#include "stdlib.h"
#include "math.h"
#include "stdio.h"

int Guass(a,b,n)
int n;
double a[],b[];
{ 
	int *js,l,k,i,j,is,p,q;
	double d,t;
	js=malloc(n*sizeof(int)); 
	l=1;
	for (k=0;k<=n-2;k++)
	{ 
		d=0.0;
		/*下面是換主元部分，即從系數(shù)矩陣A的第K行，第K列之下的部分選出
		絕對值最大的元，交換到對角線上。*/
		for (i=k;i<=n-1;i++)
			for (j=k;j<=n-1;j++)
			{ 
				t=fabs(a[i*n+j]); /*fabs()定義在math.h中，含義是求一個浮點(diǎn)數(shù)的絕對值。*/
				if (t>d) { d=t; js[k]=j; is=i;}
			}
		if (d+1.0==1.0) l=0;  /*主元為0*/
		/*主元不為0的時候*/
		else 
		{ 
			if (js[k]!=k) 
				for (i=0;i<=n-1;i++)
				{ 
					p=i*n+k; q=i*n+js[k];
					t=a[p]; a[p]=a[q]; a[q]=t;
				}
			if (is!=k)
			{ 
				for (j=k;j<=n-1;j++)
				{ 
					p=k*n+j; q=is*n+j;
					t=a[p]; a[p]=a[q]; a[q]=t;
				}
				t=b[k]; b[k]=b[is]; b[is]=t;
			}
		}
		if (l==0)
		{ 
			free(js); printf("fail\n");
			return(0);
		}
		d=a[k*n+k];
		/*下面為歸一化部分*/
		for (j=k+1;j<=n-1;j++)
		{ 
			p=k*n+j; a[p]=a[p]/d;
		}
		b[k]=b[k]/d;
		/*下面為矩陣A,B消元部分*/
		for (i=k+1;i<=n-1;i++)
		{ 
			for (j=k+1;j<=n-1;j++)
			{ 
				p=i*n+j;
				a[p]=a[p]-a[i*n+k]*a[k*n+j];
			}
			b[i]=b[i]-a[i*n+k]*b[k];
		}
	}

	d=a[(n-1)*n+n-1];
	/*矩陣無解或有無限多解*/
	if (fabs(d)+1.0==1.0) 
	{ 
		free(js); printf("該矩陣為奇異矩陣\n");
		return(0);
	}
	b[n-1]=b[n-1]/d;
	/*下面為迭代消元*/
	for (i=n-2;i>=0;i--)
	{ 
		t=0.0;
		for (j=i+1;j<=n-1;j++)
			t=t+a[i*n+j]*b[j];
		b[i]=b[i]-t;
	}
	js[n-1]=n-1;
	for (k=n-1;k>=0;k--)
		if (js[k]!=k)
		{ t=b[k]; b[k]=b[js[k]]; b[js[k]]=t;}
	free(js);
	return(1);
}

main()
{ 
	int i;
	static double a[4][4]= /*系數(shù)矩陣A*/
		{ {0.2368,0.2471,0.2568,1.2671},
		{0.1968,0.2071,1.2168,0.2271},
		{0.1581,1.1675,0.1768,0.1871},
		{1.1161,0.1254,0.1397,0.1490} };
	static double b[4]={1.8471,1.7471,1.6471,1.5471}; /*代數(shù)方程組Ax=B的B部分*/
	if (Guass(a,b,4)!=0) /*調(diào)用Guass消去，1為計(jì)算成功*/
		for (i=0;i<=3;i++) /*打印結(jié)果*/
			printf("x(%d)=%e\n",i,b[i]);
}