title: Using `mutate_at` and `na_if` together to replace zeros with NA for only some columns
body: <p>My data takes this format:</p>
  
  <pre><code>library(tidyverse)
  df &lt;- mtcars
  df &lt;- df %&gt;% mutate(vs_doubled = vs * 2) %&gt;% select(mpg, cyl, vs, am, vs_doubled)
  
  head(df)
  
  
  #&gt;    mpg cyl vs am vs_doubled
  #&gt; 1 21.0   6  0  1          0
  #&gt; 2 21.0   6  0  1          0
  #&gt; 3 22.8   4  1  1          2
  #&gt; 4 21.4   6  1  0          2
  #&gt; 5 18.7   8  0  0          0
  #&gt; 6 18.1   6  1  0          2
  </code></pre>
  
  <p>I'm trying to use <code>mutate_at</code> and <code>na_if</code> to set 0 values as NA--but only for specific columns ("vs" and "am"). I would like to leave the column "vs_doubled" with zeros in it.</p>
  
  <p>I haven't quite got it right, because the following line doesn't work:</p>
  
  <pre><code>df &lt;- df %&gt;% mutate_at(.vars = c("vs", "am"), .funs = na_if(y = 0))
  </code></pre>