# [Boost-bugs] [Boost C++ Libraries] #12157: Dividing zero by zero produces one in boost::multiprecision::cpp_dec_float

Subject: [Boost-bugs] [Boost C++ Libraries] #12157: Dividing zero by zero produces one in boost::multiprecision::cpp_dec_float
Date: 2016-04-26 17:05:41

#12157: Dividing zero by zero produces one in boost::multiprecision::cpp_dec_float
------------------------------+----------------------------
Type: Bugs | Status: new
Milestone: To Be Determined | Component: multiprecision
Version: Boost 1.60.0 | Severity: Problem
Keywords: divide |
------------------------------+----------------------------
When using Boost multiprecision, dividing 0/0 produces 1. I would expected
the result to be inf. When dividing other numbers by zero it does produce
inf.

{{{
#include<iostream>
#include<boost/multiprecision/cpp_dec_float.hpp>

typedef
boost::multiprecision::number<boost::multiprecision::cpp_dec_float<0>>
BigFloat;

int main() {
BigFloat zero(0);
BigFloat one(1);
std::cout << zero / zero << std::endl;
std::cout << one / zero << std::endl;
}

}}}

I looked at operator/ from here:
http://www.boost.org/doc/libs/1_60_0/boost/multiprecision/cpp_dec_float.hpp
I think adding the case "if(iszero() && v.iszero())" as below could solve
this issue.

{{{
template <unsigned Digits10, class ExponentType, class Allocator>
cpp_dec_float<Digits10, ExponentType, Allocator>& cpp_dec_float<Digits10,
ExponentType, Allocator>::operator/=(const cpp_dec_float<Digits10,
ExponentType, Allocator>& v)
{
if(iszero() && v.iszero())
{
*this = inf();
return *this;
}

const bool u_and_v_are_finite_and_identical = ( (isfinite)()
&& (fpclass == v.fpclass)
&& (exp == v.exp)
&& (cmp_data(v.data) == static_cast<boost::int32_t>(0)));

if(u_and_v_are_finite_and_identical)
{
if(neg != v.neg)
{
*this = one();
negate();
}
else
*this = one();
return *this;
}
else
{
if(iszero())
{
if((v.isnan)() || v.iszero())
{
return *this = v;
}
return *this;
}
cpp_dec_float t(v);
t.calculate_inv();
return operator*=(t);
}
}
}}}

```--
Ticket URL: <https://svn.boost.org/trac/boost/ticket/12157>
Boost C++ Libraries <http://www.boost.org/>
Boost provides free peer-reviewed portable C++ source libraries.
```

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