//a= coefficient matrix
//b= rhs column vector
//n= # of equations
//x= solution vector
//tol= tolerance (detection of values close to zero)
//er=determine if singular (if so er=-1)

double s[n];
int er = 0;
for(int i=0; i<n; i++)
{
	s[i]=fabs(a[i][1]);
	for(int j=1; j<n; j++)
	{
		if(fabs(a[i][j])>s[i])
			s[i]=fabs(a[i][j]);
	}
}
//Eliminate
for(int k = 0; k < n-1; k++)
{
	// go to pivot
       int p=k;
	double big = fabs(a[k][k]/s[k]);
	for(int ii = k+1; k<n)
       {
       	dummy = fabs(a[ii][k]/s[ii]);
       	if(dummy>big)
       {
       		big=dummy;
       p=ii;
		}
	}
	if(p != k)
	{
		for(int jj = k; jj < n; jj++)
		{
			dummy = a[p][jj];
			a[p][jj] = a[k][jj];
			a[k][jj] = dummy;
		}
		dummy = b[p];
		b[p] = b[k];
		b[k] = dummy;
		dummy = s[p];
		s[p] = s[k];
		s[k] = dummy;
	}
	if(fabs(a[k][k]/s[k])<tol)
	{
		er = -1;
		break;
	}
	for(int i = k+1; i<n; i++)
       {
       	factor = a[i][k]/a[k][k];
       	for(int j = k+1; j<n; j++)
       {
       	a[i][j] -= factor*a[k][j];
       }
       b[i] -= factor*b[k];
	}
}



if(er != -1)
{