#include "stdio.h"
extern yearday(int year,int month,int day);
int monthday[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}};
   
main()
{
    int y,m,d;
    printf("please give year:");
    scanf("%d \ n",&y);
    printf("please give month:");
    scanf("%d \ n",&m);
    printf("please give day:");
    scanf("%d \ n",&d);
    printf("The day of the year is = %d.",yearday(y,m,d));        
    getch();
}
int yearday(int year,int month,int day)
{
    int i,leap;
    leap = year%4 == 0 && year%100 !=0 || year%400==0;
    for(i=1;i<month;i++)
        day+=monthday[leap][i];
    return(day);
}

编译成功，运行不对，请各位帮忙也测试一下。

[[italic] 本帖最后由 stevending 于 2007-11-28 22:14 编辑 [/italic]]

不明白你的题要做什么~并且有两个地方看的不是很清楚~extern yearday(int year,int month,int day);用extern 定义是什么意思~

  leap = year%4 == 0 && year%100 !=0 || year%400==0;
放在这里有什么用~又没括号~也没if语句~

去掉scanf语句中的 n就OK了， 这是一个输入年月日，计算这一天是这一年的第多少天的小程序。

你说的那一句是求其是否为闰年的公式，运算符号有优先级别，故无需加括号区隔开。

/* 输入：年、月、日，输出该年过去了几天 */

#include<stdio.h>

int main(void)
{
    int year, month, day, ndays, i;
    int ds[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
   
    printf("Please input year, month, day: ");
    while(scanf("%d %d %d", &year, &month, &day) == 3)
    {
        while(getchar() != '\n');
        
        if(year % 4 == 0)
            ds[1] == 29;
        else
            ds[2] == 28;

        if(year < 0 || month < 1 || month > 12 || day > ds[month - 1])
        {
            printf("Error!\n\nPlease input year,month,day: ");
            continue;
        }
        
        for(i = ndays = 0; i < month - 1; i++)
            ndays += ds[i];
        ndays += day;
        printf(" : %d\n\n", ndays);
        printf("Please input year, month, day: ");
    }

    puts("Done.");
    return 0;
}



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