title: Summing # of occurrences of column values conditional to values in another column
body: <p>I have a large data frame (<code>g1</code>) consisting of columns <code>count</code>, <code>cdr3</code>, and <code>length</code>, with <code>cdr3</code> and <code>length</code> being pertinent to my problem.</p>
  
  <p>The column <code>cdr3</code> consists of a number of strings, with column length giving the length of the string. There are, as such, 70 unique values for length.</p>
  
  <p>What's a quick and clean way to aggregate the number of strings (column <code>cdr3</code> values) that are of a particular length?</p>