Compiler Design: Parse Tree and Syntax Tree
In the field of computer science, compiler design refers to the process of creating a compiler, which is a software program that translates source code written in a high-level programming language into machine code that can be executed by a computer. One of the important aspects of compiler design is the representation of the structure and meaning of the source code. This is done through the use of parse trees and syntax trees.
Parse Tree
A parse tree, also known as a concrete syntax tree or derivation tree, is a graphical representation of the syntactic structure of a program. It shows how the input string of the program is divided into grammatical components according to the rules of a formal grammar. The parse tree represents the hierarchical relationships between the different components of the program.
Let’s consider an example to understand the concept of a parse tree. Suppose we have the following simple arithmetic expression:
5 + 3 * 2
The parse tree for this expression would look like this:
+/5*/32
In this parse tree, the non-terminal symbol ‘+’ is the root, and its children are the terminal symbols ‘5’ and ‘*’. The ‘*’ node has ‘3’ and ‘2’ as its children. This representation clearly shows the hierarchical structure of the expression.
Syntax Tree
A syntax tree, also known as an abstract syntax tree (AST), is a more abstract representation of the structure and meaning of a program. It captures the essential elements of the program’s syntax while abstracting away some of the details present in the parse tree. The syntax tree focuses on the semantics of the program rather than the specific grammar rules.
Continuing with the previous example, the syntax tree for the expression 5 + 3 * 2 would look like this:
+/5*/32
As you can see, the syntax tree is the same as the parse tree in this case. However, in more complex examples, the syntax tree may differ from the parse tree. The syntax tree provides a more concise representation of the program’s structure and meaning.
Example: Parsing an If-Else Statement
Let’s consider another example to illustrate the use of parse trees and syntax trees. Suppose we have the following if-else statement:
if (x > 5) {y = 10;} else {y = 5;}
The parse tree for this if-else statement would look like this:
if/>{/\x5=y/10;
In this parse tree, the non-terminal symbol ‘if’ is the root. Its children are the comparison operator ‘>’, the variable ‘x’, the number ‘5’, the opening brace ‘{‘, the assignment operator ‘=’, the variable ‘y’, the number ’10’, and the semicolon ‘;’. The closing brace ‘}’ is also a child of the root, indicating the end of the if-else statement.
The syntax tree for the same if-else statement would look like this:
if/>=/\x5y/10;
In this syntax tree, the non-terminal symbol ‘if’ is still the root, but the unnecessary details such as the opening and closing braces are removed. The syntax tree focuses on the essential elements of the if-else statement, which are the condition ‘x > 5’ and the assignment ‘y = 10’ in the ‘if’ branch, and the assignment ‘y = 5’ in the ‘else’ branch.
Conclusion
Parse trees and syntax trees are important tools in compiler design for representing the structure and meaning of a program. The parse tree provides a detailed graphical representation of the syntactic structure of the program, while the syntax tree abstracts away some of the details and focuses on the essential elements of the program’s syntax. These trees help in the analysis and transformation of source code during the compilation process, enabling the compiler to generate efficient and correct machine code.