round_f for ARM with math

static inline float round_f(float value)
{
    float rep;
    asm volatile ("vrinta.f32 %0,%1" : "=t"(rep) : "t"(value));
    return rep;
}

没有数学的ARM的round_f

union f__raw {
    struct {
        uint32_t massa  :23;
        uint32_t order  :8;
        uint32_t sign   :1;
    };
    int32_t     i_raw;
    float       f_raw;
};

float round_f(float value)
{
    union f__raw raw;
    int32_t exx;
    uint32_t ex_mask;
    raw.f_raw = value;
    exx = raw.order - 126;
    if (exx < 0) {
        raw.i_raw &= 0x80000000;
    } else if (exx < 24) {
        ex_mask = 0x00ffffff >> exx;
        raw.i_raw += 0x00800000 >> exx;
        if (exx == 0) ex_mask >>= 1;
        raw.i_raw &= ~ex_mask;
    };
    return  raw.f_raw;
};

你可以四舍五入到n位精度:

double round( double x )
{
const double sd = 1000; //for accuracy to 3 decimal places
return int(x*sd + (x<0? -0.5 : 0.5))/sd;
}

值得注意的是，如果想要从舍入中得到整数结果，则不需要通过上下限或上下限。也就是说,

int round_int( double r ) {
    return (r > 0.0) ? (r + 0.5) : (r - 0.5); 
}

它在cmath中从c++ 11开始提供(根据http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf)

#include <cmath>
#include <iostream>

int main(int argc, char** argv) {
  std::cout << "round(0.5):\t" << round(0.5) << std::endl;
  std::cout << "round(-0.5):\t" << round(-0.5) << std::endl;
  std::cout << "round(1.4):\t" << round(1.4) << std::endl;
  std::cout << "round(-1.4):\t" << round(-1.4) << std::endl;
  std::cout << "round(1.6):\t" << round(1.6) << std::endl;
  std::cout << "round(-1.6):\t" << round(-1.6) << std::endl;
  return 0;
}

输出:

round(0.5):  1
round(-0.5): -1
round(1.4):  1
round(-1.4): -1
round(1.6):  2
round(-1.6): -2

round_f for ARM with math

static inline float round_f(float value)
{
    float rep;
    asm volatile ("vrinta.f32 %0,%1" : "=t"(rep) : "t"(value));
    return rep;
}

没有数学的ARM的round_f

union f__raw {
    struct {
        uint32_t massa  :23;
        uint32_t order  :8;
        uint32_t sign   :1;
    };
    int32_t     i_raw;
    float       f_raw;
};

float round_f(float value)
{
    union f__raw raw;
    int32_t exx;
    uint32_t ex_mask;
    raw.f_raw = value;
    exx = raw.order - 126;
    if (exx < 0) {
        raw.i_raw &= 0x80000000;
    } else if (exx < 24) {
        ex_mask = 0x00ffffff >> exx;
        raw.i_raw += 0x00800000 >> exx;
        if (exx == 0) ex_mask >>= 1;
        raw.i_raw &= ~ex_mask;
    };
    return  raw.f_raw;
};

Boost中还实现了某种类型的舍入:

#include <iostream>

#include <boost/numeric/conversion/converter.hpp>

template<typename T, typename S> T round2(const S& x) {
  typedef boost::numeric::conversion_traits<T, S> Traits;
  typedef boost::numeric::def_overflow_handler OverflowHandler;
  typedef boost::numeric::RoundEven<typename Traits::source_type> Rounder;
  typedef boost::numeric::converter<T, S, Traits, OverflowHandler, Rounder> Converter;
  return Converter::convert(x);
}

int main() {
  std::cout << round2<int, double>(0.1) << ' ' << round2<int, double>(-0.1) << ' ' << round2<int, double>(-0.9) << std::endl;
}

注意，这仅在执行到整数的转换时有效。