title: Get data.frame row by ID
body: <p>I have a data.frame $X$ obtained from a data.frame $Y$ by filtering out some rows. 
  I am now interested in accessing a row with ID $i$ (By ID I mean the value appearing in the very first column, generated by R). I am only aware of the $X[i,]$ command, which, as far as I understand, gives me the $i$-th row in $X$ but not the row with ID $i$. </p>
  
  <p>Is there any other command to access a row by its id?</p>