完整源代码:

#include<stdio.h>
#include"4.17.h"
#include <stdio.h>
struct Date //定义一个结构体变量Date
{
	int year;
	int month;
	int day;
}date;

int days(int year, int month, int day) //days函数判断该日是本年的第几天
{
	int a[12] = { 31,28,31,30,31,30,31,31,30,31,30,31 }; //数组a中存放12个月的天数
	int sum = 0, i;//sum为天数,i实现天数累加
	for (i = 0; i < month - 1; i++)//如:五月则对1-4月天数求和
	{
		sum += a[i];
	}
	sum += day;
	if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
	//判断该年是否是闰年, 如果是闰年则在天数上加1
	{
		if (month > 2)
			sum = sum + 1;
		else
			sum = sum + 0;
	}
	else
			sum = sum + 0;
	return sum; 
}
int main()
{
	int sum = 0;
	printf("请输入年 - 月 - 日\n");
	scanf_s("%d-%d-%d", &date.year, &date.month, &date.day);
	sum = days(date.year, date.month, date.day); 
	printf("该日是%d年的第%d天", date.year,sum);
}