title: Counting number of entries by ID in R?
body: <p>So I'm trying to count the number of entries by ID in R, I'll use a modified version of mtcars to get my point across.  Here's the data:</p>
  
  <pre><code>car type            mpg cyl  disp  hp drat    wt  qsec vs am gear carb
  Mazda RX4           21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4
  Mazda RX4           21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4
  Datsun 710          22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1
  Datsun 710          21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1
  </code></pre>
  
  <p>I want to end up with a table that counts the number of entries for each ID, so that my results are:</p>
  
  <pre><code>Mazda RX4  2
  Datsun 710 2
  </code></pre>
  
  <p>Should be a fairly simple and straightforward solution, but I'm new to R and can't quite figure it out.  Should I use "Aggregate"?</p>