title: R. replacing null value representation with NA
body: <p>I have tried all the methods that I have found on stackoverflow regarding this topic and nothing worked.</p>
  
  <p>Here is a sample of my dataset called TEST:</p>
  
  <pre><code>x2000 x2001 x2002
  
  100   1200   230
  200   2002   280
  :     1980   :  
  </code></pre>
  
  <p>":" represents a missing value. The problem is that I cannot replace this colon with R-accepted NA.</p>
  
  <p>What I have tried:</p>
  
  <pre><code>sum(TEST %in c(":"))
  returns: [1] 0
  
  TEST[TEST==":"] &lt;-NA #does nothing
  </code></pre>
  
  <p>I tried to save the file as .csv, replace the values with "NA" in excel and it still does nothing. The columns are not factors. if the column contains the value of ":" then the column is "chr" otherwise it is "int".  </p>