\begin{Schunk}
\begin{Sinput}
 dat_legs
\end{Sinput}
\begin{Soutput}
  animal legs
1    dog    4
2   cats    4
3  human    2
4  snake    0
5   tree    0
\end{Soutput}
\begin{Sinput}
 dat_fur
\end{Sinput}
\begin{Soutput}
  animal    fur
1    dog    yes
2   cats Mostly
3  human     No
4   bird     No
\end{Soutput}
\begin{Sinput}
 merge(x = dat_fur, y = dat_legs, by = "animal", all.x = TRUE)
\end{Sinput}
\begin{Soutput}
  animal    fur legs
1   bird     No   NA
2   cats Mostly    4
3    dog    yes    4
4  human     No    2
\end{Soutput}
\end{Schunk}
