title: how to read huge csv file into R by row condition?
body: <p>I have a huge csv file about 15 million row, with size around 3G. </p>
  
  <p>I would like to read this file into R by piece, each time only choose those rows fit into certain condition.</p>
  
  <p>e.g. one of the column is called product type, so I only need to read one type of product into R, and process it then output the result, after that I move to another type of product...</p>
  
  <p>so far I have read about different methods, such as upload the big file into database, or read column by column by colbycol, or read a chunk of rows by ff ...</p>
  
  <p>is any pure R solution can solve my problem?</p>