code: def getValue(self):
      ret = None
      if self.windowFull():
          prev = self.getValues()[0]
          actual = self.getValues()[-1]
          if actual is not None and prev is not None:
              diff = float(actual - prev)
              if diff == 0:
                  ret = float(0)
              elif prev != 0:
                  ret = diff / prev
      return ret
  
file_path: pyalgotrade/pyalgotrade/technical/roc.py
imports: from pyalgotrade import technical
label_desc: Use image node_id 2 for calling the ROCEventWindow obj's underlying member method code with example usage: obj.getValue() and returns: ret