title: Count using dplyr
body: <p>With this data frame:</p>
  
  <pre><code>df = data.frame(mes = c(1,1,2,2,3,3), 
                  ano = c(1981, 1982,1983), 
                  x1 = c(95,8,10,NA,NA,98), 
                  x2 = c(NA, NA, 89, 48, NA, 10))
  &gt; df
    mes  ano x1 x2
  1   1 1981 95 NA
  2   1 1982  8 NA
  3   2 1983 10 89
  4   2 1981 NA 48
  5   3 1982 NA NA
  6   3 1983 98 10
  </code></pre>
  
  <p>I want to get this:</p>
  
  <pre><code>  mes x1_n x2_n
  1   1    2    0
  2   2    1    2
  3   3    1    1
  </code></pre>
  
  <p>I mean, for each <code>mes</code> unique value I want to know how many non <code>NA</code> values there are. I was trying to work with <code>dplyr::count()</code> but I get this:</p>
  
  <pre><code>&gt; count(df,mes)
  # A tibble: 3 × 2
      mes     n
    &lt;dbl&gt; &lt;int&gt;
  1     1     2
  2     2     2
  3     3     2
  </code></pre>
  
  <p>Any thoughts? Thanks.</p>