title: Melting data, keeping certain columns paired
body: <p>I have data as follows:</p>
  <pre><code>DT &lt;- structure(list(ECOST = c(&quot;Choice_01&quot;, &quot;Choice_02&quot;, &quot;Choice_03&quot;, 
  &quot;Choice_04&quot;, &quot;Choice_05&quot;, &quot;Choice_06&quot;, &quot;Choice_07&quot;, &quot;Choice_08&quot;, 
  &quot;Choice_09&quot;, &quot;Choice_10&quot;, &quot;Choice_11&quot;, &quot;Choice_12&quot;), control = c(18, 
  30, 47, 66, 86, 35, 31, 46, 55, 39, 55, 41), treatment = c(31, 
  35, 46, 68, 86, 36, 32, 42, 52, 39, 58, 43), control_p = c(0.163636363636364, 
  0.272727272727273, 0.427272727272727, 0.6, 0.781818181818182, 
  0.318181818181818, 0.281818181818182, 0.418181818181818, 0.5, 
  0.354545454545455, 0.5, 0.372727272727273), treatment_p = c(0.319587628865979, 
  0.360824742268041, 0.474226804123711, 0.701030927835051, 0.88659793814433, 
  0.371134020618557, 0.329896907216495, 0.43298969072165, 0.536082474226804, 
  0.402061855670103, 0.597938144329897, 0.443298969072165)), row.names = c(NA, 
  -12L), class = c(&quot;tbl_df&quot;, &quot;tbl&quot;, &quot;data.frame&quot;))
  
  # A tibble: 12 x 5
     ECOST     control treatment control_p treatment_p
     &lt;chr&gt;       &lt;dbl&gt;     &lt;dbl&gt;     &lt;dbl&gt;       &lt;dbl&gt;
   1 Choice_01      18        31     0.164       0.320
   2 Choice_02      30        35     0.273       0.361
   3 Choice_03      47        46     0.427       0.474
   4 Choice_04      66        68     0.6         0.701
   5 Choice_05      86        86     0.782       0.887
   6 Choice_06      35        36     0.318       0.371
   7 Choice_07      31        32     0.282       0.330
   8 Choice_08      46        42     0.418       0.433
   9 Choice_09      55        52     0.5         0.536
  10 Choice_10      39        39     0.355       0.402
  11 Choice_11      55        58     0.5         0.598
  12 Choice_12      41        43     0.373       0.443
  </code></pre>
  <p>I want to melt this data, but I want the columns <code>control</code> and <code>control_p</code> to stay together, and the columns <code>treatment</code> and <code>treatment_p</code> to stay together, creating a table with 24 rows and 4 columns.</p>
  <p>Desired result:</p>
  <pre><code># A tibble: 12 x 5
     ECOST     count   percentage  group
     &lt;chr&gt;       &lt;dbl&gt;     &lt;dbl&gt;   
   1 Choice_01      18     0.164   control
   2 Choice_02      30     0.273   control
   3 Choice_03      47     0.427   control
   4 Choice_04      66     0.6     control 
   5 Choice_05      86     0.782   control  
   6 Choice_06      35     0.318   control
   7 Choice_07      31     0.282   control   
   8 Choice_08      46     0.418   control  
   9 Choice_09      55     0.5     control   
  10 Choice_10      39     0.355   control  
  11 Choice_11      55     0.5     control    
  12 Choice_12      41     0.373   control      
  13 Choice_01      18     0.320   treatment
  14 Choice_02      30     0.361   treatment
  15 Choice_03      46     0.474   treatment
  16 Choice_04      68     0.701   treatment
  17 Choice_05      86     0.887   treatment
  18 Choice_06      36     0.371   treatment
  19 Choice_07      32     0.330   treatment
  20 Choice_08      42     0.433   treatment
  21 Choice_09      52     0.536   treatment
  22 Choice_10      39     0.402   treatment
  23 Choice_11      58     0.598   treatment
  24 Choice_12      43     0.443   treatment
  </code></pre>