Parsers in Compilers: Top-Down vs. Bottom-Up Parsing Techniques

Explore top-down and bottom-up parsing techniques used in compilers. This guide explains how parsers analyze code, create parse trees, and the strengths and weaknesses of different parsing approaches (recursive descent, shift-reduce).

Parsers in Compilers: Top-Down and Bottom-Up Parsing

Understanding Parsers

A parser is a crucial component of a compiler, responsible for analyzing the stream of tokens (produced by the lexical analyzer) to determine if the input code conforms to the rules of the programming language’s grammar and creating a structured representation of the code (usually a parse tree). This structured representation makes it easier for subsequent stages of the compiler (like semantic analysis and code generation) to process and translate the code into machine instructions.

Types of Parsing

Two main approaches exist for parsing:

1. Top-Down Parsing:

Top-down parsing (also called recursive descent or predictive parsing) builds the parse tree from the root (start symbol) downwards, towards the leaves (input symbols). The parser predicts the production rule to use based on the current input and works its way down the tree, creating nodes that represent the grammar rules. This method is suitable when the grammar is relatively simple and the parsing algorithm can easily predict the production rule to apply at each step.

Diagram illustrating top-down parsing

2. Bottom-Up Parsing:

Bottom-up parsing (also known as shift-reduce parsing) constructs the parse tree from the leaves (input symbols) upwards, towards the root (start symbol). It essentially traces the rightmost derivations of the input string in reverse. The parser starts with the input string and applies grammar rules to reduce the string, building up the parse tree from the leaves to the root. This method is often more flexible than top-down parsing and is well-suited for handling more complex grammars.

Diagram illustrating bottom-up parsing

Example Production Rules

Consider this simple grammar:

Grammar Rules

E → T
T → T * F
T → id
F → T
F → id

Classification of Bottom-Up Parsing

Several variations exist within bottom-up parsing:

  • Shift-Reduce Parsing
  • Operator Precedence Parsing
  • Table-Driven LR Parsing (LR(1), SLR(1), CLR(1), LALR(1))