用C语音实现找出1~1000 内的所有完数，并按以下格式输出，6 its factors are 1,2,3。

完数是指一个数恰好等于它的因子之和，例如6是完数，因为6的因子为1、2、3，且1+2+3=6。

以下是用C语言实现找出1~1000内的所有完数，并按照题目要求输出的代码：

#include <stdio.h>

int main()
{
    int i, j, sum;
    printf("Perfect numbers between 1 and 1000:\n");
    for(i=1; i<=1000; i++){
        sum = 0;
        for(j=1; j<i; j++){
            if(i%j == 0){
                sum += j;
            }
        }
        if(sum == i){
            printf("%d its factors are ", i);
            for(j=1; j<i; j++){
                if(i%j == 0){
                    printf("%d,", j);
                }
            }
            printf("\b.\n");  // 去掉最后一个逗号
        }
    }
    return 0;
}

运行结果：

Perfect numbers between 1 and 1000:
6 its factors are 1,2,3.
28 its factors are 1,2,4,7,14.
496 its factors are 1,2,4,8,16,31,62,124,248.