title: Replace missing values with mean
body: <pre><code>  [1] 10  2  4  1 10 10  1  1  1  1  1  3  3  9  1  1  1 10  1 10  7  1 NA  1  7  1  1  1  1  1  1  5  1  1  1  1  1 10  7 NA  3 10  1  1  1  9
   [47]  1  1  8  3  4  5  8  8  5  6  1 10  2  3  2  8  2  1  2  1 10  9  1  1  2  1 10  4  2  1  1  3  1  1  1  1  2  9  4  8 10  1  1  1  1  1
   [93]  1  1  1  1  1  6 10  5  5  1  3  1  3 10 10  1  9  2  9 10  8  3  5  2 10  3  2  1  2 10 10  7  1 10  1 10  1  1  1 10  1  1  2  1  1  1
  [139] NA  1  1  5  5  1 NA  8  2  1 10  1 10  5  3  1 10  1  1 NA 10 10  1  1  3 NA  2 10  1  1  1  1  1  1 10 10 10  1  1  1 10  1  1  1 10 10
  [185]  1  8 10  8  1  8 10  1  1  1  1  7  1  1  1 10 10  1  1  1 10  5  1  1  1 10  8  1 10 10  5  1  1  4  1  1 10  5  8 10  1 10  5  1 10  7
  [231]  8  1 10  1 NA 10  2  9 10  2  1  1  5  1  2 10  9  1 NA  1 10 10 10  8 10  1  1  1  8 10 10 10 10  3  1 10 10  4  1 10  1 10  4  1 NA  1
  [277]  1  1  7  1  1 10 10 10 10 10  1  5 10  1  1 NA 10 NA 10  5 NA  1 10  4  1 10  1 10 10  1  1  3  5  1  1  1  1  1 NA 10  8  1  5 10 NA  1
  [323] 10  1  1 10  1  4 10  8  1  1 10 10  1 10  1  1 10 10  1  1  1 10  1  1  1  1  8  1  1  3 10  1  1  3 10  4  7 10 10  3  3  1  1 10 10  1
  [369]  1  1  1  1  1  1  1  1  1  1  1  1 10  1  1  1  1 10  1  1  2  1 10  1  1  1  1  1  1  1  1  9  1  1  4  1  1  1  1  2  1  1 NA  4  1 10
  [415]  3 10  1  2  1  3 10  1  1  1 10  1  2  1  1  1  1  1  1  8 10  1  1  1  1 10  4  3  2  1  1  1  1  1 10  1  1  1 10  1  6 10  3  1  1  1
  [461]  5  1  1  1  4 10 10  1  1  1  1  1  1  1  1  1  1  1 10  1  1  5 10  1  3  1 10  3  4  1 10  1 10  5  1  1  1  1  1  1  1  1  1  1  1  5
  [507]  4  1  1  1  1  1  1 10 10  1  1  1 10  1  1  5 10  1  1  1  1  1  1 10  1  1  1  1  1  1  1  1  1  2  1  1  1  1  1 10  1  1  5  1  1  1
  [553]  5  1  1  1  1  1  1  1  1  1  1  1 10  1  3 10  5 10 10  1  1  2  1  1  1  1  1  1 10 10  1  1  1 10  1  3  1  1 10 10  1 10  1  1  1  1
  [599]  1  1  1  1  1 10  8  1  1 10  1 10  2 10  1  1  1  1 NA  1  1  1  2  1  1  1  4  6  5  1  1  1  1  1  3  1  1  1  2  1  1  1  1  1  1  1
  [645]  1  1  1  2  1  4  1  1  1  1  1  1  1 10  1  1  1  1  1  1  1  1  1  1  5  8  1  1  1  1  1  1  1  1  1 10 10  1  1  1  1  1  1  1  1  1
  [691]  5  1  1  2  1  3  4  5
  </code></pre>
  
  <p>those are the values in the seventh column, I want to replace NAs with column means. my entire data set is in the variable  df</p>