id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
12408	cpp_dec_float_50 x = x*x*x gives wrong result!!!	Dave Nalepa <denalepa1@…>	John Maddock	"I'm a beginner with C++ and with boost so I won't be surprised if I'm making some blunder but in case there's a real problem here, here goes:
Using the multiprecision cpp_dec_float_50 I get a enormous error when I cube a variable.
I'm using Microsoft VS2015 Community and making a console program (since my attempts at using boost with a cli program failed miserably). 
The core code (I think) is
#include <boost/multiprecision/cpp_dec_float.hpp> 
using boost::multiprecision::cpp_dec_float_50;
// both at the global level = before main(){...
I set f2 to this and then use the line
f2 = f2*f2*f2; //to compute (f2)³ 
when f2 = 1.0335055959... I get the new f2 as 1.140909... which isn't even close to the correct value of 1.10392227...
When I use a temporary variable, f3, it works just fine
f3= f2*f2*f2 // f3 = 1.1039...
It is probably some error the compiler is making, I don't know. This is obviously a worrisome bug. Simple arithmetic shouldn't give results that are off by 3% !!
I'm not skilled enough in VS2015 to know what else to tell you about the switches that are or are not selected; I compile and run in the VS IDE.
//===================================== here's the entire program (stub)
// I've shortened 1.03559... but I still get the wrong value.
#include <iomanip>
#include <iostream>
#include <boost/multiprecision/cpp_dec_float.hpp>

using boost::multiprecision::cpp_dec_float_50;

int main()
{
	cpp_dec_float_50 f2 = (cpp_dec_float_50)(1.03559);
	f2 = f2*f2*f2;
	std::cout << ""f2 = "" << f2 << std::endl;
// gives me 1.15.... which is wrong should be 1.1106...
// it is actually the value of f2*f2*f2*f2, or close to it.
    return 0;
}"	Bugs	closed	To Be Determined	multiprecision	Boost 1.61.0	Problem	fixed