方法一：

#include <stdio.h> 
#include <math.h> 

int main(void)
{
    int n;
    scanf("%d", &n);

    int i;
    double sum = 0;
    double sign;

    for (i=1; i<=n; i++)
    {
        sign = pow(-1, i+1);
        sum = sum + 1.0/(sign*i);
    }

    printf("f(%d) = %f\n", n, sum);

    return 0;
}
 

方法二：

#include <stdio.h> 

int main(void)
{
    int n;
    scanf("%d", &n);

    int i;
    double sum = 0;
    double sign = 1.0;

    for (i=1; i<=n; i++)
    {
        sum = sum + sign/i;
        sign = -sign;
    }

    printf("f(%d) = %f\n", n, sum);

    return 0;
}