コード:

#include <stdio.h>

int getIteration(void)
{
    int n;

    do {
        printf("0以上の整数を入力＞ ");
        scanf("%d", &n);
        if (n >= 0) break;
        printf("入力エラー\n");
    } while (n < 0);
    return n;
}

double calcPI_4(int n)
{
    double sum, x;
    int i;

    for (sum = i = 0; i <= n; i++) {
        x = 2 * i + 1;
        if (i % 2 != 0) x = -x;
        sum += 1 / x;
    }
    return sum;
}

void printAnswer(double s)
{
    printf("%f\n", s);
}

int main(void)
{
    printAnswer(calcPI_4(getIteration()));
    return 0;
}

コード:

sum += (1/sign);

コード:

#include <stdio.h>

int main(void)
{
    int i, n, sign;
    double sum;
    n = 0;
    while (1) {
        printf("0以上の整数を入力＞ ");
        scanf("%d", &n);
        if (n>0) {
            break;
        } else {
            printf("入力エラー\n");
    
    }    
    sign = 1;
    sum = 0;
    for (sign=0; sign<=n ; sign++) {
        sum += (1/sign);
        sign *= -1;
    }
    printf("%f\n", sum);
    return 0;
}