\begin{frame}{Top-down Parsing} \begin{goal}{} \emph{Top-down parsing} tries to derive the input word from the starting variable $S$. \end{goal} \pause\bigskip \begin{block}{} \emph{Simple leftmost strategy:} \begin{itemize} \item Always expand the leftmost variable $A$.\\ (Replace $A$ by $u$ if there is a rule $A \to u$.) \item Backtrack when a mismatch with the input string is found.\\ (Then try another rule.) \end{itemize} \pause\smallskip \alert{Disadvantage}: backtracking is expensive and difficult. \end{block} \end{frame}