Recommender System Approach

https://en.wikipedia.org/wiki/Recommender_system
http://divakalife.blogspot.tw/2010/04/data-mining-collaborative-filtering.html

1. Collaborative filtering (User-based, Item-based)
2. Content-based filtering

Collaborative filtering can be consider as 2-D array:
User-based: sum of product of ratings and user-similarity
Item-based: sum of product of ratings and item-similarity
So, it's very dependent on the definition of similarity...

Content-based filtering use detailed item features to come out the suggestion...