What is the Difference Between Top Down and Bottom Up Parsing?
🆚 Go to Comparative Table 🆚The main difference between top-down and bottom-up parsing lies in the approach they take to build the parse tree and the type of derivation they use. Here are the key differences between the two parsing techniques:
Top-Down Parsing:
- Starts from the top (start symbol) of the parse tree and works down to match the input.
- Uses leftmost derivation, which means it attempts to find the left-most derivation for an input string.
- Often used for LL(k) grammars.
- Has good error reporting, is predictable, and easier to implement.
- Examples of top-down parsing techniques include Recursive Descent parser.
Bottom-Up Parsing:
- Starts from the input terminals (bottom of the parse tree) and constructs the parse tree by reducing them according to grammar rules.
- Uses rightmost derivation, which means it attempts to reduce the input string to the start symbol of a derivation tree.
- Typically used for a broader range of grammars, including LR(k).
- Handles a broader grammar, is less predictable, more complex to implement, and involves challenging error recovery.
- Examples of bottom-up parsing techniques include Shift-Reduce parser.
In summary, top-down parsing starts from the top of the parse tree and works down, using leftmost derivation, while bottom-up parsing starts from the input terminals and works up, using rightmost derivation. Top-down parsing is more predictable and easier to implement, but it is limited for complex grammars. Bottom-up parsing can handle a broader range of grammars but is more complex to implement and has more challenging error recovery.
Comparative Table: Top Down vs Bottom Up Parsing
Here is a table comparing the differences between top-down and bottom-up parsing:
| Feature | Top-Down Parsing | Bottom-Up Parsing |
|---|---|---|
| Approach | Starts from the highest level of the parse tree and works down to the input string | Starts from the input string and works up to the start symbol |
| Strategy | Analyzing unknown data relationships by hypothesizing general parse tree structures | Analyzing unknown data and inferring higher-order structures from them |
| Parsing Technique | Left Most Derivation | Right Most Derivation |
| Main Decision | Selecting the production rule to use in order to construct the string | Selecting when to use a production rule to reduce the string to get the start symbol |
| Example Algorithm | Recursive Descent Parsing | Shift-Reduce Parser |
In summary, the main difference between top-down and bottom-up parsing lies in their approach to constructing the parse tree. Top-down parsing starts from the start symbol and works down to the input string, while bottom-up parsing starts from the input string and works up to the start symbol.
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