我自己写的一个计算一年中某一天在该年是第几天：
#include<stdio.h>
void main()
{
    int y,m,d,total=0;
    printf("please input y空格m空格d:");
    scanf("y/m/d=%d%d%d",&y,&m,&d);
    if(m==1)
        printf("total=%d",d);

    if(m==2)
        printf("total=%d",d+31);

    if(m==3)
    {
        if(y%4==0&&(y%100!=0)||y%400==0)
            printf("total=%d",d+31+29);
        else
            printf("total=%d",d+31+28);
    }

        if(m==4)
    {
        if(y%4==0&&y%100!=0||y%400==0)
            printf("total=%d",d+31+29+31);
        else
            printf("total=%d",d+31+28+31);
    }

        if(m==5)
    {
        if(y%4==0&&y%100!=0||y%400==0)
            printf("total=%d",d+31+29+31+30);
        else
            printf("total=%d",d+31+28+31+30);
    }

        if(m==6)
    {
        if(y%4==0&&y%100!=0||y%400==0)
            printf("total=%d",d+31+29+31+30+31);
        else
            printf("total=%d",d+31+28+31+30+31);
    }

        if(m==7)
    {
        if(y%4==0&&y%100!=0||y%400==0)
            printf("total=%d",d+31+29+31+30+31+30);
        else
            printf("total=%d",d+31+28+31+30+31+30);
    }

        if(m==8)
    {
        if(y%4==0&&y%100!=0||y%400==0)
            printf("total=%d",d+31+29+31+30+31+30+31);
        else
            printf("total=%d",d+31+28+31+30+31+30+31);
    }

        if(m==9)
    {
        if(y%4==0&&y%100!=0||y%400==0)
            printf("total=%d",d+31+29+31+30+31+30+31+31);
        else
            printf("total=%d",d+31+28+31+30+31+30+31+31);
    }

        if(m==10)
    {
        if(y%4==0&&y%100!=0||y%400==0)
            printf("total=%d",d+31+29+31+30+31+30+31+31+30);
        else
            printf("total=%d",d+31+28+31+30+31+30+31+31+30);
    }

        if(m==11)
    {
        if(y%4==0&&y%100!=0||y%400==0)
            printf("total=%d",d+31+29+31+30+31+30+31+31+30+31);
        else
            printf("total=%d",d+31+28+31+30+31+30+31+31+30+31);
    }

        if(m==12)
    {
        if(y%4==0&&y%100!=0||y%400==0)
            printf("total=%d",d+31+29+31+30+31+30+31+31+30+31+30);
        else
            printf("total=%d",d+31+28+31+30+31+30+31+31+30+31+30);
    }

}

运行不起啊