title: using pandas to select a column as a condition for another column
body: <p>I am currently working on my Health data from the iPhone health app. I've made a pandas dataframe using the data, but I want to condense it a but further.
  I have the following columns for my steps taken data: unit, startDate, endDate, value. (Please exclude the last column.)
  Now, the iPhone records data per minute, a few minutes etc. 
  I would like to sum up the counts of the steps per day using the dates of course, but I'm not quite sure how to do that. 
  Thank you in advance.
  <a href="https://i.stack.imgur.com/cRlcu.png" rel="nofollow noreferrer">Pandas Dataframe</a></p>