title: Subset data.frame based on a column that matches a separate vector
body: <p>once again stumped here.</p>
  
  <p>I have a data.frame of 4 columns:</p>
  
  <pre><code>Col1      Col2      Col3      Col4
  1         1.lsm     0.43      0.34
  2         1.lsm     0.47      0.30
  3         1.lsm     0.27      0.85
  1         2.lsm     0.35      0.55
  2         2.lsm     0.71      0.46
  3         2.lsm     0.53      0.37
  4         2.lsm     0.63      0.34
  </code></pre>
  
  <p>Col1 is the cell number for cells that have been tracked over time.</p>
  
  <p>I have a vector containing integers pertaining to which cells I want to keep:</p>
  
  <pre><code>keep=c(3, 4)
  </code></pre>
  
  <p>Now, I want to use the vector "keep" to decide which rows of the data.frame are kept and output a new data.frame with all columns but keep only the relevant rows.</p>
  
  <p>I.e. the ideal output here would be:</p>
  
  <pre><code>Col1      Col2      Col3      Col4
  3         1.lsm     0.27      0.85
  3         2.lsm     0.53      0.37
  4         2.lsm     0.63      0.34
  </code></pre>