Context-Free Grammar In NLP

S
→ NP VP
→ Det N VP
→ the boy VP
→ the boy V NP
→ the boy plays NP
→ the boy plays PP
→ the boy plays P NP
→ the boy plays with NP
→ the boy plays with Det N
→ the boy plays with a ball

import nltk
from nltk import CFG

# 1. Define the CFG grammar
# The fix: added 'V PP' to the VP rule to allow for a verb followed by a prepositional phrase
grammar = CFG.fromstring("""
S -> NP VP
NP -> Det N | Det N PP | N
VP -> V NP | VP PP | V PP
PP -> P NP

Det -> 'the' | 'a'
N -> 'boy' | 'ball'
V -> 'plays'
P -> 'with'
""")

# 2. Input sentence (must match grammar exactly!)
sentence = ['the', 'boy', 'plays', 'with', 'a', 'ball']

# 3. Create the parser
parser = nltk.ChartParser(grammar)

# 4. Try parsing and show derivation
found = False
for tree in parser.parse(sentence):
    found = True
    print("Parse tree found!\n")
    print(tree)  # Bracketed form
    tree.pretty_print()  # Nice aligned print
    print("\nProductions (Derivation sequence):")
    for prod in tree.productions():
        print(prod)

if not found:
    print("No valid parse found. Check grammar or sentence.")

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