title: dplyr how to do the rowwise conditional mutate operation
body: <p>I want to have additional column to the iris table, so that within the same Species, first row is NA, and the rest rows are 1 if Petal.Width is the same as the last row (the row right above), or 0 if Petal.Width is different from last row.</p>
  
  <p>How do I achieve this with dplyr? Change column below is desired.</p>
  
  <pre><code>   Sepal.Length Sepal.Width Petal.Length Petal.Width Species Change
  1          5.1         3.5          1.4         0.2  setosa   NA
  2          4.9         3.0          1.4         0.2  setosa   0
  3          4.7         3.2          1.3         0.2  setosa   0
  4          4.6         3.1          1.5         0.2  setosa   0
  5          5.0         3.6          1.4         0.2  setosa   0
  6          5.4         3.9          1.7         0.4  setosa   1
  </code></pre>