Tokens, patterns, and lexemes in Compiler Design - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the basic unit of a programming language.

Compiler Design

A [1] is the smallest unit of a program that has a meaning.

Drag options to blanks, or click blank then click option'

Atoken

Bpattern

Clexeme

Dsyntax

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing token with lexeme or pattern.

Thinking syntax is the smallest unit.

A token is the smallest meaningful unit in a programming language, such as keywords, identifiers, or symbols.

2fill in blank

medium

Complete the code to define the role of a pattern in token recognition.

Compiler Design

A [1] defines the rules or structure that a sequence of characters must follow to form a token.

Drag options to blanks, or click blank then click option'

Alexeme

Bsyntax

Cpattern

Dgrammar

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing pattern with lexeme or syntax.

Thinking grammar defines token structure.

A pattern is a rule or description (often a regular expression) that defines how characters combine to form a token.

3fill in blank

hard

Fix the error in the statement about lexemes.

Compiler Design

A [1] is the actual sequence of characters in the source code that matches a pattern.

Drag options to blanks, or click blank then click option'

Apattern

Blexeme

Csyntax

Dtoken

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up lexeme with token or pattern.

Thinking lexeme is a rule rather than actual text.

A lexeme is the actual text in the source code that matches a pattern and forms a token.

4fill in blank

hard

Fill both blanks to complete the dictionary comprehension that maps tokens to their lexemes if the lexeme length is greater than 3.

Compiler Design

{token: lexeme for token, lexeme in token_lexeme_list if len(lexeme) [1] 3 and token [2] 'IDENTIFIER'}

Drag options to blanks, or click blank then click option'

A>

B==

C!=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than instead of greater than for length.

Using inequality instead of equality for token check.

The comprehension filters lexemes longer than 3 characters and tokens equal to 'IDENTIFIER'.

5fill in blank

hard

Fill all three blanks to create a dictionary of tokens and lexemes where lexemes start with a letter and tokens are not 'KEYWORD'.

Compiler Design

{ [1]: [2] for [3], lexeme in token_lexeme_pairs if lexeme[0].isalpha() and [3] != 'KEYWORD' }

Drag options to blanks, or click blank then click option'

Atoken

Blexeme

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping keys and values in the dictionary.

Using wrong variable names in the loop.

This comprehension creates a dictionary mapping tokens to lexemes where lexemes start with a letter and tokens are not keywords.

Practice

1. What is a token in compiler design?

easy

A. The exact sequence of characters in the source code

B. A category or label for a group of characters

C. A rule describing valid sequences of characters

D. A syntax error found during compilation

Solution

1. Step 1: Understand the role of tokens

   Tokens act like words in a sentence, grouping characters into meaningful categories.

2. Step 2: Differentiate tokens from lexemes and patterns

   Lexemes are the actual text, patterns are rules, tokens are the labels for those texts.

3. Final Answer:

   A category or label for a group of characters -> Option B

4. Quick Check:

   Token = label/category [OK]

Hint: Tokens are like word types, not the words themselves [OK]

Common Mistakes:

• Confusing tokens with lexemes
• Thinking tokens are the exact text
• Mixing tokens with syntax errors

2. Which of the following correctly describes a pattern in lexical analysis?

easy

A. The exact characters matched in the source code

B. A syntax error detected during compilation

C. The output of the parser after syntax analysis

D. A rule or regular expression defining valid tokens

Solution

1. Step 1: Define what a pattern is

   Patterns describe the rules or formats that valid tokens must follow, often using regular expressions.

2. Step 2: Eliminate incorrect options

   Exact characters are lexemes, parser output is unrelated, syntax errors are not patterns.

3. Final Answer:

   A rule or regular expression defining valid tokens -> Option D

4. Quick Check:

   Pattern = rule for tokens [OK]

Hint: Patterns are rules, not actual text [OK]

Common Mistakes:

• Confusing patterns with lexemes
• Thinking patterns are output of parsing
• Mixing patterns with errors

3. Given the source code snippet: int count = 10; Which of the following is a correct lexeme for the token identifier?

medium

A. count

B. =

C. 10

D. int

Solution

1. Step 1: Identify the token 'identifier'

   An identifier token represents names like variables or functions.

2. Step 2: Match lexemes to token types

   In the snippet, 'count' is the variable name, so it is the lexeme for the identifier token.

3. Final Answer:

   count -> Option A

4. Quick Check:

   Identifier lexeme = count [OK]

Hint: Identifiers are names, not keywords or symbols [OK]

Common Mistakes:

• Choosing keywords like 'int' as identifiers
• Selecting numbers or operators as identifiers
• Confusing lexemes with token types

4. Identify the error in this tokenization process for the input var_1 = 25;: Tokens identified are var (identifier), _1 (identifier), = (assignment), 25 (number), ; (semicolon).

medium

A. The underscore and digit should be part of the same identifier token

B. The assignment operator should be tokenized as two separate tokens

C. The number 25 should be split into two tokens '2' and '5'

D. The semicolon should not be a token

Solution

1. Step 1: Understand identifier token rules

   Identifiers can include letters, digits, and underscores as one token, not split.

2. Step 2: Analyze the given tokenization

   Splitting 'var_1' into 'var' and '_1' is incorrect; it should be one identifier token.

3. Final Answer:

   The underscore and digit should be part of the same identifier token -> Option A

4. Quick Check:

   Identifier tokens include underscores and digits [OK]

Hint: Keep underscores and digits inside one identifier token [OK]

Common Mistakes:

• Splitting identifiers incorrectly
• Misinterpreting assignment operator tokenization
• Ignoring semicolon as a token

5. You want to design a lexical analyzer that recognizes tokens for integers and identifiers. Which pattern correctly matches an identifier but not an integer?

hard

A. [a-zA-Z0-9]+

B. [0-9]+

C. [a-zA-Z_][a-zA-Z0-9_]*

D. [0-9]*[a-zA-Z]+

Solution

1. Step 1: Understand identifier and integer patterns

   Identifiers start with a letter or underscore, followed by letters, digits, or underscores. Integers are digits only.

2. Step 2: Eliminate incorrect options

   A matches integers like '123'; B matches only integers; D fails to match identifiers ending in digits like 'var1'.

3. Final Answer:

   [a-zA-Z_][a-zA-Z0-9_]* -> Option C

4. Quick Check:

   Identifier pattern starts with letter/underscore [OK]

Hint: Identifiers start with letter/underscore, not digit [OK]

Common Mistakes:

• Allowing digits as first character in identifiers
• Confusing integer patterns with identifiers
• Using patterns that match both integers and identifiers