# CYK Parsing Algorithm for General Context-Free Grammars

Given a context-free grammar , how to check if ?

• recursive descent gives efficient answer when is LL(1)
• we now see how to do it for arbitrary context-free grammar Conventions:

• S will always denote the start symbol of the grammar
• rules of grammar are always of the form where is a string of terminals and non-terminals

## Chomsky Normal Form

A grammar is in Chomsky normal form if it has only these kinds of productions:

X ::= Y Z
X ::= t
S ::= ""

where

• X,Y,Z denote non-terminals
• t denotes terminals
• S is the start non-terminal
• if S::=“” appears, then S does not appear on right-hand side of another rule

Observe:

• the empty string can only occur for the start non-terminal
• terminals occur only by themselves on right-hand side
• in parse tree, each non-terminal leads either to terminal or to two other non-terminals

## Parsing a Chomsky Normal Form Grammar

Example Grammar

S ::= L R | S S | L X
X ::= S R
L ::= "{"
R ::= "}"

Consider an input string

{ { } { } { } }

For each terminal t in input, for which non-terminal X is it the case that X ?

{ { } { } { } }   length of substring
L L R L R L R R      1
S   S   S         2
X        3
S   S           4
X          5
S             6
X           7
S             8

For each string w of length 2 in input, for which non-terminal X is it the case that X ?

Dynamic programming algorithm: for each substring, determine which non-terminals can generate it.

Let be input word

Let d(i)(j) denote non-terminals deriving substring of from i to j. CYK algorithm:

  INPUT: word w, grammar G in Chomsky normal form
OUTPUT: true iff (w in L(G))
N = |w|
var d : Array[N][N]
forall i != j : d(i)(j) = {}
d(i)(i) = {X | G contains X->w(i)}

for k = 2 to N // substring length
for i = 0 to N-k // initial position
for j = 1 to k-1 // length of first half
for each (X::=Y Z) in G
if Y in d(i)(i+j-1) and Z in d(i+j)(i+k-1)
d(i)(j) = d(i)(j) union {X}
return (S in d(0,N-1))

## Example of Parsing

Consider this fragment of a grammar of language with references and procedure calls

statement ::= assign | call
assign ::= expr "=" expr
call ::= expr "." ID "(" expr ")"
expr ::= ID | expr "." expr

Is this grammar LL(1)?

What is its Chomsky Normal Form?