Answer:
Step-by-step explanation:
The two grammar characteristics that prohibit them from being used as the basis for a top-down parser are:
1. Left Recursion: Left recursion occurs when a non-terminal in a grammar can be derived directly or indirectly from itself. For example, consider the grammar rule A → Aα | β. Here, A can derive itself through A → Aα → Aαα →..., leading to an infinite loop. A top-down parser that tries to expand A first will never terminate, as it will continually expand A into itself. Thus, left recursion is not suitable for top-down parsers.
2. Ambiguity: Ambiguity occurs when a grammar rule has more than one valid parse tree. For example, consider the grammar rule A → BCD | Bε. Here, if we have the input "B", there are two ways to derive it: A → Bε or A → BCD → B. A top-down parser may not know which of these two paths to follow and may end up traversing both, leading to an exponential increase in the size of the parsing tree. This costly ambiguity is another reason top-down parsers are generally not used when the grammar contains ambiguities.