title: R-read.csv only read in certain rows without having to modify code everytime
body: <p>I would like to only use the rows in my <code>csv</code> file that correspond to a specific date. I have seen many good ways to do this, however, they all require you to <em>actually know that certain date and specify it in your code</em>. Since I will be running this program frequently, what I am looking for is a fully automated process where I would not have to continue to change the specific date in my code. My data set looks something like this (Fortunately, I always want to read from the bottom, so I can use tail if need be):</p>
  
  <pre><code>Date      Ticker
  ...        ....
  2015-12-31 TIF
  2016-01-31 DD
  2016-01-31 ADP
  </code></pre>
  
  <p>Essentially, I am asking if there is a way to say <code>read.csv("df.csv", *only rows with same date as last row*)</code>.</p>
  
  <p>I know that subsetting based on date is possible or there may be some way to do it like this: </p>
  
  <pre><code>x &lt;-tail(df, *only rows with same date as last row*)
  </code></pre>
  
  <p>however, after some time, my data set will get quite large, and I don't think I would like to continue to read in such a large data set every time.</p>