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div运算符做浮点除法运算, mod运算符做求余运算, floor函数返回不大于参数的最大整数(趋近于正无穷), ceiling返回不小于参数的最小整数(趋近于负无穷)
 
//BBB[position() mod 2 = 0 ]
选择偶数位置的BBB元素

     <AAA>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <CCC/>
          <CCC/>
          <CCC/>
     </AAA>
在XLab中打开实例 | 树视图 (JPG)
 
//BBB[ position() = floor(last() div 2 + 0.5) or position() = ceiling(last() div 2 + 0.5) ]
选择中间的BBB元素

     <AAA>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <CCC/>
          <CCC/>
          <CCC/>
     </AAA>
在XLab中打开实例 | 树视图 (JPG)
 
//CCC[ position() = floor(last() div 2 + 0.5) or position() = ceiling(last() div 2 + 0.5) ]
选择中间的CCC元素

     <AAA>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <BBB/>
          <CCC/>
          <CCC/>
          <CCC/>
     </AAA>
在XLab中打开实例 | 树视图 (JPG)