title: How to remove `NA` from set of vectors
body: <p>I have <code>n</code>-size vectors <code>w1</code>, <code>w2</code>, <code>w3</code>. I need to remove <code>NA</code> from each column of corresponded elements.
  For <code>6</code>-size vectors:</p>
  
  <pre><code>w1 &lt;- c(NA, NA, NA, NA,  2,  1) 
  w2 &lt;- c(NA, NA,  4,  3, NA, NA)
  w3 &lt;- c(6,  5, NA, NA, NA, NA)
  </code></pre>
  
  <p>expected result is  <code>(6, 5, 4, 3, 2, 1)</code></p>
  
  <p>I save vectors in <code>data.frame</code>:</p>
  
  <pre><code>df &lt;- data.frame(w1, w2, w3)
  remove &lt;- c("NA")
  </code></pre>
  
  <p>And then try to remove <code>NA</code> row-by-row</p>
  
  <pre><code>w &lt;- c()
  k &lt;- nrow(df)
  
  for(i in 1:k)
  {
  w &lt;- c(w, df[i,] [! df[i,] %in% remove])
  }
  
  df &lt;- data.frame(df, w=unlist(w))
  </code></pre>
  
  <p><strong>My result is in df$w:</strong></p>
  
  <pre><code>df
    w1 w2 w3 w
  1 NA NA  6 6
  2 NA NA  5 5
  3 NA  4 NA 4
  4 NA  3 NA 3
  5  2 NA NA 2
  6  1 NA NA 1
  </code></pre>
  
  <p><strong>Question.</strong> Could someone guide me how to rewrite code in the vector form without <code>for-loop</code>?</p>