Re: [Boost-users] more Boost::Units questions

Subject: Re: [Boost-users] more Boost::Units questions
From: Mark Volkmann (mark_at_[hidden])
Date: 2009-01-12 17:36:39

On Jan 12, 2009, at 4:23 PM, Steven Watanabe wrote:

> AMDG
>
> Mark Volkmann wrote:
>>>> Okay, I've looked at the code related to conversions.  
>>>> conversion_factor.cpp sets up a bunch of double values that I  
>>>> assume are used to convert values in one unit to values in  
>>>> another. However, they aren't used in any code.
>>>
>>> conversion_factor isn't used for conversions.  It just returns the  
>>> value that
>>> would be used for a conversion.
>>
>> I thought that was the case. However, I haven't yet worked out how  
>> to create a conversion_factor for miles to kilometers. That should  
>> be just a few lines I think. Here's what I tried:
>>
>> #include <boost/units/base_units/us/mile.hpp>
>> ...
>> #typedef bu::us::mile_base_unit::unit_type mile;
>> ...
>> double kilometer = 1000 * si::meter;
>> double mile_to_km = conversion_factor(mile, kilometer);
>>
>> That last line doesn't compile. Do you see what I'm doing wrong?
>
> Only the last line fails?  The only line of that which should compile
> is the #include.
>
> conversion_factor return the conversion between two different units.
> kilometer in your example is not a unit.
>
> #include <boost/units/systems/si/prefixes.hpp>
>
> double mile_to_km = conversion_factor(mile(), si::kilo*si::meter);
>
>>> http://www.boost.org/doc/html/boost_units/Examples.html#boost_units.Examples.ConversionExample
>>> The example uses SI and CGS, but converting between any units is  
>>> similar.
>>
>>
>> Thanks. I've read that page several times already and still don't  
>> get it. I suspect my lack of experience with C++ templates is the  
>> issue. I see this:
>>
>> <snip>
>>
>> However, I don't know what to specify in place of cgs::length in  
>> order to use miles instead of centimeters.
>>
>> I wish that page contained an example of converting from some U.S.  
>> unit (such as miles) to some SI unit (such as meters).
>
> The first template argument of quantity<> is supposed to
> be a unit.  Once you have the correct quantity/unit types
> conversions are easy.
>
> typedef bu::us::mile_base_unit::unit_type mile;
>
> quantity<mile> m(5.90e4 * si::meters);

Oh my goodness, I actually have working code now demonstrating two  
ways to perform conversions! Thanks!

   // Convert miles to kilometers.
   double mile_to_km = conversion_factor(mile(), si::kilo*si::meter);
   cout << mile_to_km << std::endl;
   double marathonMiles = 26.2;
   double marathonKMs = marathonMiles * mile_to_km;
   cout << "marathonKMs = " << marathonKMs << endl;

   // Convert kilometers to miles.
   quantity<mile> miles(10 * si::kilo * si::meter);
   cout << "10K miles = " << miles.value() << endl;

---
Mark Volkmann