forked from mpusz/mp-units
45 lines
1.9 KiB
ReStructuredText
45 lines
1.9 KiB
ReStructuredText
FAQ
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===
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General
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-------
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Why all UDLs are prefixed with ``q_`` instead of just using unit symbol?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Usage of only unit symbols in UDLs would be a preferred approach (less to type,
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easier to understand and maintain). However, while increasing the coverage for
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the library we learned that there are a lot unit symbols that conflict with
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built-in types or numeric extensions. A few of those are: ``F`` (farad),
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``J`` (joule), ``W`` (watt), ``K`` (kelvin), ``d`` (day),
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``l`` or ``L`` (litre), ``erg``, ``ergps``. For a while for those we used ``_``
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prefix to make the library work at all, but at some point we had to unify the
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naming and we came up with ``q_`` prefix which results in a creation of
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quantity of a provided unit.
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Why dimensions depend on units and not vice versa?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Most of the libraries define units in terms of dimensions and this was also an
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initial approach for this library. However it turns out that for such a design
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it is hard to provide support for all the required scenarios.
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The first of them is to support multiple unit systems (like SI, CGS, ...) where
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each of can have a different base unit for the same dimension. Base quantity of
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dimension length in SI has to know that it should use ``m`` to print the unit
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symbol to the text output, while the same dimension for CGS should use ``cm``.
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Also it helps in conversions among those systems.
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The second one is to support natural units where more than one dimension can be
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measured with the same unit (i.e. ``GeV``). Also if someone will decide to
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implement a systems where SI quantities of the same kind are expressed as
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different dimensions (i.e. height, width, and depth) all of them will just be
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measured in meters.
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Why do we spell ``metre`` instead of ``meter``?
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Well, this is how ISO 80000 defines it (British English spelling by default).
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