title: Use condition in summarise_at in dplyr
body: <p>I would like to be able to use a condition to filter what gets summarised in summarise_at calls. I know this can be done when calling dplyr:: summarise() directly but I would like to avoid having to specify this for each variable.</p>
  
  <p>I have provided the below code thinking that this call to summarise_at would give me the correct data. But it does not. I would like the code to return the data set labelled final data. Where there is a row for each individual with each variable summed ONLY when the row matches with the filtering condition. When it does not match or there is nothing to sum I would like it to return 0 or NA.</p>
  
  <pre><code>Data = tibble(Group = c(1,1,1,1,1,1,1,1),
                Week = c(1,2,3,4,1,2,3,4),
                Condition = c(1,1,1,0,0,1,1,0),
                Var1 = 1:8,
                Var2 = 9:16,
                Var3 = 17:24)
  
  Data %&gt;% 
      group_by(Group, Week, Condition) %&gt;% 
      summarise_at(vars(Var1, Var2, Var3), ~sum(., na.rm = T)) %&gt;%
      ungroup()
  
  
  Final.Data = tibble(Group = c(1,1,1,1),
                      Week = c(1,2,3,4),
                      Var1 = c(1,8,10,0),
                      Var2 = c(9,24,26,0),
                      Var3 = c(17,40,42,0))
  </code></pre>