From: Andy Seaborne <andy.seaborne@epimorphics.com>

Date: Tue, 31 Jul 2012 16:56:27 +0100

Message-ID: <5018002B.6090509@epimorphics.com>

To: public-rdf-dawg-comments@w3.org

Date: Tue, 31 Jul 2012 16:56:27 +0100

Message-ID: <5018002B.6090509@epimorphics.com>

To: public-rdf-dawg-comments@w3.org

On 31/07/12 16:32, David Booth wrote: > On Tue, 2012-07-31 at 15:52 +0100, Andy Seaborne wrote: >> >> On 31/07/12 15:05, David Booth wrote: >>> Hi Andy, >>> >>> On Tue, 2012-07-31 at 09:04 +0100, Andy Seaborne wrote: >>>> David, >>>> >>>> Thank you for your comment on serializing xsd:decimal, xsd:float, >>>> xsd:double. >>>> >>>> XSD defines the operations for these datatypes including rules for >>>> precision. For xsd:decimal, the precision required is: >>>> >>>> [[ http://www.w3.org/TR/xmlschema11-2/ >>>> All ·minimally conforming· processors must support decimal values whose >>>> absolute value can be expressed as i / 10k, where i and k are >>>> nonnegative integers such that i < 1016 and k ≤ 16 (i.e., those >>>> expressible with sixteen total digits). >>>> ]] >>>> >>>> [[ http://www.w3.org/TR/xmlschema-2/ >>>> Note: All ·minimally conforming· processors ·must· support decimal >>>> numbers with a minimum of 18 decimal digits >>>> ]] >>>> >>>> SPARQL uses the work of XQuery/XPath Functions and Operators and the >>>> requirements for handling these numeric datatypes derive from that body >>>> of work. The SPARQL specification links to each operator required. >>>> >>>> RDF defines a literal as a lexical form and a datatype. It does not >>>> consider the datatype to modify the behavior of forming the RDF literal. >>> >>> Should I understand this to mean that when a minimally conforming SPARQL >>> server creates a value of type xsd:decimal, then at least 18 decimal >>> digits of precision must be retained in *serializing* that value to >>> lexical form? I understand that the computations must be performed with >>> at least 18 decimal digits of precision, but serialization is >>> (potentially) a different matter. >> >> Serialization can't truncate - it must represent the value and there is >> no flexibility in the serialization step of forming the literal. If the >> calculation says the value is X, then the serialization must represent X. >> >> The serialization represents the value. In some datatypes like >> xsd:decimal, there are multiple ways to write the same value. It's a >> feature of the datatype, not RDF itself. xsd:decimal does not record >> precision with trailing zeros. > > If I've understood correctly, this means that a minimally conforming > SPARQL server MUST retain at least 18 decimal digits of precision when > an xsd:decimal is serialized into lexical form, though trailing zeros > (after the decimal point) are optional. Thus, the value 3 may be > serialized as any of these > > "3.0"^^xsd:decimal > "3.00000000000000000"^^xsd:decimal > "3.00000000000000000000000000000000000000000000"^^xsd:decimal > > but the value of pi must NOT be serialized as > > "3.14159"^^xsd:decimal > > because that serialization does not retain at least 18 digits of > precision. Let's avoid pi as that gets into what the constant is set to, which will not be exactly pi. Your example was: 0.049508196721311475409836 which has 16 significant places: 0495081967213115 (if rounded in the last place - I believe that ..114 would also be right, if less right, as the two 16 digit closest numbers) (XML schema 1.1) so "0.0495081967"^^xsd:decimal and "0.050"^^xsd:decimal do not serialize that value as they represent distinctly different values on the number line. >>>> All ·minimally conforming· processors must support decimal values whose >>>> absolute value can be expressed as i / 10k, where i and k are >>>> nonnegative integers such that i < 1016 and k ≤ 16 (i.e., those >>>> expressible with sixteen total digits). > > If this is the case, then I am satisfied with this response. > > Thank you! > David > >> >> See http://www.w3.org/TR/xsd-precisionDecimal/ (W3C Working Group Note) >> which is not a required datatype in SPARQL (or even an XSD processor) >> >> Andy >> >>> >>> Thanks, >>> David >>> >>> >>>> >>>> The working group is not planning to make any changes in this area. >>>> >>>> I would be grateful if you reply to this message to confirm that the >>>> working group has responded to your comment. >>>> >>>> Yours, on behalf of the SPARQL Working Group, >>>> >>>> Andy >>>> >>>> >>>> On 20/07/12 16:19, David Booth wrote: >>>>> It is currently difficult to compare the results of two different SPARQL >>>>> servers when xsd:decimal, xsd:float or xsd:double are used, because >>>>> there does not seem to be any standard way to control the number of >>>>> decimal places that are serialized. For example, if the decimal >>>>> 0.049508196721311475409836 is computed, one server may serialize this as >>>>> >>>>> "0.0495081967"^^xsd:decimal >>>>> >>>>> and another may serialize it as >>>>> >>>>> "0.050"^^xsd:decimal >>>>> >>>>> One might (erroneously) attempt to round both of the serialized values >>>>> to one decimal place in order to compare them, but if they have already >>>>> been rounded once before serialization (as above) then this won't work, >>>>> because 0.050 rounds to 0.1, while 0.0495081967 rounds to 0.0. >>>>> >>>>> I think the SPARQL standard should provide some "SHOULD" guidance, such >>>>> as specifying that a SPARLQ server SHOULD (by default) serialize a >>>>> specified number of decimal places (perhaps all available), and MAY >>>>> provide a standard means of controlling the number of decimal places >>>>> that are serialized. >>>>> >>>>> If it is too late in the WG process to consider this for SPARQL 1.1 (and >>>>> I assume it is) then please add this to the wish list for the next >>>>> version. >>>>> >>>>> Thanks! >>>>> >>>> >>>> >>> >> >> >Received on Tuesday, 31 July 2012 15:57:01 GMT

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