Good day,
I'm working on a C library (for myself, code: https://github.com/BattlestarSC/matrixLibrary.git) to handle matrix functions. This is mostly a learning/practice activity. One of my challenges is to take the determinant of a matrix efficiently. As my current attempts have failed, I wanted to take a different approach. I was reading though this method from MIT docs: http://web.mit.edu/18.06/www/Spring17/Determinants.pdf and it made a lot of sense. The issue I'm having is how to get to said point. As the Gaussian elimination method is good for multi-variable systems of equations, my matricies are not built from equations, and therefor are not part of a system. As in, each equation has no set result and does not fit into the form from this paper here:https://math.oregonstate.edu/home/programs/undergrad/CalculusQuestStudyGuides/vcalc/gauss/gauss.html
From this point, I'm at a loss as far as how to proceed with this method.
It makes a lot of sense to take the pivot point from each set of equations as described in the MIT paper, but how should I set up my matricies to make said result valid?
When you perform a Gaussian elimination, you swap rows and repeatedly subtract a multiple of one row from another to produce an upper triangular form.
When you do this on a system of equations or an "augmented matrix", you do not use any information from the result column. The decisions about which rows to swap and which to subtract with what multiplier would be exactly the same no matter what numbers are in the result column.
Because the "result column" is not used, you can perform the same procedure on a normal square matrix. Since the operations don't change the determinant (if you negate one row whenever you swap), you end up with an upper triangular matrix with the same det as the original.
The MIT author calls a function lu to do this in the example near the start. This does L-U decomposition on the matrix, which returns the Gaussian-eliminated matrix in the 'U' part: https://en.wikipedia.org/wiki/LU_decomposition.
L-U decomposition is pretty cool. It's like doing Gaussian elimination to solve all systems with the same "matrix part" all at once, which again you can do because the process doesn't need to see the result column at all.
Starting with a matrix M, you get L and U such that LU = M. That means, if you want to solve:
Mx = y
... where (x an y are column vectors), you have:
LUx = y
Solve Lv=y, which is easy (just substitution) because L is lower-triangular. Then you have:
Ux = v
... which is easy to solve because U is upper-triangular.
GEM is not very good for computers as it needs to reorder the rows so the algo leads to a valid result that adds relatively big overhead and potential instability (if ordered badly). The GEM is much better suited for humans and paper/pencil as we instinctively reorder/chose rows ...
So you should go with the (sub)Determinant approach as you wanted in the first place. Is faster and safer. I know its a bit tricky to learn it from papers. If it helps this is mine ancient matrix.h class (but in C++) I wrote when I was still a rookie (so there might be some hidden bugs I do not know of haven't use this for ages):
//--- matrix ver: 2.1 -------------------------------------------------------
#ifndef _matrix_h
#define _matrix_h
//---------------------------------------------------------------------------
double fabs(double x)
{
if (x<0) x=-x;
return x;
}
//---------------------------------------------------------------------------
class matrix
{
private:double **p;
int xs,ys;
double zeroacc;
public: matrix() { p=NULL; xs=0; ys=0; resize(1,1); zeroacc=1e-10; }
~matrix() { free(); }
void free();
int resize(int _xs,int _ys);
matrix& operator=(const matrix &b);
matrix& operator+();
matrix& operator-();
matrix& operator+(matrix &b);
matrix& operator-(matrix &b);
matrix& operator*(matrix &b);
matrix& operator+=(matrix &b);
matrix& operator-=(matrix &b);
matrix& operator*=(matrix &b);
matrix& operator!();
double& operator()(int y,int x);
double* operator[](int y) { return p[y]; }
void one();
int get_xs() { return xs; }
int get_ys() { return ys; }
double get_zeroacc() { return zeroacc; }
void set_zeroacc(double _zeroacc) { zeroacc=_zeroacc; if (zeroacc<0) zeroacc=-zeroacc; }
void ld(int y,double x0=0.0,double x1=0.0,double x2=0.0,double x3=0.0,double x4=0.0,double x5=0.0,double x6=0.0,double x7=0.0,double x8=0.0,double x9=0.0);
void prn(TCanvas *scr,int x0,int y0);
void lxch(int y1,int y2);
void lcom(int y1,int y2,double k);
void lmul(int y,double k);
void ldiv(int y,double k);
int gaus(matrix &b);
matrix& matrix::submatrix(int _x,int _y);
double determinant();
double subdeterminant();
matrix& inv_det();
matrix& inv_gaus();
};
//---------------------------------------------------------------------------
void matrix::free()
{
int y;
if (p!=NULL)
for (y=0;y<ys;y++)
delete[] p[y];
delete[] p;
p=NULL;
xs=0;
ys=0;
}
//---------------------------------------------------------------------------
int matrix::resize(int _xs,int _ys)
{
int y;
free();
if (_xs<1) _xs=1;
if (_ys<1) _ys=1;
xs=_xs;
ys=_ys;
p=new double*[ys];
if (p==NULL)
{
xs=0;
ys=0;
return 0;
}
for (y=0;y<ys;y++)
{
p[y]=new double[xs];
if (p[y]==NULL)
{
if (y>0)
for (y--;y>=0;y--)
delete p[y];
delete p;
p=NULL;
xs=0;
ys=0;
return 0;
}
}
return 1;
}
//---------------------------------------------------------------------------
matrix& matrix::operator=(const matrix &b)
{
int x,y;
if (!resize(b.get_xs(),b.get_ys())) return *this;
if (b.p)
for (y=0;y<ys;y++)
for (x=0;x<xs;x++)
p[y][x]=b.p[y][x];
return *this;
}
//---------------------------------------------------------------------------
matrix& matrix::operator+()
{
static matrix c;
int x,y;
c.resize(xs,ys);
for (y=0;y<ys;y++)
for (x=0;x<xs;x++)
c.p[y][x]= p[y][x];
return c;
}
//---------------------------------------------------------------------------
matrix& matrix::operator-()
{
static matrix c;
int x,y;
c.resize(xs,ys);
for (y=0;y<ys;y++)
for (x=0;x<xs;x++)
c.p[y][x]=-p[y][x];
return c;
}
//---------------------------------------------------------------------------
matrix& matrix::operator+(matrix &b)
{
static matrix c;
int x,y;
c.free();
if (xs!=b.get_xs()) return c;
if (ys!=b.get_ys()) return c;
c.resize(xs,ys);
for (y=0;y<ys;y++)
for (x=0;x<xs;x++)
c.p[y][x]=p[y][x]+b.p[y][x];
return c;
}
//---------------------------------------------------------------------------
matrix& matrix::operator-(matrix &b)
{
static matrix c;
int x,y;
c.free();
if (xs!=b.get_xs()) return c;
if (ys!=b.get_ys()) return c;
c.resize(xs,ys);
for (y=0;y<ys;y++)
for (x=0;x<xs;x++)
c.p[y][x]=p[y][x]-b.p[y][x];
return c;
}
//---------------------------------------------------------------------------
matrix& matrix::operator*(matrix &b)
{
static matrix c;
int i,j,k,ii,jj,kk;
c.free();
ii=ys;
jj=b.get_xs();
kk=b.get_ys();
if (kk!=xs) return c;
if (!c.resize(jj,ii)) return c;
for (i=0;i<ii;i++)
for (j=0;j<jj;j++)
c.p[i][j]=0.0;
for (i=0;i<ii;i++)
for (j=0;j<jj;j++)
for (k=0;k<kk;k++)
c.p[i][j]+=p[i][k]*b.p[k][j];
return c;
}
//---------------------------------------------------------------------------
matrix& matrix::operator+=(matrix &b)
{
int x,y;
if (xs!=b.get_xs()) { free(); return *this; }
if (ys!=b.get_ys()) { free(); return *this; }
for (y=0;y<ys;y++)
for (x=0;x<xs;x++)
p[y][x]+=b.p[y][x];
return *this;
}
//---------------------------------------------------------------------------
matrix& matrix::operator-=(matrix &b)
{
int x,y;
if (xs!=b.get_xs()) { free(); return *this; }
if (ys!=b.get_ys()) { free(); return *this; }
for (y=0;y<ys;y++)
for (x=0;x<xs;x++)
p[y][x]-=b.p[y][x];
return *this;
}
//---------------------------------------------------------------------------
matrix& matrix::operator*=(matrix &b)
{
matrix c;
int i,j,k,ii,jj,kk;
c.free();
ii=ys;
jj=b.get_xs();
kk=b.get_ys();
if (kk!=xs) { *this=c; return *this; }
if (!c.resize(jj,ii)) { *this=c; return *this; }
for (i=0;i<ii;i++)
for (j=0;j<jj;j++)
c.p[i][j]=0.0;
for (i=0;i<ii;i++)
for (j=0;j<jj;j++)
for (k=0;k<kk;k++)
c.p[i][j]+=p[i][k]*b.p[k][j];
*this=c; return *this;
}
//---------------------------------------------------------------------------
matrix& matrix::operator!()
{
// return inv_det();
return inv_gaus();
}
//---------------------------------------------------------------------------
double& matrix::operator()(int y,int x)
{
static double _null;
if (x<0) return _null;
if (y<0) return _null;
if (x>=xs) return _null;
if (y>=ys) return _null;
return p[y][x];
}
//---------------------------------------------------------------------------
void matrix::one()
{
int x,y;
for (y=0;y<ys;y++)
for (x=0;x<xs;x++)
if (x!=y) p[y][x]=0.0;
else p[y][x]=1.0;
}
//---------------------------------------------------------------------------
void matrix::ld(int y,double x0,double x1,double x2,double x3,double x4,double x5,double x6,double x7,double x8,double x9)
{
int x;
if (y<0) return;
if (y>=ys) return;
x=0;
if (x<xs) p[y][x]=x0; x++;
if (x<xs) p[y][x]=x1; x++;
if (x<xs) p[y][x]=x2; x++;
if (x<xs) p[y][x]=x3; x++;
if (x<xs) p[y][x]=x4; x++;
if (x<xs) p[y][x]=x5; x++;
if (x<xs) p[y][x]=x6; x++;
if (x<xs) p[y][x]=x7; x++;
if (x<xs) p[y][x]=x8; x++;
if (x<xs) p[y][x]=x9; x++;
}
//---------------------------------------------------------------------------
void matrix::prn(TCanvas *scr,int x0,int y0)
{
int x,y,xx,yy,dx,dy;
dx=50;
dy=13;
yy=y0;
for (y=0;y<ys;y++)
{
xx=x0;
for (x=0;x<xs;x++)
{
scr->TextOutA(xx,yy,AnsiString().sprintf("%.4lf",p[y][x]));
xx+=dx;
}
yy+=dy;
}
}
//---------------------------------------------------------------------------
void matrix::lxch(int y1,int y2)
{
int x;
double a;
if (y1<0) return;
if (y2<0) return;
if (y1>=ys) return;
if (y2>=ys) return;
for (x=0;x<xs;x++) { a=p[y1][x]; p[y1][x]=p[y2][x]; p[y2][x]=a; }
}
//---------------------------------------------------------------------------
void matrix::lcom(int y1,int y2,double k)
{
int x;
if (y1<0) return;
if (y2<0) return;
if (y1>=ys) return;
if (y2>=ys) return;
for (x=0;x<xs;x++) p[y1][x]+=p[y2][x]*k;
}
//---------------------------------------------------------------------------
void matrix::lmul(int y,double k)
{
int x;
if (y<0) return;
if (y>=ys) return;
for (x=0;x<xs;x++) p[y][x]*=k;
}
//---------------------------------------------------------------------------
void matrix::ldiv(int y,double k)
{
int x;
if (y<0) return;
if (y>=ys) return;
if ((k> zeroacc)||(k<-zeroacc)) k=1.0/k; else k=0.0;
for (x=0;x<xs;x++) p[y][x]*=k;
}
//---------------------------------------------------------------------------
int matrix::gaus(matrix &b)
{
int x,y;
double a;
if (xs!=ys) return 0;
if (ys!=b.ys) return 0;
for (x=0;x<xs;x++)
{
a=p[x][x]; // je aktualny prvok (x,x) na diagonale = 0 ?
if (a<0) a=-a;
if (a<=zeroacc)
for (y=0;y<ys;y++) // ak hej najdi nejaky nenulovy riadok v aktualnom stlpci (x)
if (x!=y)
{
a=p[y][x];
if (a<0) a=-a;
if (a>=zeroacc) // ak sa nasiel tak ho pripocitaj k aktualnemu riadku co zrusi tu nulu
{
b.lcom(x,y,1.0);
lcom(x,y,1.0);
break;
}
}
a=p[x][x]; // este raz otestuj ci na diagonale neni nula
if (a<0) a=-a;
if (a<=zeroacc) return 0; // ak je tak koniec
b.ldiv(x,p[x][x]); // sprav na diagonale 1-tku
ldiv(x,p[x][x]);
for (y=0;y<ys;y++) // a vynuluj zvysne riadky v stlpci(x)
if (y!=x)
{
b.lcom(y,x,-p[y][x]);
lcom(y,x,-p[y][x]);
}
}
return 1;
}
//---------------------------------------------------------------------------
//---------------------------------------------------------------------------
//---------------------------------------------------------------------------
matrix& matrix::submatrix(int _x,int _y)
{
static matrix c;
int x,y,xx,yy;
c.resize(xs-1,ys-1);
yy=0; for (y=0;y<ys;y++)
if (y!=_y)
{
xx=0; for (x=0;x<xs;x++)
if (x!=_x)
{
c.p[yy][xx]=p[y][x];
xx++;
}
yy++;
}
return c;
}
//---------------------------------------------------------------------------
double matrix::determinant()
{
double D;
matrix a;
int x,y,s;
D=0;
if (xs!=ys) return D;
if (xs==1) { D=p[0][0]; return D; }
y=0;
s=y&1;
for (x=0;x<xs;x++)
{
a=submatrix(x,y);
if (s) D-=a.determinant()*p[y][x];
else D+=a.determinant()*p[y][x];
s=!s;
}
return D;
}
//---------------------------------------------------------------------------
double matrix::subdeterminant()
{
double D;
matrix a,b;
int x,y,s;
D=0;
if (xs!=ys) return D;
if (xs==1) { D=p[0][0]; return D; }
b=this[0];
for (y=0;y<ys;y++)
for (x=0;x<xs;x++)
{
a=b.submatrix(x,y);
p[y][x]=a.determinant();
}
y=0;
s=y&1;
for (x=0;x<xs;x++)
{
if (s) D-=p[y][x]*b.p[y][x];
else D+=p[y][x]*b.p[y][x];
s=!s;
}
return D;
}
//---------------------------------------------------------------------------
matrix& matrix::inv_det()
{
int x,y,s;
double D;
static matrix a,b;
a=this[0];
b=this[0];
D=b.subdeterminant();
if (fabs(D)>zeroacc) D=1.0/D;
for (y=0;y<ys;y++)
for (x=0;x<xs;x++)
{
s=(x+y)&1;
if (s) a.p[y][x]=-b.p[x][y]*D;
else a.p[y][x]= b.p[x][y]*D;
}
return a;
}
//---------------------------------------------------------------------------
matrix& matrix::inv_gaus()
{
static matrix a,b;
a=*this;
b.resize(xs,ys);
b.one();
a.gaus(b);
return b;
}
//---------------------------------------------------------------------------
#endif
//---------------------------------------------------------------------------
Both GEM inv_gaus and (sub)determinant inv_det approaches are present so just extract/compare from it what you need.
BTW lately I needed some math stuff for N-dimensional space and once I was at it I also coded a square matrix as template where the (sub)Determinant approach is done as recursive template nd_math.h:
//--- N-Dimensional math ver: 1.002 -----------------------------------------
#ifndef _ND_math_h
#define _ND_math_h
//---------------------------------------------------------------------------
#include <math.h>
//---------------------------------------------------------------------------
#ifndef _rep4d_h
double divide(double a,double b) { if (fabs(b)<1e-30) return 0.0; return a/b; }
#endif
//---------------------------------------------------------------------------
template <const DWORD N> class vector
{
public:
double a[N];
vector() {}
vector(vector& a) { *this=a; }
~vector() {}
vector* operator = (const vector<N> *a) { *this=*a; return this; }
//vector* operator = (vector<N> &a) { ...copy... return this; }
double& operator [](const int i) { return a[i]; }
vector<N> operator + () { return *this; } // =+v0
vector<N> operator - () { int i; vector<N> q; for ( i=0;i<N;i++) q.a[i]= -a[i]; return q; } // =-v0
vector<N> operator + (vector<N> &v) { int i; vector<N> q; for ( i=0;i<N;i++) q.a[i]=a[i]+v.a[i]; return q; } // =v0+v1
vector<N> operator - (vector<N> &v) { int i; vector<N> q; for ( i=0;i<N;i++) q.a[i]=a[i]-v.a[i]; return q; } // =v0-v1
double operator * (vector<N> &v) { int i; double q; for (q=0.0,i=0;i<N;i++) q +=a[i]*v.a[i]; return q; } // =(v0.v1) dot product
vector<N> operator + (const double &c) { int i; vector<N> q; for ( i=0;i<N;i++) q.a[i]=a[i]+c; return q; } // =v0+(c,c,c,c,...)
vector<N> operator - (const double &c) { int i; vector<N> q; for ( i=0;i<N;i++) q.a[i]=a[i]-c; return q; } // =v0-(c,c,c,c,...)
vector<N> operator * (const double &c) { int i; vector<N> q; for ( i=0;i<N;i++) q.a[i]=a[i]*c; return q; } // =v0*c
vector<N> operator / ( double c) { int i; vector<N> q; c=divide(1.0,c); for ( i=0;i<N;i++) q.a[i]=a[i]*c; return q; } // =v0/c
vector<N> operator +=(vector<N> &v) { this[0]=this[0]+v; return *this; }; // v0+=v1
vector<N> operator -=(vector<N> &v) { this[0]=this[0]-v; return *this; }; // v0-=v1
vector<N> operator +=(const double &c) { this[0]=this[0]+c; return *this; }; // v0+=(c,c,c,c,...)
vector<N> operator -=(const double &c) { this[0]=this[0]-c; return *this; }; // v0-=(c,c,c,c,...)
vector<N> operator *=(const double &c) { this[0]=this[0]*c; return *this; }; // v0*=c
vector<N> operator /=(const double &c) { this[0]=this[0]/c; return *this; }; // v0/=c
AnsiString str() { int i; AnsiString q; for (q="( ",i=0;i<N;i++) q+=AnsiString().sprintf("%6.3lf ",a[i]); q+=")"; return q; }
double len() { int i; double l; for (l=0.0,i=0;i<N;i++) l+=a[i]*a[i]; return sqrt(l); } // get size
double len2() { int i; double l; for (l=0.0,i=0;i<N;i++) l+=a[i]*a[i]; return l; } // get size^2
void len(double l) { int i; l=divide(l,len()); for (i=0;i<N;i++) a[i]*=l; } // set size
void unit() { len(1.0); } // set unit size
void zero() { int i; for (i=0;i<N;i++) a[i]=0.0; } // set zero vector
void rnd() { int i; for (i=0;i<N;i++) a[i]=(2.0*Random())-1.0; } // set random unit vector
void set(double c) { int i; for (i=0;i<N;i++) a[i]=c; } // (c,c,c,c,...)
// i x j = k | | i j k |
// j x k = i | a x b = det | a0 a1 a2 | = + i*det | a1 a2 | - j*det | a0 a2 | + k*det | a0 a1 |
// k x i = j | | b0 b1 b2 | | b1 b2 | | b0 b2 | | b0 b1 |
void cross(const vector<N> *v)
{
int i,j;
matrix<N> m0;
matrix<N-1> m;
for (i=1;i<N;i++)
for (j=0;j<N;j++)
m0.a[i][j]=v[i-1].a[j];
for (j=0;j<N;j++)
{
m=m0.submatrix(0,j);
if (int(j&1)==0) a[j]=+m.det();
else a[j]=-m.det();
}
}
void cross(vector<N> **v)
{
int i,j;
matrix<N> m0;
matrix<N-1> m;
for (i=1;i<N;i++)
for (j=0;j<N;j++)
m0.a[i][j]=v[i-1]->a[j];
for (j=0;j<N;j++)
{
m=m0.submatrix(0,j);
if (int(j&1)==0) a[j]=+m.det();
else a[j]=-m.det();
}
}
void cross(vector<N> &v0) { vector<N> *v[ 1]={&v0}; cross(v); }
void cross(vector<N> &v0,vector<N> &v1) { vector<N> *v[ 2]={&v0,&v1}; cross(v); }
void cross(vector<N> &v0,vector<N> &v1,vector<N> &v2) { vector<N> *v[ 3]={&v0,&v1,&v2}; cross(v); }
void cross(vector<N> &v0,vector<N> &v1,vector<N> &v2,vector<N> &v3) { vector<N> *v[ 4]={&v0,&v1,&v2,&v3}; cross(v); }
void cross(vector<N> &v0,vector<N> &v1,vector<N> &v2,vector<N> &v3,vector<N> &v4) { vector<N> *v[ 5]={&v0,&v1,&v2,&v3,&v4}; cross(v); }
void cross(vector<N> &v0,vector<N> &v1,vector<N> &v2,vector<N> &v3,vector<N> &v4,vector<N> &v5) { vector<N> *v[ 6]={&v0,&v1,&v2,&v3,&v4,&v5}; cross(v); }
void cross(vector<N> &v0,vector<N> &v1,vector<N> &v2,vector<N> &v3,vector<N> &v4,vector<N> &v5,vector<N> &v6) { vector<N> *v[ 7]={&v0,&v1,&v2,&v3,&v4,&v5,&v6}; cross(v); }
void cross(vector<N> &v0,vector<N> &v1,vector<N> &v2,vector<N> &v3,vector<N> &v4,vector<N> &v5,vector<N> &v6,vector<N> &v7) { vector<N> *v[ 8]={&v0,&v1,&v2,&v3,&v4,&v5,&v6,&v7}; cross(v); }
void cross(vector<N> &v0,vector<N> &v1,vector<N> &v2,vector<N> &v3,vector<N> &v4,vector<N> &v5,vector<N> &v6,vector<N> &v7,vector<N> &v8) { vector<N> *v[ 9]={&v0,&v1,&v2,&v3,&v4,&v5,&v6,&v7,v8}; cross(v); }
void cross(vector<N> &v0,vector<N> &v1,vector<N> &v2,vector<N> &v3,vector<N> &v4,vector<N> &v5,vector<N> &v6,vector<N> &v7,vector<N> &v8,vector<N> &v9) { vector<N> *v[10]={&v0,&v1,&v2,&v3,&v4,&v5,&v6,&v7,v8,v9}; cross(v); }
void ld(const double &a0) { a[0]=a0; }
void ld(const double &a0,const double &a1) { a[0]=a0; a[1]=a1; }
void ld(const double &a0,const double &a1,const double &a2) { a[0]=a0; a[1]=a1; a[2]=a2; }
void ld(const double &a0,const double &a1,const double &a2,const double &a3) { a[0]=a0; a[1]=a1; a[2]=a2; a[3]=a3; }
void ld(const double &a0,const double &a1,const double &a2,const double &a3,const double &a4) { a[0]=a0; a[1]=a1; a[2]=a2; a[3]=a3; a[4]=a4; }
void ld(const double &a0,const double &a1,const double &a2,const double &a3,const double &a4,const double &a5) { a[0]=a0; a[1]=a1; a[2]=a2; a[3]=a3; a[4]=a4; a[5]=a5; }
void ld(const double &a0,const double &a1,const double &a2,const double &a3,const double &a4,const double &a5,const double &a6) { a[0]=a0; a[1]=a1; a[2]=a2; a[3]=a3; a[4]=a4; a[5]=a5; a[6]=a6; }
void ld(const double &a0,const double &a1,const double &a2,const double &a3,const double &a4,const double &a5,const double &a6,const double &a7) { a[0]=a0; a[1]=a1; a[2]=a2; a[3]=a3; a[4]=a4; a[5]=a5; a[6]=a6; a[7]=a7; }
void ld(const double &a0,const double &a1,const double &a2,const double &a3,const double &a4,const double &a5,const double &a6,const double &a7,const double &a8) { a[0]=a0; a[1]=a1; a[2]=a2; a[3]=a3; a[4]=a4; a[5]=a5; a[6]=a6; a[7]=a7; a[8]=a8; }
void ld(const double &a0,const double &a1,const double &a2,const double &a3,const double &a4,const double &a5,const double &a6,const double &a7,const double &a8,const double &a9) { a[0]=a0; a[1]=a1; a[2]=a2; a[3]=a3; a[4]=a4; a[5]=a5; a[6]=a6; a[7]=a7; a[8]=a8; a[9]=a9; }
};
//---------------------------------------------------------------------------
template <DWORD N> class matrix // square matrix
{
public:
vector<N> a[N];
matrix() {}
matrix(matrix& a) { *this=a; }
~matrix() {}
matrix* operator = (const matrix<N> *a) { *this=*a; return this; }
//matrix* operator = (matrix<N> &a) { ...copy... return this; }
vector<N>& operator [](const int i) { return a[i]; }
matrix<N> operator + () { return *this; }
matrix<N> operator - () { matrix<N> q; int i,j; for (i=0;i<M;i++) for (j=0;j<N;j++) q[i][j]=-a[i][j]; return q; } // = -m0
matrix<N> operator * (const matrix &m)
{
matrix<N> q;
int i,j,k;
for (i=0;i<N;i++)
for (j=0;j<N;j++)
for (q.a[i][j]=0.0,k=0;k<N;k++)
q.a[i].a[j]+=a[i].a[k]*m.a[k].a[j];
return q;
}
vector<N> operator * (vector<N> &v)
{
vector<N> q;
int i,j;
for (i=0;i<N;i++)
for (q.a[i]=0.0,j=0;j<N;j++)
q.a[i]+=a[i][j]*v.a[j];
return q;
}
matrix<N> operator * (const double &c)
{
matrix<N> q;
int i,j;
for (i=0;i<N;i++)
for (j=0;j<N;j++)
q.a[i].a[j]=a[i].a[j]*c;
return q;
}
matrix<N> operator / (const double &c)
{
return this[0]*divide(1.0,c);
}
matrix<N> operator *=(matrix<N> &m) { this[0]=this[0]*m; return *this; };
vector<N> operator *=(vector<N> &v) { this[0]=this[0]*v; return *this; };
matrix<N> operator *=(const double &c) { this[0]=this[0]*c; return *this; };
matrix<N> operator /=(const double &c) { this[0]=this[0]/c; return *this; };
AnsiString str() { int i,j; AnsiString q; for (q="",i=0;i<N;i++,q+="\r\n") { for (q+="( ",j=0;j<N;j++) q+=AnsiString().sprintf("%6.3lf ",a[i][j]); q+=")"; } return q; }
void unit() { int i,j; for (i=0;i<N;a[i][i]=1.0,i++) for (j=0;j<N;j++) a[i][j]=0.0; } // set unit matrix
void zero() { int i,j; for (i=0;i<N;i++) for (j=0;j<N;j++) a[i][j]=0.0; } // set zero matrix
void rnd() { int i,j; for (i=0;i<N;i++) for (j=0;j<N;j++) a[i][j]=(2.0*Random())-1.0; } // set random <-1,+1> matrix
void set(double c) { int i,j; for (i=0;i<N;i++) for (j=0;j<N;j++) a[i][j]=c; } // (c,c,c,c,...)
void orthonormal() // convert to orthonormal matrix
{
int i,j;
vector<N> *pV[N],*pp;
for (i=0;i<N;i++) { a[i].unit(); pV[i]=a+i; }
for (i=1;i<N;i++)
{
pV[0]->cross(pV+1);
pp=pV[0]; for (j=1;j<N;j++) pV[j-1]=pV[j]; pV[N-1]=pp;
}
}
matrix<N> transpose()
{
int i,j;
matrix<N> M;
for (i=0;i<N;i++)
for (j=0;j<N;j++)
M[i][j]=a[j][i];
return M;
}
matrix<N> inverse()
{
return adjugate()/det();
}
matrix<N> adjugate()
{
matrix<N> C;
double s;
int i,j;
for (i=0;i<N;i++)
for ((i&1)?s=-1.0:s=+1.0,j=0;j<N;j++,s=-s)
C[j][i]=minor(i,j)*s;
return C;
}
matrix<N> cofactor()
{
matrix<N> C;
double s;
int i,j;
for (i=0;i<N;i++)
for ((i&1)?s=+1.0:s=-1.0,j=0;j<N;j++,s=-s)
C[i][j]=minor(i,j)*s;
return C;
}
double minor(int i,int j)
{
return submatrix(i,j).det();
}
matrix<N-1> submatrix(int i,int j)
{
matrix<N-1> m;
int i0,i1,j0,j1;
for (i0=0,i1=0;i1<N;i1++)
if (i1!=i){ for (j0=0,j1=0;j1<N;j1++)
if (j1!=j){ m.a[i0][j0]=a[i1][j1]; j0++; } i0++; }
return m;
}
double det();
};
//---------------------------------------------------------------------------
double matrix<1>::det() { return a[0][0]; }
double matrix<2>::det() { return (a[0][0]*a[1][1])-(a[0][1]*a[1][0]); }
template <DWORD N> double matrix<N>::det()
{
double d=0.0; int j;
matrix<N-1> m;
for (j=0;j<N;j++)
{
m=submatrix(0,j);
if (int(j&1)==0) d+=a[0][j]*m.det();
else d-=a[0][j]*m.det();
}
return d;
}
//---------------------------------------------------------------------------
#endif
//---------------------------------------------------------------------------
But as you can see that code is a bit more complicated to follow as I am in a different coding level now (look for inverse)...
If you need also results then compute it as matrix equation:
A*X = Y
X = inv(A)*Y
Where X are unknowns (vector) , Y are knowns (vector) and A is the matrix.
来源：https://stackoverflow.com/questions/55434303/gaussian-elimination-without-result-for-acceleration