请教一道C语言数列求和编程

Question

函数fun是根据输入n，求下面分母为n以内(包含n)的素数构成的数列的和 ：s=1/2-2/3+3/5-4/7+5/11-...，输入10，输出：-0.138095。
要求在/Begin/和/End/之间编写程序，其它部分不得增删语句。

#include<stdio.h>
#include<stdlib.h>
double fun(int n)
{
/Begin/

/End/
}
int main()
{
int n;
double s,x;
FILE *in,*out;
printf("输入n:");
scanf("%d",&n);
s=fun(n);
printf("%lf\n",s);

in=fopen("in2019-2-1-2.dat","r");
out=fopen("out2019-2-1-2.dat","w");
while(!feof(in))
{
fscanf(in,"%d",&n);
fprintf(out,"%lf\n",fun(n));
}
system("pause");

Answer 1

供参考：

#include<stdio.h>
#include<stdlib.h>
double fun(int n)
{
    /*Begin*/
    int i, j, k = 1, t = 1;
    double s = 0;
    if (n < 2)  return s;
    for (i = 2; i <= n; i++)
    {
        for (j = 2; j < i; j++)
            if (i % j == 0) break;
        if (i == j) {
            s += t * k * 1.0 / i;
            k++;
            t *= -1;
        }
    }
    return s;
    /*End */
}
int main()
{
    int n;
    double s, x;
    FILE* in, * out;
    printf("输入n:");
    scanf("%d", &n);
    s = fun(n);
    printf("%lf\n", s);

    in = fopen("in2019-2-1-2.dat", "r");
    out = fopen("out2019-2-1-2.dat", "w");
    while (!feof(in))
    {
        fscanf(in, "%d", &n);
        fprintf(out, "%lf\n", fun(n));
    }
    system("pause");
    return 0;
}