Excluding certain values from an xsl:sum equation

Question

Within my stylesheet, I have an xsl:variable that looks into the XML, looks at a list of objects and returns the sum of the price value from each object, my code for that is exampled below:

  <xsl:variable name="ExampleTotal">
      <xsl:value-of select="sum(/ListOfObjects/Object/PriceOfObject)"/>
  </xsl:variable>

And this works fine, My problem is that I don't need the sum of every one of these objects in the list, only the ones that contain a value of being true.

Example XML:

<ListOfObjects>
  <Object>
    <PriceOfObject>35.00</PriceOfObject>
    <PriceNeeded>true</PriceNeeded>
    <OtherData>example</OtherData>
  </Object>
  <Object>
    <PriceOfObject>36.00</PriceOfObject>
    <PriceNeeded>true</PriceNeeded>
    <OtherData>example</OtherData>
  </Object>
  <Object>
    <PriceOfObject>35.00</PriceOfObject>
    <PriceNeeded>true</PriceNeeded>
    <OtherData>example</OtherData>
  </Object>
  <Object>
    <PriceOfObject>33.00</PriceOfObject>
    <PriceNeeded>false</PriceNeeded>
    <OtherData>example</OtherData>
 </Object>
</ListOfObjects>   

is it possible to look through the list of objects and only sum up the total of the ones that have the value as true?

Answer 1

Try this:

<xsl:variable name="ExampleTotal">
  <xsl:value-of select="sum(/ListOfObjects/Object[PriceNeeded='true']/PriceOfObject)"/>
</xsl:variable>

Above code changed as suggested by Michael Kay,

<xsl:variable name="ExampleTotal" 
  select="sum(/ListOfO‌​bjects/Object[PriceNe‌​eded='true']/PriceOfO‌​bject)"/>