哥德巴赫曾猜想

#include<stdio.h>#include<math.h>int isprime(int n){int a=0;int k=(int)sqrt(n);for(int i=2;i<=k;i++){if(n%i==0){a=1;break;}}return a;}int main(){float p;int a[20000]={0};int i,j,q,n,m,l;int x=0;int y=0;int sum=0;for(i=2;i<=20000;i++){q=0;         p=sqrt(i); for(j=2;j<=p;j++) { if(i%j==0) {    q=1;  break; } } if(q==0) {             a[x++]=i; }}/*for(int k=0;k<x;k++){printf("%d ",a[k]);}*/scanf("%d",&n);m=n/2;    while(a[y]<=m){         l=n-a[y]; if(isprime(l)==0) { sum++; } y++;}printf("%d",sum);return 0;    }