#include<stdio.h>
void main()
{
char q;	
int i=1,c,m;
double a[100],b[100],d[100],x,y=0,j=1,p,sum=1;

printf("當Xi和Yi同時輸入911就結束輸入節點\n");

printf("x1=");
scanf("%lf",&a[1]);
printf("y1=");
scanf("%lf",&b[1]);
while(a[i]!=911||b[i]!=911)
{i++;
printf("x%d=",i);
scanf("%lf",&a[i]);
printf("y%d=",i);
scanf("%lf",&b[i]);
if(i>100)
printf("error");
}
c=i-1;

for(i=c;i>=1;i--)
{p=a[i];
   if(p==a[1])
   {for(m=c;m>=2;m--)
     sum=sum*(p-a[m]);
   }
   else{
    for(m=c;m>=1;m--)
{if(p==a[m])
	m=m-1;
sum=sum*(p-a[m]);
}
   }

d[i]=sum;
sum=1;
}

ander:printf("x=");
scanf("%lf",&x);

for(i=c;i>=1;i--)
{if(x==a[i])
{i=i+c;
break;
}
}

if(i>c)
printf("y=%f\n",b[i-c]);
else
{
for(i=c;i>=1;i--)
j=j*(x-a[i]);

for(i=c;i>=1;i--)
{

y=y+(b[i]*j)/((x-a[i])*d[i]);
}
printf("y=%lf\n",y);

j=1;
y=0;
 
}	

printf("是否繼續(y/n)?\n");
getchar();
scanf("%c",&q);
if(q=='y')
goto ander;
else 
printf("謝謝使用\n"); 

}