title: Delete character vectors from df that contain the same string
body: <p>I want to identify (and subsequently delete) character vectors from a dataset, that consist of entries, that are all equal (e.g. do not have any variation): </p>
  
  <pre><code>test_data &lt;- tibble(a = c("A", "B", "C"), b = c("A", "A", "A"), c = c("", "", ""), d = 1:3)
  
  test_data
  
  # A tibble: 3 x 4
    a     b     c         d
    &lt;chr&gt; &lt;chr&gt; &lt;chr&gt; &lt;dbl&gt;
  1 A     A     ""        1
  2 B     A     ""        2
  3 C     A     ""        3
  </code></pre>
  
  <p>I want the result to be something like this:</p>
  
  <pre><code># A tibble: 3 x 2
    a         d
    &lt;chr&gt; &lt;dbl&gt;
  1 A         1
  2 B         2
  3 C         3
  </code></pre>
  
  <p>Of course I can achieve that by doing:</p>
  
  <pre><code>out &lt;- c("b", "c")
  test_data %&gt;% select(- one_of((out)))
  </code></pre>
  
  <p>But as I have a lot of those columns and also a lot of rows, I'd prefer not to have to do it "manualy".</p>
  
  <p>I found this <a href="https://stackoverflow.com/questions/48425289/get-column-names-with-zero-variance-using-dplyr">but it only works for numeric vectors.</a></p>