强制cpp_dec_float向下舍入 [英] Force cpp_dec_float to round down
问题描述
我使用 .str(n,std :: ios_base :: scientific)
打印 ccp_dec_float
我注意到它向上舍入。
我使用 cpp_dec_float
为会计,所以我需要向下舍入。如何做到这一点?
它不四舍五入。事实上,它确实是银行家的:看到 Live On Coliru
#include< boost / multiprecision / number.hpp>
#include< boost / multiprecision / cpp_int.hpp>
#include< boost / multiprecision / cpp_dec_float.hpp>
#include< iostream>
命名空间mp = boost :: multiprecision;
int main()
{
使用Dec = mp :: cpp_dec_float_50;
(Dec d:{
Dec(3.34),Dec(3.35),Dec(3.38),
Dec(2.24 (-2.28),Dec(-2.28),Dec(-2.28),Dec(-2.28 3.34),Dec( - 3.35),Dec( - 3.38),
})
{
std :: cout< d.str(2,std :: ios_base :: fixed)
<< - ><< d.str(1,std :: ios_base :: fixed)<< \\\
;
}
}
列印:
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
所以如果你想要另一种舍入, p>
以下是一种通用方法( 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;
//通过ADL找到ceil / floor,并使用表达式模板进行优化
if(v <0)
return ceil(v * scale)
else
//通过ADL找到floor,并使用表达式模板进行优化
return floor(v * scale)/ scale;
}
这可能会由于静态已知的比例因子和使用的Boost Multiprecision库中的表达式模板。
I am using .str(n, std::ios_base::scientific)
to print ccp_dec_float
s.
I've noticed that it rounds up.
I am using cpp_dec_float
for accounting, so I need to round downward. How can this be done?
It doesn't round up. In fact, it does banker's round: See it 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";
}
}
Prints:
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'd want to write it explicitly
Here's a generic 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 hopefully compiles down to optimal code due to statically known scale factor and the use of expression templates in Boost Multiprecision library.
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