title: R: how to easily replace the missing values in data frame into "."
body: <p>I am trying to manipulate a dataframe.
  The data frame contains missing value (NA), and I would like to replace every NAs' into "." (string).</p>
  
  <p>How can I do that? Below is an example:
  consider the following data frame</p>
  
  <pre><code>   dataset1
     no      name
      1      John
      2      Paul
      3        NA
      4    Sophie
  </code></pre>
  
  <p>How can I change this dataset into:</p>
  
  <pre><code>   dataset1
     no      name
      1      John
      2      Paul
      3         .
      4     Sophie
  </code></pre>
  
  <p>Thank you,</p>