Tokens, patterns, and lexemes in Compiler Design - Step-by-Step Execution

Concept Flow - Tokens, patterns, and lexemes

Source Code Text

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Lexical Analyzer

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Pattern Matching

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Identify Lexemes

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Generate Tokens

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Pass Tokens to Parser

The source code is read by the lexical analyzer, which uses patterns to find lexemes and then generates tokens to pass to the parser.

Execution Sample

Compiler Design

int x = 10;
// Sample code line

This code line is scanned to identify lexemes and generate tokens like 'int', 'x', '=', '10', and ';'.

Analysis Table

Step	Input Text Segment	Pattern Matched	Lexeme Identified	Token Generated
1	"int"	Keyword pattern	"int"	KEYWORD_INT
2	" " (space)	Whitespace	Ignored	null
3	"x"	Identifier pattern	"x"	IDENTIFIER
4	" " (space)	Whitespace	Ignored	null
5	"="	Operator pattern	"="	ASSIGN_OP
6	" " (space)	Whitespace	Ignored	null
7	"10"	Number pattern	"10"	NUMBER
8	";"	Delimiter pattern	";"	SEMICOLON
9	\n	Newline	Ignored	null

💡 All input text processed; tokens generated for all lexemes except whitespace and newline which are ignored.

State Tracker

Variable	Start	After Step 1	After Step 3	After Step 5	After Step 7	Final
Current Position	0	3	5	7	10	11
Current Lexeme	""	"int"	"x"	"="	"10"	";"
Tokens List	[]	[KEYWORD_INT]	[KEYWORD_INT, IDENTIFIER]	[KEYWORD_INT, IDENTIFIER, ASSIGN_OP]	[KEYWORD_INT, IDENTIFIER, ASSIGN_OP, NUMBER]	[KEYWORD_INT, IDENTIFIER, ASSIGN_OP, NUMBER, SEMICOLON]

Key Insights - 3 Insights

Why are spaces and newlines not turned into tokens?

Spaces and newlines are recognized as whitespace patterns and ignored during token generation, as shown in steps 2, 4, 6, and 9 in the execution_table.

How does the lexical analyzer know where one lexeme ends and another begins?

It uses pattern matching and checks for delimiters like spaces, operators, or punctuation to separate lexemes, as seen between steps 1 and 3 where space separates 'int' and 'x'.

What is the difference between a lexeme and a token?

A lexeme is the actual substring from the source code (like 'int' or '10'), while a token is the category or type assigned to that lexeme (like KEYWORD_INT or NUMBER), as shown in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what token is generated at step 5?

A IDENTIFIER

B ASSIGN_OP

C NUMBER

D SEMICOLON

💡 Hint

Check the 'Token Generated' column at step 5 in the execution_table.

At which step does the lexical analyzer identify the lexeme "x"?

A Step 5

B Step 1

C Step 3

D Step 7

💡 Hint

Look at the 'Lexeme Identified' column in the execution_table for "x".

If the input had no spaces, how would the lexical analyzer separate tokens?

A It uses patterns like operators and delimiters to separate lexemes

B It treats the entire line as one token

C It would fail to separate tokens

D It ignores all characters

💡 Hint

Refer to the key_moments explanation about how lexemes are separated.

Concept Snapshot

Tokens, patterns, and lexemes:
- Lexical analyzer reads source code text.
- Patterns define how lexemes (actual text pieces) are recognized.
- Tokens are categories assigned to lexemes (e.g., KEYWORD, IDENTIFIER).
- Whitespace and comments are usually ignored.
- Tokens are passed to the parser for syntax analysis.

Full Transcript

In compiler design, the lexical analyzer reads the source code text and uses patterns to find lexemes, which are actual substrings like 'int' or 'x'. Each lexeme is then assigned a token, which is a category like KEYWORD_INT or IDENTIFIER. Spaces and newlines are recognized as whitespace and ignored. The process continues until all input text is processed, generating a list of tokens for the parser to use. This step-by-step execution shows how the analyzer moves through the input, identifies lexemes, and generates tokens accordingly.