Re: [Boost-users] Multiprecision -- cubing a number gives unexpected result

Subject: Re: [Boost-users] Multiprecision -- cubing a number gives unexpected result
From: John Maddock (jz.maddock_at_[hidden])
Date: 2017-01-19 14:23:56

It's this bug: https://svn.boost.org/trac/boost/ticket/12408

Fixed in boost-1.62.

HTH, John.

On 19/01/2017 17:39, Craig Henderson wrote:
> Hello,
> I am new to Boost.Multiprecision and have come across a result that I 
> cannot understand.
>
> I have the following code to calculate the sum of three cubes,
>     using bigint_t = boost::multiprecision::mpz_int;
>     bigint_t a = 2220422932;
>     bigint_t b = 2218888517; b = -b;
>     bigint_t c =  283059965; c = -c;
>     std::cout << '(' << a << ")^3+(" << b << ")^3+(" << c << ")^3 = ";
>     bigint_t a1 = a * a * a;
>     bigint_t b1 = b * b * b;
>     bigint_t c1 = c * c * c;
>     std::cout << '(' << a1 << ")+(" << b1 << ")+(" << c1 << ") = ";
>     std::cout << bigint_t(a1+b1+c1) << '\n';
>
> and it produces the correct result
> (2220422932)^3+(-2218888517)^3+(-283059965)^3 = 
> (10947302325566084787191541568)+(-10924622727902378924946084413)+(-22679597663705862245457125) 
> = 30
>
> However, if I use this implementation, with a,b and c assigned the 
> result of the multiplication,
>     a = a * a * a;
>     b = b * b * b;
>     c = c * c * c;
>     std::cout << '(' << a << ")+(" << b << ")+(" << c << ") = ";
>     std::cout << bigint_t(a+b+c) << '\n';
> ... I get a very strange result:
> (24307641127223864542936438774018437376)+(24240519923499804093545671548118385521)+(6419686120902663137493915211500625) 
> = 48554580736844571299619604237348323522
>
> What is going on? Is this an invalid use of the multiplication that I 
> have missed?
> I get the same result with cpp_int and gmp;
>     using bigint_t = boost::multiprecision::cpp_int;
>     using bigint_t = boost::multiprecision::mpz_int;
>
> I'm using g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609, Boost 1.58
>
> Many thanks
> Craig
>
>
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