#include<stdio.h>
int main()
{
    int duichen(int data[],int length);
    int a[4]={1,2,2,1};
    duichen(a,4);
    return 0;
}
int duichen( int data[],int length)//函数的作用是判断数组是否对称
{         
         int i=0;
         int m,j;
         if(length/2!=0)//当length为奇数时
             printf("不对称!\n");
          else
          {
              m=length/2;
              for(i=0;i<m;i++)
              for(j=length-1;j>=m;j--)
              {
                  if(data[i]==data[j] && length/2==0 )
                      continue;
              }
                  printf("是对称的!\n");
            
          }
                  return 0;
          }

if(length/2!=0)  改为 if(length%2!=0)

#include<stdio.h>
int main()
{
    int duichen(int data[],int length);
    int a[4]={1,2,2,1};
    duichen(a,4);
    return 0;
}
int duichen( int data[],int length)//函数的作用是判断数组是否对称
{         
         int i=0,flag=1;              //我设置了一个标志位，判断是否对称
         int m,j;
         if(length%2!=0)//当length为奇数时             是求余，不是除
             printf("不对称!\n");
          else
          {
              m=length/2;
              for(i=0;i<m;i++)             //你这里的比较是否对称的算法是错误的，仔细想想，你的内循环j值在变，而i没有变化时什么情况？
              {
                  if(data[i]==data[length-i-1])       //这是我改的
                      continue;
                  else
                  {
                      flag=0;
                      break;
                  }
              }
            
          }
          if(flag)
              printf("是对称的！\n");
          else
              printf("不对称！\n");
          return 0;
}

楼上正解