编写一个程序使它打印如下图形；例如，如果程序读入的边长是4，那么它应该显示：

* * * *
*     *
*     *
* * * *

请问如何编写，请各位好心人指导.

#include <stdio.h>
int main()
{
    int num, j, i;
    printf("请输入一边长:\n");
    scanf("%d", &num);
    for(i = 1; i <= num; i++)
    {
        if(i == 1 || i == num)
        {
            for(j = 1; j <= num; j++)
            {
                printf("* ");
            }
            printf("\n");
        }
        else
        {
            for(j = 1; j <= num; j++)
            {
                if(j == 1)
                {
                    printf("* ");
                }
                else if(j == num)
                {
                    printf("*");
                }
                else
                {
                    printf("  ");
                }
            }
            printf("\n");
        }
    }
}

thank you very much