c - 在 c 编程中舍入长十进制值

Question

我被这个问题困住了。
我在一些变量中得到双值，如下所示：

a = 0.76271469999999997000
b = 0.66698279999999999000
c = 0.34262199999999998000

我需要将这些四舍五入

rounded_a = 0.762714700000000
rounded_b = 0.666982800000000
rounded_c = 0.342622000000000

我该怎么做呢？

Answer 1

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

double round_n(double x, int n){
    double wk;
    wk = pow(10.0, (double)n);
    return round(x * wk)/wk;//c99 function
}

int main(void){
    double a, b, c;
    a = 0.76271469999999997000;
    b = 0.66698279999999999000;
    c = 0.34262199999999998000;
    a = round_n(a, 7);
    b = round_n(b, 7);
    c = round_n(c, 7);
    printf("a=%.8lf\nb=%.8lf\nc=%.8lf", a, b, c);
    return 0;
}

Answer 2

这是我使用的代码：

int64_t __pow10_arr[18] = { 1l, 10l, 100l, 1000l, 10000l, 100000l,
                            1000000l, 10000000, 100000000l, 1000000000l, 10000000000l, 100000000000l,
                            1000000000000l, 10000000000000l, 100000000000000l, 1000000000000000l, 10000000000000000l, 100000000000000000l };

double roundToNfractions ( double val, int n )
{
  if (n<0 || n >= ( sizeof (__pow10_arr)/ sizeof (int64_t) ) ) {
    // log error however you wish to
    return val;
  }
  val *= __pow10_arr[n];
  val += 0.5;
  val = (uint64_t) val;
  val /= __pow10_arr[n];
  return val;
}