Second class 1007 test of PAT And The feasible fast algorithm to compute all prim numbers in a range

Second class 1007 test of PAT And The feasible fast algorithm to compute all prim numbers in a range

Topic：

   “素数对猜想”认为“存在无穷多对相邻且差为2的素数”。现给定任意正整数N(<10^​5​​ )，请计算不超过N的满足猜想的素数对的个数。

Code:

#include <stdio.h>
//
int main()
{int n;int i;char *arr = NULL;int prim = 2;int counter = 0;scanf("%d", &n);arr = (char *)calloc(sizeof(char), n);for(i = 2; i <= n; i++){if(arr[i] == 0){int j;for(j = i + i; j < n; j += i)arr[j] = 1;if(i - prim == 2)counter++;prim = i;}}printf("%d", counter);free(arr);char *b = (char *)calloc(sizeof(char), 100000);free(b);
}

Time : 2020.6.10