#include <iostream>
#include <time.h>
#include <cmath>
#include <stdio.h>
using namespace std;
double max(double x[],int n)
{
    double max_=abs(x[0]);
    for(int i=1;i<n;i++)
        if(abs(x[i])>max_)
            max_=abs(x[i]);
    return max_;
}

int main()
{
    double a[4][4]={{5,1,-1,-2},
                 {2,8,1,3},
                 {1,-2,-4,-1},
                 {-1,3,2,7}};
    double b[4]={-2,-6,6,12},x[4]={1,1,1,1},diff[4],s,w=1.15;//取鬆弛因子為1.15
    int i,j,count=1;
    do
    {
        for(i=0;i<4;i++)
        {
            s=0;
            for(j=0;j<4;j++)
                s=s+a[i][j]*x[j];
            diff[i]=w*(b[i]-s)/a[i][i];
            x[i]+=diff[i];
        }
        printf("%-3d: ",count++);
        for(i=0;i<4;i++)
            printf("%-10lf  ",x[i]);
        cout<<endl;
    }while(max(diff,4)>10e-6);
    return 0;
}