算法：输入一个十进制数，根据输入的数输出所对应的英文星期单词，若输入的数小于1或大于7，则输出ERROR。

#include "stdio.h"

int main(void)
{
int day = 0;

printf("请输入1~7之间的数字：");
scanf_s("%d", &day);

switch (day)
{
case 1:
printf("Monday!\n");break;
case 2:
printf("Tuesday!\n");break;
case 3:
printf("Wednesday!\n");break;
case 4:
printf("Thursday!\n");break;
case 5:
printf("Friday!\n");break;
case 6:
printf("Saturday!\n");break;
case 7:
printf("Sunday!\n");break;
default:
printf("ERROR!\n");break;
}

return 0;
}