On Aug 19, 2006, at 2:04 PM, Andy Little wrote:
"Matthias Troyer" <troyer@itp.phys.ethz.ch> wrote in message news:09128591-A3C8-4432-A934-9C01B9042AF7@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