[Rd] Troubles with the function rmultinom.c of the R's Random Number Generator

[Hin-Tak Leung]

Sophie Ancelet wrote:
> Thank you for the answer. However, I sought in Doc. Writing R
> extensions, in particular in the paragraph 5.6 "Calling C from FORTRAN
> and vice versa" (page 67) but I did not find anything which could help me to
> correct my code. Indeed, rmultinom.c is a particular function
> since arrays are passed in arguments. Has somebody ever
> written a wrapper for this function?

You probably haven't managed to digest what Prof Ripley was writing: 
"All arguments to functions called from C by Fortran are pointers
(or should be: yours are not)" - here is a more hand-holding kind
of answer, which is exactly the same advice, really - you need to
be doing something like this (note the pointers) instead:

 >>>void F77_SUB(sarmultinom)(int *n,
 >>>                         double** prob,
 >>>                         int *K,
 >>>                         int** rN){
 >>>rmultinom(n, prob, K, rN);}

I'll be interested to see what breaks - do you mind sending me
both of test-multinom.f wrapper.c direct for me to have a look?

HTL

> At 17:06 20/01/06 +0000, Prof Brian Ripley wrote:
> 
>>All arguments to functions called from C by Fortran are pointers
>>(or should be: yours are not).  The error is within your own code.
>>
>>You don't want to call rndstart and rndend around every call, only before 
>>the first and after the last.
>>
>>This is not the list for advice om mixed Fortran/C programming, though.
> 
> 
> 
> 
> 
>>On Fri, 20 Jan 2006, Sophie Ancelet wrote:
>>
>>
>>>Hi,
>>>
>>>I'm simulating a Markov chain in Fortran interfaced with R-2.2.1 in order
>>>to generate data according to a Markov Random Field called the Potts model.
>>>
>>>R Version:
>>>platform i686-pc-linux-gnu
>>>arch     i686
>>>os       linux-gnu
>>>system   i686, linux-gnu
>>>status
>>>major    2
>>>minor    2.1
>>>year     2005
>>>month    12
>>>day      20
>>>svn rev  36812
>>>
>>>
>>>
>>>
>>>Each loop of my Fortran calls the function rmultinom.c of the R's Random
>>>Number Generator through the wrapper:
>>>
>>>#include <R.h>
>>>#include <Rmath.h>
>>>void F77_SUB(sarmultinom)(int n,
>>>                         double* prob,
>>>                         int K,
>>>                         int* rN){
>>>rmultinom(n, prob, K, rN);}
>>>
>>>
>>>
>>>My fortran program is:
>>>
>>>subroutine testsarmultinom(n,prob,K,rN)
>>>implicit none
>>>integer n,K,rN(K)
>>>double precision prob(K)
>>>
>>>call rndstart()
>>>call sarmultinom(n,prob,K,rN)
>>>call rndend()
>>>end
>>>
>>>
>>>In order to understand better how the function rmultinom.c works, I have
>>>written an R code which calls this fortran subroutine as follows:
>>>
>>>system("R CMD SHLIB test-multinom.f wrapper.c")
>>>dyn.load("~/Package/test/test-multinom.so")
>>>
>>>n=1
>>>prob=c(0.6,0.1,0.3)
>>>K=3
>>>rN=c(1,0,0)
>>>res<- .Fortran("testsarmultinom",
>>>              as.integer(n),
>>>              as.double(prob),
>>>              as.integer(K),
>>>              as.integer(rN))
>>>
>>>
>>>Unfortunately, I have some trouble with the results. First, this command
>>>always returns 0 values. In other words, I always get:
>>>
>>>
>>>>res[[4]]
>>>
>>>[1] 0 0 0
>>>
>>>
>>>Moreover, if I run this R code a second time, an error message appears:
>>>Segmentation fault.
>>>
>>>Has somebody ever used rmultinom.c and encountered these problems? My code
>>>must be wrong but I don't know where. In this case, what is the correct way
>>>to call the C function rmultinom.c?
>>>
>>>Thanks in advance,
>>>
>>>Sophie.
> 
> 
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