1, .5, log=T, lower.tail=FALSE))
[1] -0.6931472 -Inf
Since "lower.tail=FALSE", Pr{X>1} = 0 in this context, and log(0) =
-Inf, consistent with the documentation.
However, the inverse of this does NOT recover 0:1:
> qbinom(pb01,1, .5, log=T, lower.tail=F)
[1] 0 NaN
Shouldn't the NaN here be 1? If yes, this is relatively easy to fix.
Consider for example the following:
qbinom. <-
function (p, size, prob, lower.tail = TRUE, log.p = FALSE){
q. <- .Internal(qbinom(p, size, prob, lower.tail, log.p))
q.[p==(-Inf)] <- 1
q.
}
> qbinom.(pb01,1, .5, log=T, lower.tail=F)
[1] 0 1
Warning message:
NaNs produced in: qbinom(p, size, prob, lower.tail, log.p)
It's also easy to eliminate the Warning. Consider for example the
following:
qbinom. <-
function (p, size, prob, lower.tail = TRUE, log.p = FALSE){
if(any(p.inf <- p==(-Inf))&&(!lower.tail)&&log.p){
n <- max(length(p), length(size), length(prob))
p <- rep(p, length=n)
size <- rep(size, length=n)
prob <- rep(prob, length=n)
q. <- size
q.[p>(-Inf)] <- .Internal(qbinom(p[!p.inf],
size[!p.inf], prob[!p.inf], lower.tail, log.p))
return(q.)
}
.Internal(qbinom(p, size, prob, lower.tail, log.p))
}
I suspect that for the right person, it would likely be easy to fix
this in the .Internal qbinom code. However, that's beyond my current R
skill level.
Thanks for all your efforts to make R what it is today.
Best Wishes,
spencer graves
--
Spencer Graves, PhD
Senior Development Engineer
PDF Solutions, Inc.
333 West San Carlos Street Suite 700
San Jose, CA 95110, USA
spencer.graves at pdf.com
www.pdf.com <http://www.pdf.com>
Tel: 408-938-4420
Fax: 408-280-7915
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