title: dplyr: mutate() and mutate_if()
body: <p>I want to change all NAs in columns to 0.
  I can do it with <code>mutate()</code>, but not with <code>mutate_if()</code>. </p>
  
  <p>This works:</p>
  
  <pre><code>&gt; test &lt;- tibble(Q3.2 = c(1,2, NA, NA, 3),
                               Q8.2 = c(2, NA, 1, NA, 4))
  
  &gt; test %&gt;% select_("Q3.2", "Q8.2") %&gt;%
      mutate(Q3.2 = ifelse(is.na(Q3.2), 0, Q3.2),
             Q8.2 = ifelse(is.na(Q8.2), 0, Q8.2))
  </code></pre>
  
  <p>But this doesn't:</p>
  
  <pre><code>&gt; test %&gt;% select_("Q3.2", "Q8.2") %&gt;%
  +     mutate_if(is.na(.), 0, .)
  Error in get(as.character(FUN), mode = "function", envir = envir) : 
    object 'p' of mode 'function' was not found
  </code></pre>