title: find the minimum value that is not zero per each group using aggregate function
body: <p>I have the following dataset:</p>
  
  <pre><code>6b9691fa    1   0   2   1   4
  6b9691fa    1   1   -1  1   5
  6b9691fa    1   2   3   0   4
  6b9691fa    2   0   1   5   3
  6b9691fa    2   1   0   5   3
  6b9691fa    2   2   3   5   3
  6b9691fa    3   0   10  1   9
  6b9691fa    3   1   10  0   9
  6b9691fa    3   2   10  1   9
  </code></pre>
  
  <p>I would like to find the minimum value in the 4th, 5th, and 6th columns per each group of the 2nd column (i.e., three groups that are 1, 2, and 3). The minimum value should not be zero. To do that, I did the following:</p>
  
  <pre><code> aggregate(dataset[,4:6], list(dataset$V2), min)
  </code></pre>
  
  <p>But that returns zero. What can I do in order to avoid zero as a minimum?</p>