Question

Possible Duplicate:
Rounding Number to 2 Decimal Places in C

I have not found a function with a signature double round(double d, int digits) like here in c.
When i try to build i get a error:

error: too many arguments to function ’round’

How can I round in C with N digits after the decimal point?

Answer 1

Using recursion (which is going to be slow for some values of digits)

#include <math.h>
double my_round(double x, unsigned int digits) {
  if (digits > 0) {
    return my_round(x*10.0, digits-1)/10.0;
  }
  else {
    return round(x);
  }
}

A method likely to be somewhat faster, but which relies on a single call to the slow pow function:

#include <math.h>

double my_round(double x, unsigned int digits) {
    double fac = pow(10, digits);
    return round(x*fac)/fac;
}

An even faster method is to precompute a lookup table with the likely powers and use that instead of pow.

#include <math.h>

double fac[];  // population of this is left as an exercise for the reader

double my_round(double x, unsigned int digits) {
    return round(x*fac[digits])/fac[digits];
}