code: @Override
      public double value(int indexAttribute) {
          int location = locateIndex(indexAttribute);
          if ((location >= 0) && (indexValues[location] == indexAttribute)) {
              return attributeValues[location];
          } else {
              // returns a NaN value which represents missing values instead of a 0 (zero)
              return Double.NaN;
          }
      }
docs: Value of the attribute in the indexAttribute position.
  If this value is absent, a NaN value (marker of missing value)
  is returned, otherwise this method returns the actual value.
  
  @param indexAttribute the index attribute
  @return the double