title: tidyverse concatenate characters across columns does not work with c_across
body: <p>I have the following data:</p>
  <pre><code>dat &lt;- data.frame(sheet      = c(&quot;a&quot;, &quot;b&quot;, &quot;c&quot;),
                    condition1 = c(&quot;k&quot;, &quot;l&quot;, &quot;m&quot;),
                    condition2 = c(&quot;x&quot;, &quot;y&quot;, &quot;z&quot;))
  </code></pre>
  <p>I now want to create a new column using the tidyverse that concatenastes these columns. However, I always get an error.</p>
  <pre><code>library(tidyverse)
  
  new_dat %&gt;%
    rowwise() %&gt;%
    mutate(marker = str_c(c_across(c(sheet, starts_with(&quot;condition&quot;))), sep = &quot;/&quot;)) %&gt;%
    ungroup()
  
  Error: Problem with `mutate()` input `marker`.
  x Input `marker` can't be recycled to size 1.
  i Input `marker` is `str_c(c_across(c(sheet, starts_with(&quot;condition&quot;))), sep = &quot;/&quot;)`.
  i Input `marker` must be size 1, not 3.
  i Did you mean: `marker = list(str_c(c_across(c(sheet, starts_with(&quot;condition&quot;))), sep = &quot;/&quot;))` ?
  i The error occurred in row 1.
  </code></pre>
  <p>Any suggestion/help?</p>
  <p>Expected output:</p>
  <pre><code>  sheet condition1 condition2 marker
  1     a          k          x  a/k/x
  2     b          l          y  b/l/y
  3     c          m          z  c/m/z
  </code></pre>
  <p>Note: I alyo tried the code without using the rowwise part and using <code>across</code> instead of <code>c_across</code> (and adapting the syntax for across), but I'm getting a similar error. Replacing <code>str_c</code> by <code>paste0</code> also doesn't work.</p>
  <p>EDIT: deleted the x column from the input data since it doesn't add any value.</p>