Applying the Algorithm to the Example Grammar

Oct 13, 2025 by wbrum.org 106 views

Introduction

In the field of formal language theory and compiler design, context-free grammars are a fundamental concept. However, one of the challenges in working with these grammars is the presence of left recursion, which can lead to infinite loops and make parsing difficult. In this article, we will explore a general algorithm for eliminating left recursion in context-free grammars and apply it to a specific example.

What is Left Recursion?

Left recursion occurs when a non-terminal symbol appears on the left-hand side of a production rule, and the same non-terminal symbol appears on the right-hand side of the rule. This can lead to infinite loops during parsing, as the parser will keep applying the same rule over and over again.

The General Algorithm

The general algorithm for eliminating left recursion is as follows:

Identify Left Recursion: Identify the non-terminal symbols that are involved in left recursion.
Create a New Non-Terminal: Create a new non-terminal symbol that will replace the original non-terminal symbol in the left-recursive production rule.
Replace the Original Non-Terminal: Replace the original non-terminal symbol in the left-recursive production rule with the new non-terminal symbol.
Add a New Production Rule: Add a new production rule that defines the new non-terminal symbol in terms of the original non-terminal symbol and any other non-terminal symbols that are involved in the left recursion.
Repeat the Process: Repeat the process for all non-terminal symbols that are involved in left recursion.

Applying the Algorithm to the Example Grammar

Let's apply the general algorithm to the example grammar:

S → SS | t | ε

Step 1: Identify Left Recursion

In this grammar, the non-terminal symbol S is involved in left recursion, as it appears on the left-hand side of the production rule S → SS.

Step 2: Create a New Non-Terminal

Let's create a new non-terminal symbol S' that will replace the original non-terminal symbol S in the left-recursive production rule.

Step 3: Replace the Original Non-Terminal

We replace the original non-terminal symbol S in the left-recursive production rule with the new non-terminal symbol S':

S' → SS

Step 4: Add a New Production Rule

We add a new production rule that defines the new non-terminal symbol S' in terms of the original non-terminal symbol S and any other non-terminal symbols that are involved in the left recursion:

S' → S S'

Step 5: Repeat the Process

We repeat the process for the new non-terminal symbol S', which is also involved in left recursion:

S'' → S' S'

However, we notice that the new non-terminal symbol S'' is also involved in left recursion, and the process will continue indefinitely. This is where the general algorithm fails, and we need to apply a different approach to eliminate the left recursion.

A Modified Approach

To eliminate the left recursion in the example grammar, we can use a modified approach that involves creating a new non-terminal symbol and replacing the original non-terminal symbol in the left-recursive production rule with a sequence of non-terminal symbols and terminal symbols.

Let's create a new non-terminal symbol S'' that will replace the original non-terminal symbol S in the left-recursive production rule:

S'' → S t | ε

We replace the original non-terminal symbol S in the left-recursive production rule with the new non-terminal symbol S'':

S → S'' | t | ε

This modified approach eliminates the left recursion in the example grammar and produces a finite set of production rules.

Conclusion

In this article, we explored a general algorithm for eliminating left recursion in context-free grammars and applied it to a specific example. However, we found that the general algorithm fails to eliminate the left recursion in some cases, and a modified approach is needed to produce a finite set of production rules. The modified approach involves creating a new non-terminal symbol and replacing the original non-terminal symbol in the left-recursive production rule with a sequence of non-terminal symbols and terminal symbols.

References

Wikipedia. (n.d.). Elimination of left recursion. Retrieved from https://en.wikipedia.org/wiki/Elimination_of_left_recursion
Aho, A. V., Sethi, R., & Ullman, J. D. (1986). Compilers: Principles, techniques, and tools. Addison-Wesley.
Hopcroft, J. E., Ullman, J. D., & Motwani, R. (2006). Introduction to automata theory, languages, and computation. Addison-Wesley.

Q: What is left recursion in a context-free grammar?

A: Left recursion occurs when a non-terminal symbol appears on the left-hand side of a production rule, and the same non-terminal symbol appears on the right-hand side of the rule. This can lead to infinite loops during parsing, as the parser will keep applying the same rule over and over again.

Q: Why is left recursion a problem in parsing?

A: Left recursion is a problem in parsing because it can lead to infinite loops, which can cause the parser to hang or crash. Additionally, left recursion can make it difficult to implement a parser, as the parser will need to keep track of the non-terminal symbols and their corresponding production rules.

Q: How can I eliminate left recursion in a context-free grammar?

A: There are several ways to eliminate left recursion in a context-free grammar, including:

Using the general algorithm for eliminating left recursion, which involves creating a new non-terminal symbol and replacing the original non-terminal symbol in the left-recursive production rule with a sequence of non-terminal symbols and terminal symbols.
Using a modified approach that involves creating a new non-terminal symbol and replacing the original non-terminal symbol in the left-recursive production rule with a sequence of non-terminal symbols and terminal symbols.
Using a parser generator tool, such as yacc or ANTLR, which can automatically eliminate left recursion in a context-free grammar.

Q: What is the general algorithm for eliminating left recursion?

A: The general algorithm for eliminating left recursion involves the following steps:

Identify Left Recursion: Identify the non-terminal symbols that are involved in left recursion.
Create a New Non-Terminal: Create a new non-terminal symbol that will replace the original non-terminal symbol in the left-recursive production rule.
Replace the Original Non-Terminal: Replace the original non-terminal symbol in the left-recursive production rule with the new non-terminal symbol.
Add a New Production Rule: Add a new production rule that defines the new non-terminal symbol in terms of the original non-terminal symbol and any other non-terminal symbols that are involved in the left recursion.
Repeat the Process: Repeat the process for all non-terminal symbols that are involved in left recursion.

Q: What is the modified approach for eliminating left recursion?

A: The modified approach for eliminating left recursion involves creating a new non-terminal symbol and replacing the original non-terminal symbol in the left-recursive production rule with a sequence of non-terminal symbols and terminal symbols.

Q: Can I use a parser generator tool to eliminate left recursion?

A: Yes, you can use a parser generator tool, such as yacc or ANTLR, to automatically eliminate left recursion in a context-free grammar.

Q: What are some common mistakes to avoid when eliminating left recursion?

A: Some common mistakes to avoid when eliminating left recursion include:

Failing to identify all non-terminal symbols that are involved in left recursion.
Failing to create a new non-terminal symbol that will replace the original non-terminal symbol in the left-recursive production rule.
Failing to replace the original non-terminal symbol in the left-recursive production rule with the new non-terminal symbol.
Failing to add a new production rule that defines the new non-terminal symbol in terms of the original non-terminal symbol and any other non-terminal symbols that are involved in the left recursion.

Q: How can I test my parser to ensure that it is working correctly?

A: You can test your parser by using a set of test cases that cover all possible inputs to the parser. You can also use a parser generator tool, such as yacc or ANTLR, to automatically generate a parser and test it with a set of test cases.

Q: What are some resources for learning more about eliminating left recursion?

A: Some resources for learning more about eliminating left recursion include:

The Wikipedia article on "Elimination of left recursion".
The book "Compilers: Principles, Techniques, and Tools" by Alfred V. Aho, Ravi Sethi, and Jeffrey D. Ullman.
The book "Introduction to Automata Theory, Languages, and Computation" by John E. Hopcroft, Jeffrey D. Ullman, and Rajeev Motwani.
Online tutorials and courses on compiler design and parsing theory.