[Rd] rmultinom.c error probability not sum to 1

[M.van_Iterson at lumc.nl]

Hi all, 

I should have given a better explanation of my problem. Here it is.

I extracted from my code the bit that gives the error. Place this in a file called test.c

#include <math.h>
#include <R.h>
#include <Rmath.h>
#include <float.h>
#include <R_ext/Print.h>

int main(){

  double prob[3] = {0.0, 0.0, 0.0};
  double prob_tot = 0.;

  prob[0] = 0.3*dnorm(2, 0, 1, 0);
  prob[1] = 0.5*dnorm(5, 0, 1, 0);
  prob[2] = 0.2*dnorm(-3, 0, 1, 0);

  //obtain prob_tot
  prob_tot = 0.;
  for(int j = 0; j < 3; j++)
    prob_tot += prob[j];

  //normalize probabilities
  for(int j = 0; j < 3; j++)
    prob[j] = prob[j]/prob_tot;

  //or this give the same error
  //prob[2] = 1.0 - prob[1] - prob[0];

  //checking indeed prob_tot not exactly 1
  for(int j = 0; j < 3; j++)
    prob_tot += prob[j];

  Rprintf("Prob_tot: %f\n", prob_tot);

  int rN[3];
  rmultinom(1, prob, 1, rN);
  return 0;
}
 
run R CMD SHLIB test.c to generate the test.so. Now from within R

> dyn.load("test.so")
> .C("main")
Prob_tot: 1.017084
Error: rbinom: probability sum should be 1, but is 0.948075

Maybe I miss some trivial C knowledge why this is not exactly one!

Thanks in advance!

Regards, 
Maarten
________________________________________
From: peter dalgaard [pdalgd at gmail.com]
Sent: Thursday, March 10, 2016 1:26 PM
To: Iterson, M. van (MOLEPI)
Cc: r-devel at r-project.org
Subject: Re: [Rd] rmultinom.c error probability not sum to 1

On 10 Mar 2016, at 12:47 , M.van_Iterson at lumc.nl wrote:

> Dear all,
>
> I have a questions regarding using the c function rmultinom.c.
>
> I got the following error message "rbinom: probability sum should be 1, but is 0.999264"
>
> Which is thrown by:
>
> if(fabs((double)(p_tot - 1.)) > 1e-7)
> MATHLIB_ERROR(_("rbinom: probability sum should be 1, but is %g"),
> (double) p_tot);
>
> I understand my probabilities do not sum to one close enough. I tried the following,
> p[2] = 1. - p[0] - p[1],  where 'p', are the probabilities but this is not sufficient to pass the error message!
>


p[0] ????


> Thanks in advance!
>
> Regards,
> Maarten
>
> (I don't think this is an issue with versions but I used R version 3.2.3 and can provide more details on my linux build if necessary.)
>
>
>
>       [[alternative HTML version deleted]]
>
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> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

--
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
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