Eliminate Left Recursion | Solved Example | Compiler Design

In this tutorial, we take a deep look at Left Recursion — a fundamental concept in top-down parsing and compiler design.

We start by understanding what left recursion is, why it causes non-termination in recursive-descent parsers, and how parse trees reveal the problem: the grammar keeps expanding leftward without ever consuming input. We contrast this with right recursion, which always progresses and terminates safely.

Next, we walk through clear, step-by-step transformations to eliminate both direct and indirect left recursion, using practical examples. We convert the recursive grammar into a right-recursive, LL-friendly form using the standard A → βA′ and A′ → αA′ method, ensuring the language stays the same.

By the end, you’ll fully understand:
Why left recursion breaks LL parsers
How right recursion avoids infinite expansion
How to systematically eliminate left recursion
How to rewrite grammars into parser-friendly forms

Perfect for compiler engineering students, programming language enthusiasts, and anyone learning LL parsing or CFG transformations.

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Chapters in the video:
0:00 Introduction
0:33 Problem Statement
1:17 What is Left Recursion
1:53 Procedure
3:18 Solution
4:28 Indirect Nature
7:26 the difference
9:03 Inference & Modifications
9:54 Conclusion

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