default:printf("一年只有十二個月")；break;

這里分號錯了，換成英文的

default:printf("一年只有十二個月");break;

if(year%400==0||(year%4==0&&year%100!=0)

這里缺了)

if(year%400==0||(year%4==0&&year%100!=0))

?printf("%d年%d月%d日是該年的第%d天"，year,month,day,sum);

這里逗號錯了，換成英文的

printf("%d年%d月%d日是該年的第%d天",year,month,day,sum);

2個retrue 0;}，刪掉一個即可

完整的代碼

#include?<stdio.h>
int?main()?
{?
????/*?定義需要計算的日期?*/
????int?year?=?2008;
????int?month?=?8;
????int?day?=?8;
????/*
?????*?請使用swtich語句，if...else語句完成本題
?????*?如有想看小編思路的，可以點擊左側(cè)任務中的“不會了怎么辦”
?????*?小編還是希望大家獨立完成哦~
?????*/
????int?sum,flag;
????switch(month)
????{
????????case?1:sum=0;break;
????????case?2:sum=31;break;
????????case?3:sum=59;break;
????????case?4:sum=90;break;
????????case?5:sum=120;break;
????????case?6:sum=151;break;
????????case?7:sum=181;break;
????????case?8:sum=212;break;
????????case?9:sum=243;break;
????????case?10:sum=273;break;
????????case?11:sum=304;break;
????????case?12:sum=334;break;
????????default:printf("一年只有十二個月");break;
????}
????sum=sum+day;
????if(year%400==0||(year%4==0&&year%100!=0))
????????flag=1;
????else
????????flag=0;
????if(flag=1&&month>2)????
????sum++;????
????printf("%d年%d月%d日是該年的第%d天",year,month,day,sum);
????return?0;
}