hc_matrix.c

/* 

small matrix inversion routines used by hc_polsol of H & C code


lifted from numerical recipes, but changed the matrix addressing
a[0...2][0...2]

this was moderately well tested.


$Id: hc_matrix.c,v 1.10 2006/03/20 05:32:48 becker Exp $

*/

#ifdef COMPILE_TEST
/* 
   test routine 
*/
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#define HC_PREC double
#define FALSE 0
#define HC_EPS_PREC 5e-15
void hc_ludcmp_3x3(HC_PREC [3][3],int,int *);
void hc_lubksb_3x3();
int 
main (){
  HC_PREC amat[3][3],bvec[3];int i,j,indx[3];
  char fstring[10];
  /* read in A from stdin */
  int n =2;

  for(i=0;i<n;i++)
    for(j=0;j<n;j++)
    fscanf(stdin,HC_FLT_FORMAT,&amat[i][j]);
  /* read in x from stdin */
  for(i=0;i<n;i++)
    fscanf(stdin,HC_FLT_FORMAT,&bvec[i]);
  /* solve A x = b, where b will be modified  */
  hc_ludcmp_3x3(amat,n,indx);hc_lubksb_3x3(amat,n,indx,bvec);
  for(i=0;i<n;i++)fprintf(stdout,"%g\n",bvec[i]);
  return 0;
}
#else
/* 

   normal compilation 

*/
#include "hc.h"
#endif

#define NR_TINY 1.0e-20;
/* 

   matrix is always 3 x 3 , solution is for full system for n == 3,
   for upper 2 x 2 only for n = 2

 */
void hc_ludcmp_3x3(HC_PREC a[3][3],int n,int *indx)
{
  int i,imax=0,j,k;
  HC_PREC big,dum,sum,temp;
  HC_PREC vv[3];
  
  for (i=0;i < n;i++) {
    big=0.0;
    for (j=0;j < n;j++)
      if ((temp = fabs(a[i][j])) > big) 
	big=temp;
    if (fabs(big) < HC_EPS_PREC) {
      fprintf(stderr,"hc_ludcmp_3x3: singular matrix in routine, big: %g\n",
	      (double)big);
      for(j=0;j <n;j++){
	for(k=0;k<n;k++)
	  fprintf(stderr,"%g ",(double)a[j][k]);
	fprintf(stderr,"\n");
      }
      exit(-1);
    }
    vv[i]=1.0/big;
  }
  for (j=0;j < n;j++) {
    for (i=0;i < j;i++) {
      sum = a[i][j];
      for (k=0;k < i;k++) 
	sum -= a[i][k] * a[k][j];
      a[i][j]=sum;
    }
    big=0.0;
    for (i=j;i < n;i++) {
      sum=a[i][j];
      for (k=0;k < j;k++)
	sum -= a[i][k] * a[k][j];
      a[i][j]=sum;
      if ( (dum = vv[i]*fabs(sum)) >= big) {
	big=dum;
	imax=i;
      }
    }
    if (j != imax) {
      for (k=0;k < n;k++) {
	dum = a[imax][k];
	a[imax][k]=a[j][k];
	a[j][k]=dum;
      }
      vv[imax]=vv[j];
    }
    indx[j]=imax;
    if (fabs(a[j][j]) < HC_EPS_PREC) 
      a[j][j] = NR_TINY;
    if (j != 2) {
      dum=1.0/(a[j][j]);
      for (i=j+1;i < n;i++) 
	a[i][j] *= dum;
    }
  }
}

#undef NR_TINY
void 
hc_lubksb_3x3 (a, n, indx, b)
HC_PREC a[3][3];
int n;
int *indx;
HC_PREC *b;
{
  int i,ii=0,ip,j;
  HC_PREC sum;
  int nm1;
  nm1 = n - 1;
  for (i=0;i < n;i++) {
    ip = indx[i];
    sum = b[ip];
    b[ip]=b[i];
    if (ii)
      for (j=ii-1;j <= i-1;j++) 
	sum -= a[i][j]*b[j];
    else if (fabs(sum)>HC_EPS_PREC) 
      ii = i+1;
    b[i]=sum;
  }
  for (i=nm1;i>=0;i--) {
    sum=b[i];
    for (j=i+1;j < n;j++) 
      sum -= a[i][j]*b[j];
    b[i] = sum/a[i][i];
  }
}