From spencer.graves at pdf.com  Wed Nov 23 23:01:29 2005
From: spencer.graves at pdf.com (Spencer Graves)
Date: Wed, 23 Nov 2005 14:01:29 -0800
Subject: [Rd] qbinom returns NaN
Message-ID: <4384E6B9.8000207@pdf.com>

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
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[Rd] qbinom returns NaN
Spencer Graves