R help - Dec 2001 - Newbie problems with R and compiled C

I'm a beginer programming C and I have the following problem:

I have the following C-code file

#include <stdlib.h>

void gen(int *n,  int *a, int *c, int *m, int *x0, int *x);

main(){
  int nn = 31;
  int aa = 3;
  int cc = 0;
  int mm = 31;
  int xx0 = 9;
  int xx[nn];
  int i;
  gen(&nn,&aa,&cc,&mm,&xx0,xx);
  for (i = 0; i <= nn-1; i++)
    printf("%d ,",xx[i]);
  printf("\n %d",nn);
  printf("\n %d",aa);
  printf("\n %d",cc);
  printf("\n %d",mm);
  printf("\n %d",xx0);
}

void gen(int *n,  int *a, int *c, int *m, int *x0, int *x){
  int i;
  x[0]=*x0;
  for (i = 1; i <= *n-1; i++)
    x[i] = (*a * x[i-1] + *c) % *m;
}

I compile it with:

gcc gen2.c -o gen2.exe (Within emacs in W2000)

and executing gen2.exe the result is the correct one:

9 ,27 ,19 ,26 ,16 ,17 ,20 ,29 ,25 ,13 ,8 ,24 ,10 ,30 ,28 ,22 ,4 ,12 ,5 ,15
,14 ,
11 ,2 ,6 ,18 ,23 ,7 ,21 ,1 ,3 ,9 ,
 31
 3
 0
 31
 9

Then I put in a library with files:

#include <stdlib.h>

void gen(int *n,  int *a, int *c, int *m, int *x0, int *x){
  int i;
  x[0] = *x0;
  for (i = 1; i <= *n-1; i++)
    x[i] = (*a * x[i-1] + *c) % *m;
}

for the code and the wrapper function:

gen <- function(n, a, c, m, x0){
  .C("gen", x=integer(n),
          n=as.integer(n),
          a=as.integer(a),
          c=as.integer(c),
          m=as.integer(m),
          x0=as.integer(x0))
}

and the result is not the desired one:
> gen(n=31, a=3, c=0, m=31, x0=9)
$x
 [1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

$n
[1] 31

$a
[1] 3

$c
[1] 0

$m
[1] 31

$x0
[1] 31

In fact $x shouldn't be 0 and $x0 should be 9.

I think I'm missing something obvious with C and R.

Thanks for any hint,

Eusebio Arenal




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On Tue, 18 Dec 2001, [Windows-1252] Eusebio Arenal Gutiérrez wrote:
> I'm a beginer programming C and I have the following problem:
>
> I have the following C-code file
>
> #include <stdlib.h>
>
> void gen(int *n,  int *a, int *c, int *m, int *x0, int *x);
>
> main(){
>   int nn = 31;
>   int aa = 3;
>   int cc = 0;
>   int mm = 31;
>   int xx0 = 9;
>   int xx[nn];
>   int i;
>   gen(&nn,&aa,&cc,&mm,&xx0,xx);
>   for (i = 0; i <= nn-1; i++)
>     printf("%d ,",xx[i]);
>   printf("\n %d",nn);
>   printf("\n %d",aa);
>   printf("\n %d",cc);
>   printf("\n %d",mm);
>   printf("\n %d",xx0);
> }
>
> void gen(int *n,  int *a, int *c, int *m, int *x0, int *x){
>   int i;
>   x[0]=*x0;
>   for (i = 1; i <= *n-1; i++)
>     x[i] = (*a * x[i-1] + *c) % *m;
> }
>
> I compile it with:
>
> gcc gen2.c -o gen2.exe (Within emacs in W2000)
>
> and executing gen2.exe the result is the correct one:
>
> 9 ,27 ,19 ,26 ,16 ,17 ,20 ,29 ,25 ,13 ,8 ,24 ,10 ,30 ,28 ,22 ,4 ,12 ,5 ,15
> ,14 ,
> 11 ,2 ,6 ,18 ,23 ,7 ,21 ,1 ,3 ,9 ,
>  31
>  3
>  0
>  31
>  9
>
> Then I put in a library with files:
>
> #include <stdlib.h>
>
> void gen(int *n,  int *a, int *c, int *m, int *x0, int *x){
>   int i;
>   x[0] = *x0;
>   for (i = 1; i <= *n-1; i++)
>     x[i] = (*a * x[i-1] + *c) % *m;
> }
>
> for the code and the wrapper function:
>
> gen <- function(n, a, c, m, x0){
>   .C("gen", x=integer(n),
>           n=as.integer(n),
>           a=as.integer(a),
>           c=as.integer(c),
>           m=as.integer(m),
>           x0=as.integer(x0))
> }
The order of the args is wrong: it needs to be

.C("gen", as,integer(n), as.integer(a), as.integer(c), as.integer(m),
   as.integer(x0), x = integer(n)$n

I think.

-- 
Brian D. Ripley,                  ripley at 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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