title: Merging two datasets in a one-to-many relationship
body: <p>Table 1 (900 rows) identifies the regulators of each cluster:</p>
  
  <pre><code>reg cluster weight
  a   0   59
  b   0   52
  e   1   46
  f   1   46
  g   2   66
  h   2   18
  </code></pre>
  
  <p>Table 2 (90 rows) identifies the genes belonging to each cluster:</p>
  
  <pre><code>gene cluster 
  1   0
  2   1
  3   0
  4   2
  5   0
  6   0
  7   1
  </code></pre>
  
  <p>I want to create a table 3 from these two that looks like this:</p>
  
  <pre><code>reg gene weight
  a   1   59
  a   3   59
  a   5   59
  a   6   59
  b   1   52
  b   3   52
  b   5   52
  b   6   52
  e   2   46
  e   7   46
  …
  </code></pre>
  
  <p>i.e., I want a list of every interaction between regs and genes. How can I do this in R?</p>