Filling blanks (missing values) in table with value above using ArcGIS Field Calculator?

I'm using ArcMap to tidy up some field collected point data. The point data is a sequential track from a handheld device. In the attribute table i have a field with differant activities that separate out each track. However, only the first occurrence/point of the activity is filled in and the rest below are blank till the next activity. .eg.

running

-

walking

-

Is there a way to filldown the entries to blank fields below? There are hundreds of blank entries for each one with a value, not sure if this makes a difference.

Answer

Assuming this structure:

create new field and run this field calculator expression on it:

def fillMe(text):
 global before
 if text=="-":
  return before
 else:
  before=text
  return before

fillMe( !CAT_DOG!)