Top-down and bottom-up parsing are two fundamental techniques used in compiler design to analyze the syntax of a program. They help determine if a given input string conforms to the grammar rules of a programming language.
Top-Down Parsing
Top-down parsing starts with the start symbol of the grammar and attempts to derive the input string by applying grammar rules in a top-down fashion. It's like building a tree from the root down, expanding the tree with production rules to match the input.
Key Characteristics:
- Goal-driven: It starts with the goal (start symbol) and works towards the input string.
- Leftmost derivation: It typically uses a leftmost derivation, focusing on the leftmost non-terminal in each step.
- Recursive descent: Recursive descent parsers are a common implementation, where functions correspond to non-terminals and recursively call each other.
Example:
Consider the grammar rule: S -> A B C.
A top-down parser would start with S and try to expand it using the rule. It would then try to expand A, B, and C in a similar way until it matches the input string.
Bottom-Up Parsing
Bottom-up parsing starts with the input string and attempts to reduce it to the start symbol by applying grammar rules in reverse. It's like building a tree from the leaves up, combining smaller parts to form larger ones.
Key Characteristics:
- Data-driven: It starts with the input data and works towards the start symbol.
- Rightmost derivation: It uses a rightmost derivation, focusing on the rightmost non-terminal in each step.
- Shift-reduce: It employs a combination of "shift" (reading input) and "reduce" (applying grammar rules) operations.
Example:
Consider the grammar rule: S -> A B C.
A bottom-up parser would start with the input string and try to identify the rightmost non-terminal that can be reduced using the grammar rules. It would then replace that non-terminal with its left-hand side, continuing this process until it reaches the start symbol.
Differences between Top-Down and Bottom-Up Parsing
| Feature | Top-Down | Bottom-Up |
|---|---|---|
| Direction | Top to bottom | Bottom to top |
| Derivation | Leftmost | Rightmost |
| Implementation | Recursive descent | Shift-reduce |
| Backtracking | Can be prone to backtracking | Less likely to backtrack |
| Efficiency | Can be less efficient | Generally more efficient |
Practical Insights
- Top-down parsing is simpler to understand and implement, but can be inefficient for some grammars.
- Bottom-up parsing is generally more efficient and handles a wider range of grammars.
- The choice between top-down and bottom-up parsing depends on the specific grammar and the performance requirements of the compiler.
