Michael Sipser Edition 3 Exercise 2 Question 27 (Page No. 157)

Question

$G$ is a natural-looking grammar for a fragment of a programming language, but $G$ is ambiguous$.$

1. Show that $G$ is ambiguous$.$
2. Give a new unambiguous grammar for the same language$.$

Answer 1

for the string

"if condition then if condition then a:=1 else a:=1" has two different parse trees

so the grammar is ambiguous

This is also called dangling else problem