facing difficulty to reading complex xml file using libxml2 in c++ - c++

Question

this is my xml file
<Example>
<text-frame name="SingleLineText 1" id="Text0" wrapping="true" direction="" suppress- empty-lines="false">
<style-ref name="default"/>
<style name="">
<alignment halign="left" valign="top"/>
<text-style text-color="Color 0" position="normal"/>
<font family-name="Times New Roman" size="14" style="Bold Italic"/>
<paragraph-style text-align="left"/>
</style>
<geometry size="absolute" position="absolute">
<location left="35" top="30"/>
<size width="165" height="45"/>
</geometry>
<condition name="" desc="">
<if>
<expression left="record1/#field3" operator="==" right="'1240 CrocoDoc Lane'"/>
<action-list>
<action type="suppress"/>
</action-list>
</if>
</condition>
<paragraph>
<style>
<paragraph-style indent-left="0" indent-right="0" indent-first="0" indent-hanging="0" line-spacing="multiple" line-spacing-at="1.2" text-align="left"/>
</style>
<text>
<style>
<font family-name="Times New Roman" size="14" style="Bold Italic"/>
<text-style text-color="Color 0" highlight="None" position="normal" character-spacing="0" case="normal"/>
</style>
<content><![CDATA[%(record1/#field3)]]></content>
</text>
</paragraph>
</text-frame>
</Example>
and my code to reading and writing is below.
what i want is to reach at the alignment tag using this and make change.. "/Example/text-frame/style/alignment" xpathexpression but its not working
xmlXPathObject * xpathObj = xmlXPathEvalExpression( (xmlChar*)"/Example/text-frame/style/alignment", xpathCtx );
//xmlXPathObject * xpathObj = xmlXPathEvalExpression( (xmlChar*)"/Example/Objects/Pet", xpathCtx );
if ( xpathObj == NULL ) throw "failed to evaluate xpath";
for(int i=0;i<=1; i++)
{
xmlNode *node = NULL;
if ( xpathObj->nodesetval && xpathObj->nodesetval->nodeTab )
{
node = xpathObj->nodesetval->nodeTab[i];
std::cout << "Found the node we want" << std::endl;
}
else
{throw "failed to find the expected node";}
xmlAttr *attr = node->properties;
while ( attr )
{
std::cout << "Attribute name: " << attr->name << " value: " << attr->children->content << std::endl;
attr = attr->next;
}
xmlSetProp( node, (xmlChar*)"age", (xmlChar*)"3" );
}