title: User based collaborative filtering issue
body: <p><img src="https://i.stack.imgur.com/CISaU.png" alt="prediction"></p>
  
  <p>In the user based Collaborative Filtering, the picture shows the formula of how to predict the rating of an item. And the NSa is the nearest neighbor set of user a. j is the item to be predicted. rij means the rating of item j by the user i in the NSa. So, my question is,what if the user i has never voted the item j? How to handle the rij? Thanks! </p>