哪里出了问题。。。题目是输入年月日得到是该年的第几天
#include<stdio.h>
int main()
{
    int a,b,c,n;
    scanf("%d,%d,%d",&a,&b,&c);
    if(b=1);
        n=c;
    else if(b=2);
        n=31+c;
    else if (b=3||b=5||b=7||b=9||b=11);
        n=28+(b-1)/2*31+(b-3)/2*30+c;
    else if (b=4||b=6||b=8||b=10||b=12);
        n=28+b/2*31+(b-4)/2*30+c;
    if (a%4==0&&a%400=0&&b>2);
        n++;
    printf("n=%d",n);
}

#include<stdio.h>
int main()
{
    int a,b,c,n;
    scanf("%d,%d,%d",&a,&b,&c);
    if(b=1);
        n=c;
    else if(b=2); 粗心大意，if(b=2);  后面还加了个分号。  b在if中是用于判断，不是用于赋值。b=2跟b==2完全是两回事。下面标记的全是这样的语法错误
        n=31+c;
    else if (b=3||b=5||b=7||b=9||b=11);
        n=28+(b-1)/2*31+(b-3)/2*30+c;
    else if (b=4||b=6||b=8||b=10||b=12);
        n=28+b/2*31+(b-4)/2*30+c;
    if (a%4==0&&a%400=0&&b>2);
        n++;
    printf("n=%d",n);
}

#include<stdio.h>
#define S_JAN 0
#define S_FEB 31
#define S_MAR 59
#define S_APR 90
#define S_MAY 120
#define S_JUN 151
#define S_JUL 181
#define S_AUG 212
#define S_SEP 243
#define S_OCT 273
#define S_NOV 304
#define S_DEC 334

int main(void)
{
    int date,year,month,day,number,n;
    printf("请按格式输入一个真实存在的日期（xxxx年xx月xx日）例如：20180411\n");
    printf("本程序不负责验证日期是否真实存在。^_^。\n");
    scanf("%d",&date);
    year = date / 10000;
    month = (date - year * 10000) / 100;
    day = (date - year * 10000 - month * 100);
    if ((year % 100 != 0 && year % 4 == 0)|| year % 400 == 0)
        {
            n = 1;
            printf("%d年是闰年。\n",year);
        }
    else
        {
            n = 0;
            printf("%d年不是闰年。\n",year);
        }
    switch (month)
        {
            case 1:number = day + S_JAN;break;
            case 2:number = day + S_FEB;break;
            case 3:number = day + S_MAR + n;break;
            case 4:number = day + S_APR + n;break;
            case 5:number = day + S_MAY + n;break;
            case 6:number = day + S_JUN + n;break;
            case 7:number = day + S_JUL + n;break;
            case 8:number = day + S_AUG + n;break;
            case 9:number = day + S_SEP + n;break;
            case 10:number = day + S_OCT + n;break;
            case 11:number = day + S_NOV + n;break;
            case 12:number = day + S_DEC + n;break;
            default : break;
        }
    printf("%d年%d月%d日是%d年的第%d天。\n",year,month,day,year,number);
    return 0;
}