题目：http://vjudge.net/problem/HDU-1202
分析：这题好坑爹啊，80 - 89 4点，让人情不自禁设置成80<=i<=89，那么89.5算不算呢？还有就是全部缺考的情况也需要考虑，除法要有意义。
1 #include <cstdio> 2 #include <cstring> 3 #include <cmath> 4 #include <iostream> 5 #include <algorithm> 6 #include <string> 7 #include <cstdlib> 8 9 using namespace std; 10 11 int main() 12 { 13 int n,i; 14 double s[10010],p[10010],sum_s,sum_p; 15 while(~scanf("%d",&n)) 16 { 17 sum_s=0,sum_p=0; 18 for(i=1;i<=n;i++) 19 { 20 scanf("%lf %lf",&s[i],&p[i]); 21 if (p[i]==-1) continue; 22 else 23 { 24 if(90<=p[i]) p[i]=4; 25 else if(80<=p[i]) p[i]=3; 26 else if(70<=p[i]) p[i]=2; 27 else if(60<=p[i]) p[i]=1; 28 else if(0<=p[i]) p[i]=0; 29 sum_s+=s[i],sum_p+=s[i]*p[i]; 30 } 31 } 32 if(sum_s) printf("%.2lf ",1.0*sum_p/sum_s); 33 else printf("-1 "); 34 } 35 return 0; 36 }