Lecture 14

Top-down Parsing

Start \(S\Rightarrow\alpha_1\Rightarrow\alpha_2\Rightarrow ...\Rightarrow w\), use the stack to store in intermediate \(\alpha_i\) in reverse, match against characters in \(w\).


Consumed input + reverse (stack contents) = \(\alpha_i\)


\(S\rightarrow A\enspace y\enspace B\)
\(A\rightarrow ab\)
\(A\rightarrow cd\)
\(B\rightarrow z\)
\(B\rightarrow wx\)

For simplicity, we use a augmented grammars for parsing, i.e.. invent two new symbols, \(\vdash\), \(\dashv\), new start symbol \(S'\)

\(S'\rightarrow \vdash S\dashv\)
\(S\rightarrow A\enspace y\enspace B\)
\(A\rightarrow ab\)
\(A\rightarrow cd\)
\(B\rightarrow z\)
\(B\rightarrow wx\)

say \(w=\vdash abywx\dashv\)

Stack Read Input UnreadInput Action
\(S'\) \(\epsilon\) \(\vdash abywx\dashv\) pop \(S'\); push \(\dashv, S, \vdash\)
\(\dashv S\vdash\) \(\epsilon\) \(\vdash abywx\dashv\) match \(\vdash\)
\(\dashv S\) \(\vdash\) \(abywx\dashv\) pop \(S\); push \(B,y,A\)
\(\dashv ByA\) \(\vdash\) \(abywx\dashv\) pop \(A\); push \(b,a\)
\(\dashv Byba\) \(\vdash\) \(abywx\dashv\) match \(a\)
\(\dashv Byb\) \(\vdash a\) \(bywx\dashv\) match \(b\)
\(\dashv By\) \(\vdash ab\) \(ywx\dashv\) match \(y\)
\(\dashv B\) \(\vdash aby\) \(wx\dashv\) pop \(B\), push \(x,w\)
\(\dashv xw\) \(\vdash aby\) \(wx\dashv\) match \(w\)
\(\dashv x\) \(\vdash abyw\) \(x\dashv\) match \(x\)
\(\dashv\) \(\vdash abywx\) \(\dashv\) match \(\dashv\)
\(\vdash abywx\dashv\) \(\epsilon\) Accept

But what if there is \(>1\) production with \(A\) on the LHS? How can we know which one to pick?

Our solution: use the next symbol of input (look ahead) to help decide.

Construct a predictor table: given non-terminal on stack and input symbol, tell which production to use.

  1. \(S'\rightarrow \vdash S\dashv\)
  2. \(S\rightarrow A\enspace y\enspace B\)
  3. \(A\rightarrow ab\)
  4. \(A\rightarrow cd\)
  5. \(B\rightarrow z\)
  6. \(B\rightarrow wx\)
\(\vdash\) \(a\) \(b\) \(c\) \(d\) \(w\) \(x\) \(y\) \(z\) \(\dashv\)
\(S'\) 1
\(S\) 2 2
\(A\) 3 4
\(B\) 6 5

Empty cell = parse error.

Descriptive: Parse error at row, column: expecting one of ... (characters for which current top of stack has entries)
e.g. if top of stack is \(A\), "Expecting \(a\) or \(c\)"

What if a cell contains more that one entry? Method breaks down.

A grammar is called \(LL(1)\) if each cell of the predictor table has at most one entry.

\(LL(1)=\)left-to-right scan of input, leftmost derivations produced, 1 symbols lookahead.

Automatically Computing the Predictor Table

\(Predict(A, a)\) - rules that apply when \(A\) on the stack, \(a\) is next input character.

\(Predict(A, a)=\{A\rightarrow\beta | a \in First(\beta)\}\)

\(First(\beta)\) - \(\beta\in V ^*\) - set of characters that can be the first letter of derivation starting from \(\beta\).

\(First(\beta)=\{a|\beta\Rightarrow ^*a\gamma\}\)

So \(Predict(A,a)=\{A\rightarrow\beta |\beta\Rightarrow ^*\alpha\gamma\}\)

So \(Predict(A,a)=\{A\rightarrow\beta |\beta\Rightarrow ^*\alpha\gamma\}\cup\{A\rightarrow\beta | Nullable(\beta), a\in Follow(A)\}\)

\(Nullable(B)=true\enspace iff\enspace \beta\Rightarrow ^*\epsilon\)
\(Follow(A)=\{b|S'\Rightarrow ^* \alpha Ab\beta\}\)

Computing \(Nullable\)

initialize \(Nullable[A]\)=false \(\forall A\)
\(\quad\) for each rule \(B\rightarrow B_1 ... B_k\)
\(\quad\quad\) if \(k=0\) or \(Nullable[B_i]\forall B_i\)
\(\quad\quad\quad\) \(Nullalbe[B]\leftarrow\) true.
until nothing changes

Computing \(First\)

initialize \(First[A]=\{\}\) \(\forall A\)
\(\quad\) for each rule \(B\rightarrow B_1 ... B_k\)
\(\quad\) for \(i=1 ... k\)
\(\quad\quad\quad\) if \(B_i\) is a terminal \(a\)
\(\quad\quad\quad\quad\) \(First[B]\cup=\{a\}\) break
\(\quad\quad\quad\) else
\(\quad\quad\quad\quad\) \(First[B]\cup=First[B_i]\)
\(\quad\quad\quad\quad\) if not \(Nullable[B_i]\) break
until nothing changes

Computing \(First ^*(\beta)\)

First set for a string of symbols

\(First ^*(Y_1, ..., Y_n)\)
\(\quad\) result\(\leftarrow\emptyset\)
\(\quad\) for \(i=1...n\)
\(\quad\quad\) if \(Y_i\not\in\Sigma\) (i.e. non-terminal)
\(\quad\quad\quad\) result \(\cup=First[Y_i]\)
\(\quad\quad\quad\) if not \(Nullable[Y_i]\) break
\(\quad\quad\) else (terminal)
\(\quad\quad\quad\) result \(\cup=\{Y_i\}\); break;
\(\quad\) return result

Computing \(Follow\)

initialize \(Follow[A]=\{\}\) \(\forall A\neq S'\)
\(\quad\) for each rule \(B\rightarrow B_1...B_n\)
\(\quad\quad\) for \(i=1...n\)
\(\quad\quad\quad\) if \(B_i\in N\)
\(\quad\quad\quad\quad\) \(Follow[B_i]\cup=First ^*(B_{i+1}...B_n)\)
\(\quad\quad\quad\quad\) if all \(B_{i+1}...B_n\) nullable (include i=n)
\(\quad\quad\quad\quad\quad\) \(Follow[B_i]\cup =Follow[B]\)
until nothing changes