title: Create counter within consecutive runs of values
body: <p>I wish to create a sequential number within each run of equal values, like a counter of occurrences, which restarts once the value in the current row is different from the previous row. </p>
  
  <p>Please find an example of input and expected output below.</p>
  
  <pre><code>dataset &lt;- data.frame(input = c("a","b","b","a","a","c","a","a","a","a","b","c"))
  dataset$counter &lt;- c(1,1,2,1,2,1,1,2,3,4,1,1)
  dataset
  
  #    input counter
  # 1      a       1
  # 2      b       1
  # 3      b       2
  # 4      a       1
  # 5      a       2
  # 6      c       1
  # 7      a       1
  # 8      a       2
  # 9      a       3
  # 10     a       4
  # 11     b       1
  # 12     c       1
  </code></pre>
  
  <hr>
  
  <p>My question is very similar to this one: <a href="https://stackoverflow.com/questions/15230446/cumulative-sequence-of-occurrences-of-values">Cumulative sequence of occurrences of values</a>. </p>