After the parser checks for grammatical correctness, it usually creates a Parse Tree (or concrete syntax tree). This tree is a very detailed representation of how the program matches the grammar rules, often including many keywords and intermediate grammatical steps that are not directly about the program's meaning.

An AST is a simplified, cleaned-up version of the parse tree. It throws away all the "syntactic noise" (like extra parentheses, semicolons, or redundant keywords from the grammar) and focuses only on the essential elements that define the program's structure and operations.

Key Characteristics that make ASTs perfect for Semantic Analysis:

• Only Meaningful Elements: Each node in an AST represents a significant construct in the programming language – like an operation, a variable, a literal value, an assignment, a loop, or a function call. It doesn't have nodes for intermediate grammatical rules.
• Hierarchical Relationship: The parent-child relationships in an AST show how different parts of the program relate to each other logically. For example, an "addition" node would have its two "operands" as children.
• More Abstract, Less Concrete: It's "abstract" because it doesn't show the exact syntax used; it shows the idea of the operation.
• Directly Usable: An AST is much easier to work with for semantic checks and code generation because you're dealing directly with concepts like "add," "assign," "if-then-else" rather than complex grammatical derivations.

Let's visualize ASTs for common programming constructs:

1. Expressions: Capturing Calculation Order
2. Source Code: result = a + b * c;
3. AST for a + b * c: + / \ a * / \ b c
4. Assignment Statements: Storing Values
5. Source Code: total = initial_value + 10;
6. AST: = / \ total + / \ initial_value 10
7. Control Flow Statements: Guiding Program Path
8. If-Else Statement: Making Decisions
   • Source Code: if (score > 90) { grade = 'A'; } else { grade = 'B'; }
   • AST: IfElse / | \ > = = / \ / \ score 90 grade 'A' grade 'B'
9. While Loop: Repeating Actions
   • Source Code: while (count < 5) { count = count + 1; }
   • AST: While / \ < = / \ / \ count 5 count + / \ count 1
10. Function Call:
    • Source Code: print_message("Hello", count);
    • AST: Call / \ print_message Arguments / "Hello" count

How ASTs are used in Semantic Analysis:

1. Walk the Tree: Semantic analysis often involves one or more "walks" (traversals) of the AST, typically a depth-first traversal (visiting children before processing the parent, or vice versa, depending on the attribute type).
2. Gather Information: As the analyzer visits each node, it gathers information. For example, when it visits an expression node, it might calculate and determine the type of that expression.
3. Update Symbol Table: When it encounters a declaration node (like int x;), it adds x to the symbol table with its type (int) and scope information. When it encounters a usage of x, it looks it up in the symbol table to retrieve its type for type checking.
4. Annotate AST Nodes: The computed attributes (like the type of an expression, or a pointer to its symbol table entry) are often stored directly on the AST nodes themselves. This enriches the AST, making it even more useful for later compilation stages.
5. Detect Errors: If a semantic rule is violated (e.g., trying to add a string and an integer), the analyzer reports an error message, often pointing to the specific line or node in the source code.

By creating and working with ASTs, the compiler moves from simply knowing how the code is written (syntax) to understanding what the code intends to do (meaning).