7_10.c

/* 【例7-10】定义函数day_of_year(year, month, day)，计算并返回年year、月month和日day对应的是该年的第几天。 */

# include <stdio.h>

/* 计算某个日期对应该年的第几天 */
int day_of_year(int year, int month, int day);

int main() {
    int day, month, year;

    printf("Enter year, month, day: ");
    scanf("%d%d%d", &year, &month, &day);

    printf("%d\n", day_of_year(year, month, day));

    return 0;
}

int day_of_year(int year, int month, int day) {
    int k, leap;

    /* 数组初始化，将每月的天数赋给数组 */
    int tab[2][13] = {
            {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
            {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
    };

    /* 判断闰年，当year是闰年时，leap=1；当year是非闰年时，leap=0 */
    leap = (year % 4 == 0 && year % 100 != 0 || year % 400 == 0);

    /* 计算天数 */
    for (k = 1; k < month; k++) {
        day = day + tab[leap][k];
    }

    return day;
}