title: How to combine multiple character columns into one columns and remove NA without knowing column numbers
body: <p>I would like to have a column that contains other columns characters without NA.
  I have tried <code>paste</code>, <code>str_c</code> and <code>unite</code>, but could not get the expected result. Maybe I used them incorrectly.</p>
  
  <p>The real case is, I could not know the column numbers in advance since each dataset can be varied in terms of years.</p>
  
  <p>i.e. some datasets contain 10 years, but some contain 20 years.</p>
  
  <p>Here is the input data:</p>
  
  <pre><code>input &lt;- tibble(
    id = c('aa', 'ss', 'dd', 'qq'),
    '2017' = c('tv', NA, NA, 'web'),
    '2018' = c(NA, 'web', NA, NA),
    '2019' = c(NA, NA, 'book', 'tv')
  )
  
  # A tibble: 4 x 4
    id    `2017` `2018` `2019`
    &lt;chr&gt; &lt;chr&gt;  &lt;chr&gt;  &lt;chr&gt; 
  1 aa    tv     NA     NA    
  2 ss    NA     web    NA    
  3 dd    NA     NA     book  
  4 qq    web    NA     tv    
  </code></pre>
  
  <p>The desired output with the ALL column is:</p>
  
  <pre><code>&gt; output
  # A tibble: 4 x 5
    id    `2017` `2018` `2019` ALL   
    &lt;chr&gt; &lt;chr&gt;  &lt;chr&gt;  &lt;chr&gt;  &lt;chr&gt; 
  1 aa    tv     NA     NA     tv    
  2 ss    NA     web    NA     web   
  3 dd    NA     NA     book   book  
  4 qq    web    NA     tv     web tv
  </code></pre>
  
  <p>Thanks for the help!</p>