[R] passing a matrix from R to C code

[Peter Langfelder]

Erin,

you seem to confuse R and C syntax a bit, among other things. See below.

On Mon, Sep 24, 2012 at 3:03 PM, Erin Hodgess <erinm.hodgess at gmail.com> wrote:
> Dear R People:
>
> I'm working on a project that will pass a matrix from an R function to
> a C subroutine.
>
> I tried to write the following:
>
> #include<R.h>
> #include<Rinternals.h>
>
>
>
>   void lu1(int *n, float *a, float *b, float *ab)
>
> {
>   int   i,k,j,p,na=*n+10;
>   float sum, l[200][200],u[200][200]z[200];

Comma missing?

>
>
>     for(i=0;i<na;i++) {
>       for(j=0;j<na;j++) {
>         l[i,j]=0.0;
>         u[i,j]=0.0;

In C you must write l[i][j] and u[i][j].
l[i,j] is actually equal to l[j] and that's an array of 200 doubles.

>       }
>       z[i] =0.0;
>     }
>
>
>
>
>     //********** LU decompositio*n *****//
>     for(k=1;k<=*n;k++)
>     {
>         u[k][k]=1;
>         for(i=k;i<=*n;i++)
>         {
>             sum=0;
>             for(p=1;p<=k-1;p++)
>                 sum+=l[i][p]*u[p][k];
>             l[i][k]=a[i][k]-sum;

a is a single-index array. You cannot index it using two indices.
Convert the two into one using something like i*dimension + k. Same
below.


HTH,

Peter

>         }
>
>         for(j=k+1;j<=*n;j++)
>         {
>             sum=0;
>             for(p=1;p<=k-1;p++)
>                 sum+=l[k][p]*u[p][j];
>             u[k][j]=(a[k][j]-sum)/l[k][k];
>         }
>     }
>
>
>     //***** FI*NDI*NG Z; LZ=b*********//
>
>     for(i=1;i<=*n;i++)
>     {                                        //forward subtitutio*n method
>         sum=0;
>         for(p=1<i;p++)
>         sum+=l[i][p]*z[p];
>         z[i]=(b[i]-sum)/l[i][i];
>     }
>     //********** FI*NDI*NG X; UX=Z***********//
>
>     for(i=*n;i>0;i--)
>     {
>         sum=0;
>         for(p=*n;p>i;p--)
>           sum+=u[i][p]*ab[p];
>         ab[i]=(z[i]-sum)/u[i][i];
>     }
>
> }
>
>
> And here is the output:
>  R CMD SHLIB lu1.c
> gcc -std=gnu99 -I/usr/share/R/include -DNDEBUG      -fpic  -O3 -pipe
> -g  -c lu1.c -o lu1.o
> lu1.c: In function ‘lu1’:
> lu1.c:15:8: error: incompatible types when assigning to type
> ‘float[200]’ from type ‘double’
> lu1.c:16:8: error: incompatible types when assigning to type
> ‘float[200]’ from type ‘double’
> lu1.c:33:25: error: subscripted value is neither array nor pointer nor vector
> lu1.c:41:26: error: subscripted value is neither array nor pointer nor vector
> make: *** [lu1.o] Error 1
> erin at ubuntu:~$
>
> I'm thinking that the matrix is hanging things up.
>
> Does this look familiar, please?