title: What's the best way to replace missing values with NA when reading in a .csv?
body: <p>I have a .csv dataset with many missing values, and I'd like R to recognize them all the same way (the "correct" way) when I read the table in. I've been using: </p>
  
  <pre><code>import = read.csv("/Users/dataset.csv", 
                    header =T, na.strings=c(""))
  </code></pre>
  
  <p>This script fills all the empty cells with something, but it's not consistant.  When I look at the data with <code>head(import)</code>, some missing cells are filled with <code>&lt;NA&gt;</code> and some missing cells are filled with <code>NA</code>.  I fear that R treats these two ways of identifying missing values differently when start analyzing the dataset, so I'd like to have the import uniformly read in those missing values.</p>
  
  <p>Finally, some of the missing values in my csv file are represented with a period only.  I would also like those periods to be represented by the correct missing value notation when I import to R.</p>