title: In R, How to calculate row count of a data frame depending on certain condition on column value?
body: <pre><code>cylinders   displacement    horsepower  weight  acceleration    model   origin  class
  1   8   307.0   130 3504    12.0    70  1   18.0
  2   8   350.0   165 3693    11.5    70  1   15.0
  3   8   318.0   150 3436    11.0    70  1   18.0
  4   8   304.0   150 3433    12.0    70  1   16.0
  5   8   302.0   140 3449    10.5    70  1   17.0
  6   8   429.0   198 4341    10.0    70  1   15.0
  7   8   454.0   220 4354    9.0 70  1   14.0
  8   8   440.0   215 4312    8.5 70  1   14.0
  9   8   455.0   225 4425    10.0    70  1   14.0
  10  8   390.0   190 3850    8.5 70  1   15.0
  11  8   383.0   170 3563    10.0    70  1   15.0
  12  8   340.0   160 3609    8.0 70  1   14.0
  13  8   400.0   150 3761    9.5 70  1   15.0
  14  8   455.0   225 3086    10.0    70  1   14.0
  15  4   113.0   95  2372    15.0    70  3   24.0
  16  6   198.0   95  2833    15.5    70  1   22.0
  17  6   199.0   97  2774    15.5    70  1   18.0
  18  6   200.0   85  2587    16.0    70  1   21.0
  19  4   97.0    88  2130    14.5    70  3   27.0
  20  4   97.0    46  1835    20.5    70  2   26.0
  21  4   110.0   87  2672    17.5    70  2   25.0
  22  4   107.0   90  2430    14.5    70  2   24.0
  23  4   104.0   95  2375    17.5    70  2   25.0
  </code></pre>
  
  <p>Suppose above is my data frame, how would I count the row number whose origin is 2? i.e. and should be 4</p>