Teknik Kompilasi - Pertemuan 6 (Algoritma LL(1) Parsing)

Summary

This video explains the LL(1) parsing algorithm, which is a top-down, deterministic algorithm used in context-free grammar parsing. The speaker covers essential topics such as the algorithm's features—predictive parsing, no backtracking, and leftmost derivation—while highlighting the importance of eliminating left recursion. Key concepts like First and Follow sets are explored, alongside steps for constructing a parsing table to guide the parsing process. The explanation also provides an example to demonstrate the parsing process, emphasizing the practical application of LL(1) parsing in syntactic analysis.

Takeaways

• 😀 LL(1) algorithm is a predictive parsing algorithm that processes input from left to right and uses a single lookahead symbol.
• 😀 LL(1) parsing eliminates the need for backtracking by using a table to guide the parsing process.
• 😀 The algorithm relies on **First** and **Follow** sets to determine which grammar rules to apply at each step of the parsing process.
• 😀 **Left recursion** must be removed from the grammar for LL(1) parsing to work. This is done by transforming recursive rules into non-recursive ones.
• 😀 **Left factoring** is used to eliminate ambiguities, ensuring that each production rule can be uniquely identified by the first symbol of the input.
• 😀 The **First set** of a non-terminal consists of all the terminals that can appear as the first symbol in any string derived from that non-terminal.
• 😀 The **Follow set** for a non-terminal consists of all the terminals that can appear immediately to the right of that non-terminal in some derivation.
• 😀 A **parsing table** is constructed with non-terminals as rows and terminals (including an end-of-input symbol) as columns. This table is used to guide the parsing process.
• 😀 The parsing table must be correctly filled based on the First and Follow sets to ensure that the parsing process is accurate.
• 😀 The parsing process involves matching the input string with the rules from the parsing table by iterating through the input and pushing/popping symbols from the stack.

Q & A

What is the LL(1) algorithm used for?

The LL(1) algorithm is used for predictive parsing in compilers, where 'LL' refers to scanning input left to right and performing leftmost derivation, while the '1' indicates that the parser looks ahead by one symbol.

What does the term 'left recursion' mean, and why must it be eliminated in LL(1) parsing?

Left recursion occurs when a non-terminal symbol refers to itself on the left side of a production rule. It must be eliminated because it can cause infinite recursion in recursive descent parsers, which LL(1) parsing avoids.

How is left recursion removed in LL(1)?

Left recursion is removed by rewriting the grammar to introduce a new non-terminal that handles recursive calls in a rightward direction. For example, the production `S -> S + A | A` becomes `S -> A S'` and `S' -> + A S' | ε`.

What are the FIRST and FOLLOW sets, and why are they important in LL(1) parsing?

The FIRST set contains all terminals that can appear at the beginning of strings derived from a non-terminal, while the FOLLOW set contains all terminals that can appear immediately after a non-terminal in any derivation. These sets help determine the correct production rule during parsing.

Can you explain the process of computing the FIRST set?

The FIRST set for a non-terminal is computed by looking at the first symbols of all possible strings that can be derived from it. If a production starts with a terminal, that terminal is added to the FIRST set. If it starts with a non-terminal, the FIRST set of that non-terminal is considered.

What does the FOLLOW set represent, and how is it computed?

The FOLLOW set represents all the terminals that can appear immediately after a non-terminal in any valid string derived from the grammar. It is computed by looking at the grammar rules and considering where a non-terminal is followed by a terminal or the end of the string.

What is the significance of constructing a LL(1) parsing table?

The LL(1) parsing table is crucial because it guides the parser in choosing the correct production rule based on the current input symbol and the top of the stack. It ensures that the parsing process can proceed efficiently with minimal backtracking.

How are the rows and columns of the LL(1) parsing table organized?

The rows of the LL(1) parsing table correspond to non-terminal symbols, and the columns correspond to terminal symbols (including the special symbol for end-of-input). Each table entry contains the production rule to apply based on the combination of the current non-terminal and input symbol.

What happens if the parsing table entry for a given combination of non-terminal and terminal is empty?

If a table entry is empty, it indicates that the grammar is not LL(1)-compatible for that combination, meaning the input cannot be parsed using the LL(1) algorithm. This usually happens if the grammar contains ambiguity or unresolved conflicts.

How does the LL(1) parser proceed once the parsing table is constructed?

Once the parsing table is constructed, the parser starts with the start symbol on the stack and the input string. It repeatedly compares the top of the stack with the current input symbol, applying the corresponding production rule from the table until both the stack and input are empty, indicating a successful parse.