UnitsML July Asynchronous Meeting: Information on default attributes

["Peter J. Linstrom" <peter.linstrom@nist.gov>]

All,

    During the teleconference portion of the July
meeting we discussed the idea of having a "dimensionless"
attribute for the "Dimension" element that could be
used to affirmatively declare that something was
dimensionless (as opposed to just leaving the "Dimension"
element empty). This attribute would be an optional
boolean value. During the discussion it was unclear
how XML Schema handles default values for attributes.
I have investigated this issue and found the relevant
paragraph from the w3c site
(http://www.w3.org/TR/xmlschema-0/):

  Default values of both attributes and elements are
  declared using the "default" attribute, although this
  attribute has a slightly different consequence in
  each case. When an attribute is declared with a default
  value, the value of the attribute is whatever value
  appears as the attribute's value in an instance
  document; if the attribute does not appear in the
  instance document, the schema processor provides the
  attribute with a value equal to that of the default
  attribute. Note that default values for attributes
  only make sense if the attributes themselves are
  optional, and so it is an error to specify both a
  default value and anything other than a value of
  optional for  use.

Based on this information it is clear that the
"dimensionless" attribute should have a default
value of zero. That way if the attribute is not
specified the dimension will not be affirmatively
declared to be dimensionless.

Peter

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Peter J. Linstrom
NIST, Physical and Chemical Properties Division
Phone: (301) 975-5422
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