title: R: fill rows in column by matching rows in another column
body: <p>This has probably been asked many times, but I only find more complicated cases than mine, and I really don't know where to start. I need to add a new column (<code>Condition</code>) to my data frame and fill the rows according to the values in the column <code>cellNr</code></p>
  
  <p>My data frame is <code>molten.pC</code>:</p>
  
  <pre><code>  cellNr     value
  1    G63  0.000000
  2    G64  8.848623
  3    G65  0.000000
  4    G66 10.788718
  5    B15  5.285402
  6    B16  0.000000
  7    B17  0.000000
  8    C10  0.000000
  9    C11  0.000000
  </code></pre>
  
  <p>I want to add a column <code>Condition</code> and fill it like this:</p>
  
  <pre><code>  cellNr     value     Condition
  1    G63  0.000000  Growth
  2    G64  8.848623  Growth
  3    G65  0.000000  Growth
  4    G66 10.788718  Growth
  5    B15  5.285402  Burst
  6    B16  0.000000  Burst
  7    B17  0.000000  Burst
  8    C10  0.000000  Cellularized
  9    C11  0.000000  Cellularized
  </code></pre>