CPSC-323: Introduction and Phases of Compilers

Compiler

• A program that translates source code

written in a high-level language (like C, Java) into machine

code (low-level language) that a computer can execute.

• It performs this translation before the program runs.

• Output is usually executable code or intermediate code.

• Ensures code is correct and safe to run

Benefits of a compiler

• Performance: Optimized machine code runs faster than interpreted

code.

• Early Error Detection: Compile-time errors catch bugs before

execution.

• Security: Invalid or malicious patterns can be caught during compilation.

• Portability: Same source code can target different architectures by changing the compiler backend

Compiler vs Interpreter

• Compiler: Translates entire code once, produces executable, faster runtime.

• Interpreter: Executes code line by line, slower but more flexible.

• Examples:

• Compiler: C, C++ programs.

• Interpreter: Python, JavaScript engines.

Another diagram of a compiler

Lexical Analysis

• 1st phase of a compiler

• It reads the raw source code character-by-character and groups those characters into meaningful units called lexemes, then converts those lexemes into tokens.

• If the compiler were a pipeline, this is the part that turns the messy text you typed into clean, structured data that the rest of the compiler can understand.

Why is Lexical Analysis needed?

• The parser can’t work directly on raw characters.

• Code needs to be broken into logical “words” before grammar rules

can apply.

• Errors like illegal characters must be caught early.

• Spaces, comments, and formatting must be ignored.

• Lexical analysis extracts meaning out of a stream of characters.

Lexeme

• The actual sequence of characters pulled directly from the source code.

• Think of it as: The raw “word” the programmer typed, A concrete substring taken from the character stream

• Examples: x, 123, while, +, "Hello"

• These are literal pieces of text.

Token

• The category or classification that a lexeme belongs to.

• A label the compiler attaches to a lexeme.

• Examples:

• ID - for variable names like x, count, sum.

• NUMBER - for 123, 45.6

• KEYWORD_WHILE or KEYWORD

• OP_PLUS

• STRING_LITERAL

• Is like saying: “This lexeme belongs to this grammatical class.”

Lexical Analyzer Tasks

• Reads the input as a continuous stream of characters.

• Example: int sum = a + 10;

• Groups characters into lexemes.

• Examples: int, sum , = , a , + , 10 , ;

• Converts each lexeme into a token.

• Examples: KEYWORD_INT, ID , ASSIGN , ID, PLUS, NUMBER, SEMICOLON

• Output:

• KEYWORD_INT, ID(sum), ASSIGN, ID(a), PLUS, NUMBER(10), SEMICOLON

What are the tokens and lexemes for x = y + 5?

What are the tokens and lexemes for int age = 21

What are the lexemes and tokens for result = (a+b)*3

What are the lexemes and tokens for while(count<10){ count=count+1; }?

What are the lexemes and tokens for total = price * quantity / 2.5;

Step 1 of a Compiler

Lexical Analysis

Step 2 of a Complier

Syntax Analysis/Parsing

Step 3 of a compiler

Semantic Analysis

Step 4 of a Compiler

Intermediate Code Generation

Step 5 of a Compiler

Code Optimization

Step 6 of a Compiler

Target Code Generation

ID

Token type for variable names

Ex:

• x

• count

• sum

NUMBER

Token type for integers, floats

EX:

• 123

• 45.6

KEYWORD

Token type for reserved words

• EX:

  • if

  • else

  • while

• Another way to write the token type is KEYWORD_{insert keyword here}

OP

A token type representing operators

EX:

• +

• -

• *

• The tokens of these would be written as {}_PLUS, {}_MINUS, and {}_MUL

STRING_LITERAL

Token type for strings

EX:

• “Hello World”

ASSIGN

Token type for =

SEMICOLON

Token type for a ;

L/RPAREN

Token type for (,)

L/RBRACE

Token type for {,}

FLOAT_LITERAL

Token type for decimals

EX:

• 2.5

Alternative token expression for operators

• +: PLUS

• -: MINUS

• *:MULTIPLY