title: Collapse several columns of data frame into one data frame
body: <p>For some reason, I have a data in which a few columns are a set of data frame consist of one column. So, I want to &quot;collapse&quot; these columns of data frame into one data frame.</p>
  <pre><code>library(tidyverse)
  df &lt;- tibble(col1=1:5, 
                col2=tibble(newcol=LETTERS[1:5]),
                col3=tibble(newcol2=LETTERS[6:10]))
  df
  # A tibble: 5 x 3
     col1 col2$newcol col3$newcol2
    &lt;int&gt; &lt;chr&gt;       &lt;chr&gt;       
  1     1 A           F           
  2     2 B           G           
  3     3 C           H           
  4     4 D           I           
  5     5 E           J  
  </code></pre>
  <p>I have tried <code>unnest()</code>, but, the function actually replicate data frame/tibble of col2 and col3 for each row of col1, which is not what I want.</p>
  <pre><code>df2 &lt;- df %&gt;% unnest(cols = c(col2, col3))
  df2
  # A tibble: 25 x 3
      col1 col2  col3 
     &lt;int&gt; &lt;chr&gt; &lt;chr&gt;
   1     1 A     F    
   2     1 B     G    
   3     1 C     H    
   4     1 D     I    
   5     1 E     J    
   6     2 A     F    
   7     2 B     G    
   8     2 C     H    
   9     2 D     I    
  10     2 E     J    
  # ... with 15 more rows
  </code></pre>
  <p>The result that I want is as below:</p>
  <pre><code>df3 &lt;- tibble(col1=1:5, 
               newcol=LETTERS[1:5],
               newcol2=LETTERS[6:10])
  df3
  # A tibble: 5 x 3
     col1 newcol newcol2
    &lt;int&gt; &lt;chr&gt;  &lt;chr&gt;  
  1     1 A      F      
  2     2 B      G      
  3     3 C      H      
  4     4 D      I      
  5     5 E      J  
  </code></pre>
  <p>Any idea how to do this? Any help is much appreciated.</p>