Force cpp_dec_float round

Question

I use .str(n, std::ios_base::scientific)print ccp_dec_floats.

I noticed that it is rounded.

I use cpp_dec_floatfor accounting, so I need to round down. How can I do that?

Answer 1

It does not round. In fact, this is a banker round: Watch Live On Coliru

#include <boost/multiprecision/number.hpp>
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <iostream>

namespace mp = boost::multiprecision;

int main()
{
    using Dec = mp::cpp_dec_float_50;

    for (Dec d : { 
            Dec( "3.34"),   Dec( "3.35"),   Dec( "3.38"),
            Dec( "2.24"),   Dec( "2.25"),   Dec( "2.28"),
            Dec("-2.24"),   Dec("-2.25"),   Dec("-2.28"),
            Dec("-3.34"),   Dec("-3.35"),   Dec("-3.38"),
            })
    {
        std::cout     << d.str(2, std::ios_base::fixed) 
            << " -> " << d.str(1, std::ios_base::fixed) << "\n";
    }
}

Print

3.34 -> 3.3
3.35 -> 3.4
3.38 -> 3.4
2.24 -> 2.2
2.25 -> 2.2
2.28 -> 2.3
-2.24 -> -2.2
-2.25 -> -2.2
-2.28 -> -2.3
-3.34 -> -3.3
-3.35 -> -3.4
-3.38 -> -3.4

So, if you want another kind of rounding, you want to write it explicitly

Here's a general approach ( Live On Coliru )

template <int decimals = 0, typename T>
T round_towards_zero(T const& v)
{
    static const T scale = pow(T(10), decimals);

    if (v.is_zero())
        return v;

    // ceil/floor is found via ADL and uses expression templates for optimization
    if (v<0)
        return ceil(v*scale)/scale;
    else
        // floor is found via ADL and uses expression templates for optimization
        return floor(v*scale)/scale;
}

which, we hope, combines to an optimal code due to the statically known scale factor and the use of expression patterns in the Boost Multiprecision library.

Answer 2

, , ?

double rounding_func(double in, double precision){
   return round(in*pow(10,precision))/pow(10,precision);              
}

, , ,

double rounding_func(double in, double precision){
   return floor(in*pow(10,precision))/pow(10,precision);              
}

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