#include<stdio.h>
#include<math.h>
int main()
{
    double A[3][3]={{10,1,2},{2,10,1},{1,2,10}};
     double b[5]={44,51,61};
     double x[3]={0,0,0}; //第k+1次迭代的结果
     double xx[3]={0,0,0}; //第k次迭代的结果
    int size=3;
    int Max=100;  //最大迭代次数
     double r=0.0;  //
     double sum=0.0;
     double dis=0.0;
     double dif=1.0;  //相邻迭代的结果差
    double p=0; //迭代精度
     for(int k=1;(k<Max)&&(dif>p);k++)
     {
          dif=0.0;
          for(int i=0;i<size;i++)
        {
               for(int j=0;j<size;j++)
               {
                if(j<i)
                {
                     sum+=A[i][j]*x[j];//本次的x[j]
                }
                if(j>i)
                    {
                     sum+=A[i][j]*xx[j];//上一次的x[j]
                }
               }
               x[i] = (b[i]-sum)/A[i][i];
               sum=0.0;
          }
          r=0.0;//计算相邻迭代的结果差
          for(int m=0;m<size;m++)
          {
            dis=fabs(x[m]-xx[m]);
               if(dis>r)
                r=dis;
          }
         dif=r;//打印第k次的结果
          printf("/n",k);
          for (int i=0;i<size;i++)
          {
               printf("%12.8f ",x[i]);
               xx[i]=x[i];
          }
          printf("\n与上次计算结果的差:%12.8f \n",dif);
     }
     printf("\n迭代计算的结果为：\n");
     for(int k=0;k<size;k++)
     {
          printf("%12.8f ",xx[k]);
     }
     printf("\n");
     return 0;
}

简单说说呀：
     for(int k=1;(k<Max)&&(dif>p);k++)
     {
          dif=0.0;
          for(int i=0;i<size;i++)
        {
               for(int j=0;j<size;j++)
               {
                if(j<i)
                {
                     sum+=A[i][j]*x[j];//本次的x[j]
                }
                if(j>i)
                    {
                     sum+=A[i][j]*xx[j];//上一次的x[j]
                }
               }
               x[i] = (b[i]-sum)/A[i][i];
               sum=0.0;
          }
          r=0.0;//计算相邻迭代的结果差
          for(int m=0;m<size;m++)
          {
            dis=fabs(x[m]-xx[m]);
               if(dis>r)
                r=dis;
          }
         dif=r;//打印第k次的结果
          printf("/n",k);                           /*里面应该是\n */
          for (int i=0;i<size;i++)                  /*这里 去掉int 因为旧的C标准不允许 */
          {
               printf("%12.8f ",x[i]);
               xx[i]=x[i];
          }
          printf("\n与上次计算结果的差:%12.8f \n",dif);
     }
     printf("\n迭代计算的结果为：\n");
     for(int k=0;k<size;k++)                /*这里 去掉int 因为旧的C标准不允许 */
     {
          printf("%12.8f ",xx[k]);
     }
     printf("\n");
     return 0;
}
迭代结果是 3 4  5

在C语言中，定义变量最好不要在for循环里面

重复定义