class_docstr: Keeps track of the difference between two data inputs skipping, memorizing
  the last non zero value if the current difference is zero
  
  Formula:
    - diff = data - data1
    - nzd = diff if diff else diff(-1)
class_docstr_tok: ["Keeps","track","of","the","difference","between","two","data","inputs","skipping",",","memorizing","the","last","non","zero","value","if","the","current","difference","is","zero","Formula",":","-","diff","=","data","-","data1","-","nzd","=","diff","if","diff","else","diff","(","-1",")"]
code: def once(self, start, end):
      d0array = self.data0.array
      d1array = self.data1.array
      larray = self.line.array
  
      prev = larray[start - 1]
      for i in range(start, end):
          d = d0array[i] - d1array[i]
          larray[i] = prev = d if d else prev
  
file_path: backtrader/backtrader/indicators/crossover.py
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  from .None import Indicator, And
label_desc: Use image node_id 4 for calling the NonZeroDifference obj's underlying member method code with example usage: obj.once(start, end) without return types