title: Sum column with a condition in R
body: <p>I have a dataframe like this:</p>
  
  <pre><code>df &lt;- data.frame(a=c(111,111,111,222,222,222,333,333,333),
                   b=c(1,0,1,1,1,1,0,0,1))
  df
      a b
  1 111 1
  2 111 0
  3 111 1
  4 222 1
  5 222 1
  6 222 1
  7 333 0
  8 333 0
  9 333 1
  </code></pre>
  
  <p>I need to get the sum of column 'b' for each 'a':</p>
  
  <pre><code>    A B
  1 111 2
  2 222 3
  3 333 1
  </code></pre>
  
  <p>How can I do that in the fastest way?</p>