Re: [boost] Boost Units library preview

From: Matthias Troyer (troyer_at_[hidden])
Date: 2006-08-20 08:29:05

On Aug 19, 2006, at 2:04 PM, Andy Little wrote:

>
> "Matthias Troyer" <troyer_at_[hidden]> wrote in message
> news:09128591-A3C8-4432-A934-9C01B9042AF7_at_itp.phys.ethz.ch...
>> Andy little wrote
>>> Eric leemings wrote
>>>> Abstract quantity?
>>>
>>> An abstract quantity is a quantity without a numeric value or a
>>> conversion
>>> factor. It still conforms to dimensional analysis and dimensional
>>> math.
>>> In fact the abstract quantity concept (small c) has now been
>>> gobbled up by the
>>> all encompassing Static Unit concept.
>>
>> I am confused. What is a quantity without a numeric value? Just a  
>> unit?
>> I don't see the reason for a quantity without a value. Just the name
>> quantity implies a value.
>
> I was taught at school to do dimensional analysis checks manually.  
> For example
>
> force = mass * acceleration;
>
> M = mass
> L = length
> T = time;
>
> force ==  M * L / T^2
> acceleration = L/T^2
>
> Therefore   (M* L/T^2) = (M) * ( L/ T^2)
>
> In this case its just dimensional math, no value is involved.

This is just a check of units. I would prefer calling them units and  
not quantities

>
>>>> Named quantity?
>>>
>>> A named quantity has a name. In practise in Quan-0_2_0 this means
>>> that, where U
>>> is a model of StaticUnit and is_named_quantity<U>::value is true,  
>>> that
>>> get_named_quantity_traits<U>::type , returns a traits class, which
>>> provides
>>> useful information, such as the name of the quantity and other
>>> information
>>> mainly useful for I/O purposes.
>>
>> I assume that this mean that the unit has a special name, and
>> additional information?
>
> Yes. The classic example is  torque and energy.  They are  
> dimensionally
> equivalent, but have different output
>
> Energy in Joules is output  as   J..
> Torque as N.m
>
> Note: I am aware that the output as it stands is pretty fixed ( It  
> is still
> useful for getting some output in quick examples and tests though),  
> but One day
> hopefully it should be possible to use locales and facets or  
> whatever and
> somehow to customise for various file formats.. I hope that  
> functionality could
> be added by someone with experience of that type of thing if Quan  
> got into boost
> , or at least I could get advice on how to go about it. This is the  
> idea anyway
> of the NamedQuantityTraits Concept.
>
>>>> Concrete quantity?
>>>
>>> A concrete quantity is a quantity which has a unit and a numeric
>>> value.
>>
>> Same confusion as with abstract quantity. Isn't abstract quantity
>> just the unit? I
>
> All this has data  been moved into the StaticUnit in the latest  
> version of Quan,
> yes.

OK

>
>>>> Anonymous quantity?
>>>
>>> An anonymous quantity doesnt have a name, kind of the opposite of a
>>> named
>>> quantity. is_named_quantity<U>::value is false for anonymous
>>> quantities in the
>>> next version of Quan, and so their  Named Quantity Traits class is
>>> effectively
>>> void.
>>
>> Does this mean that the name will be derived, like in m/s^2 ?
>
> Yes.
>
> Or that
>> there  is no unit symbol at all that can be printed?
>
> No, it defaults to something like the above for an anonymous quantity.

ok, so named quantity has a special unit symbol, and anonymous  
quantity a default unit symbol?

>
>  Again this seems
>> to be a property of the unit of the quantity. Wouldn't unnamed unit
>> be a better name?
>
> I prefer anonymous FWIW. Someone who writes a letter anonymously  
> does have a
> name, we just don't know what it is, so that is not quite the same  
> as someone
> not having a name. Anonymous quantities do represent a particular  
> quantity, but
> because they arise from calculations, then it is not possible to  
> know which
> quantity is intended, so it is not possible to know what their name  
> is.
> The programmer can 'name' a quantity by assigning or initialising a  
> named
> quantity from an anonymous quantity, when they need to be explicit  
> about what
> quantity it represents.

Wait, if I multiply quantities (which have units) then also the  
result will have a unit. E.g. if I divide 10 meters by 2 seconds then  
I get 5 m/s . I wouldn't call that anonymous since I know the unit: m/s

>
>>>> Static unit?
>>>
>>> This is a proper C++ Concept. Effectively its a compile time  
>>> database
>>> which can be queried about various aspects of a particular
>>> fixed_quantity. It is
>>> a template parameter to fixed_quantity:
>>>
>>> fixed_quantity<StaticUnit,NumericType>
>>>
>>>> United value?
>>>
>>> It is a detail of the implementation. Has it leaked into the docs
>>> somewhere? If
>>> so I will remove it as it may change without notice.
>>>
>>>> A
>>>> general-purpose library should use simple terms and a bare minimum
>>>> of names.
>>
>> If I understand the above concepts correctly then we only need three
>> terms:
>>
>> - named unit
>> - unnamed unit
>> - quantity (value + unit)
>
> Strictly speaking AFAIK the  'value' of a quantity  is the numeric  
> value + unit.

OK

>
> Otherwise that is quite close to the latest thinking in the latest  
> CVS Version
> of Quan. It is (almost) summed up in the fixed_quantity definition.
>
> fixed_quantity<StaticUnit,NumericType>
>
> (A Static Unit is just a Unit where all the data is available at  
> compile time.)

OK, this sounds reasonable enough, except that I'm still confused  
what exactly an anonymous quantity is.

Matthias