Querying Data Field Against Data Value

tclotworthy · April 8th, 2011, 11:22 AM

Hello,

In the below code fragment, I have a need to build an expression that can determine (as an example) if velocity is > 0. The DataArray fields relate to the comma-delimited ns1:values element directly. As an example, since the 5th array element is "velocity", the 5th value in the ns1:values (0.0) is the value of the velocity. By the way, I cannot assume that velocity is always the 5th value.

My question: can an expression be built that compares a DataArray field to its corresponding value in ns1:values? If so, would anyone be able to provide me with some guidance on how this would be done? Thanks for any response!

Code:

                          <ns1:DataArray xmlns:ns1="http://www.opengis.net/swe/1.0.1">
                                <ns1:elementCount>
                                    <ns1:Count>
                                    <ns1:value>1</ns1:value>
                                    </ns1:Count>
                                </ns1:elementCount>
                                <ns1:elementType name="Components">
                                    <ns:DataRecord xmlns:ns="http://www.opengis.net/swe/1.0.1">
                                    <ns:field name="Time">
                                    <ns:Time definition="urn:ogc:data:time:iso8601"/>
                                    </ns:field>
                                    <ns:field name="feature">
                                    <ns:Text definition="urn:ogc:data:feature"/>
                                    </ns:field>
                                    <ns:field name="position">
                                    <ns:Position definition="urn:ogc:def:property:OGC:1.0.30:position"/>
                                    </ns:field>
                                    <ns:field name="altitude">
                                    <ns:Quantity definition="urn:ogc:def:phenomenon:OGC:1.0.30:altitude">
                                    <ns:uom code="m"/>
                                    </ns:Quantity>
                                    </ns:field>
                                    <ns:field name="velocity">
                                    <ns:Quantity definition="urn:ogc:def:phenomenon:OGC:1.0.30:velocity">
                                    <ns:uom code="m/h"/>
                                    </ns:Quantity>
                                    </ns:field>
                                    <ns:field name="heading">
                                    <ns:Quantity definition="urn:ogc:def:phenomenon:OGC:1.0.30:heading">
                                    <ns:uom code="deg"/>
                                    </ns:Quantity>
                                    </ns:field>
                                    <ns:field name="source">
                                    <ns:Category definition="urn:ogc:def:phenomenon:OGC:1.0.30:source"/>
                                    </ns:field>
                                    <ns:field name="destination">
                                    <ns:Category definition="urn:ogc:def:phenomenon:OGC:1.0.30:destination"/>
                                    </ns:field>
                                    <ns:field name="target">
                                    <ns:Category definition="urn:ogc:def:phenomenon:OGC:1.0.30:target"/>
                                    </ns:field>
                                    <ns:field name="symbol">
                                    <ns:Category definition="urn:ogc:def:phenomenon:OGC:1.0.30:symbol"/>
                                    </ns:field>
                                    <ns:field name="symboltable">
                                    <ns:Category definition="urn:ogc:def:phenomenon:OGC:1.0.30:symboltable"/>
                                    </ns:field>
                                    </ns:DataRecord>
                                </ns1:elementType>
                                <ns1:encoding>
                                    <ns1:TextBlock blockSeparator=";"
                                    decimalSeparator="." tokenSeparator=","/>
                                </ns1:encoding>
                                <ns1:values>2011-03-30T15:35:41.638Z,aprs-w4brm-1,POINT(38.92216666666667 -77.66333333333333)#4326,0.0,0.0,0,w4brm-1,APRS,,r,/;</ns1:values>
                            </ns1:DataArray>

Martin Honnen · April 8th, 2011, 11:36 AM

Do you use XSLT 2.0 or 1.0? With XSLT 2.0 you could easily tokenize the comma separated list in ns1: values e.g.

Code:

<xsl:variable name="values" select="tokenize(ns1:values, ',')"/>

then you can select the position of the velocity field and index the values variable e.g.

Code:

<xsl:variable name="pos" select="count(ns1:elementType/ns:DataRecord/ns:field[@name = 'velocity']/preceding-sibling::ns:field) + 1"/>
<xsl:variable name="velocity" select="$values[$pos]"/>

tclotworthy · April 8th, 2011, 12:41 PM

Thanks for reply. I should have clarified. I must use XSLT 1.0. Any reasonable way to do it in 1.0? Thanks for any reply!

Martin Honnen · April 8th, 2011, 12:52 PM

Check whether your XSLT 1.0 processor supports http://www.exslt.org/str/functions/tokenize/index.html, then you can use the same approach, only instead of the XPath 2.0 tokenize function you use the EXSLT one.

tclotworthy · April 8th, 2011, 02:05 PM

thanks for reply.. Hmm, I should clarify that these expressions have to be written as xpath (the tool I am relying on contains an xpath processor to which I pass xpath-based expressions).

The code sample I gave is an example of a sensor notification the xpath processor would process my xpath expression against. Unfortunately, I do not know how to determine whether the xpath expression can properly process the tokenizer you mention.

Is it possible to simply assume that it doesn't support that tokenizer (I know it is not 2.0 compliant in general), and someone help me define an xpath expression that would work?

Again, any reply appreciated. Thanks for your responses!

Martin Honnen · April 9th, 2011, 07:26 AM

I doubt it is possible with pure XPath 1.0 to find the right substring if you don't know its position but need to compute that too.
If you know the position then you need to use substring-before/after often enough to extract the right substring e.g.

Code:

substring-before(substring-after(substring-after(substring-after(substring-after(ns1:values, ','), ','), ','), ','), ',')

should find the fifth substring.

tclotworthy · April 9th, 2011, 01:21 PM

Thank you Martin. I appreciate your help. I will get by with this for now.