Recursive descent parsing in Compiler Design - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a recursive descent parser function for a non-terminal symbol.

Compiler Design

def parse_[1]():
    if lookahead == 'a':
        match('a')
        parse_[1]()
    else:
        return

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

Aexpr

Bfactor

Cterm

Dstatement

Attempts:

3 left

💡 Hint

Common Mistakes

Using a terminal symbol as the function name.

Using a function name unrelated to the grammar.

The function name should match the non-terminal symbol being parsed. Here, expr is a common non-terminal in recursive descent parsers.

2fill in blank

medium

Complete the code to match the current token and advance the lookahead in a recursive descent parser.

Compiler Design

def match(expected_token):
    global lookahead
    if lookahead == [1]:
        lookahead = next_token()
    else:
        error()

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

A';'

B'+'

Cexpected_token

D'id'

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing lookahead to a fixed token instead of the expected one.

Not advancing the lookahead after a successful match.

The match function checks if the current token matches the expected token and then advances the lookahead.

3fill in blank

hard

Fix the error in the recursive descent parser function to correctly parse an expression with addition.

Compiler Design

def parse_expr():
    parse_term()
    while lookahead == [1]:
        match('+')
        parse_term()

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

A'+'

B'*'

C'-'

D'/'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong operator in the loop condition.

Not matching the operator inside the loop.

The loop should continue while the lookahead is the addition operator '+' to parse expressions with additions.

4fill in blank

hard

Fill both blanks to complete the parse_factor function that handles parentheses and identifiers.

Compiler Design

def parse_factor():
    if lookahead == [1]:
        match('(')
        parse_expr()
        match([2])
    else:
        match('id')

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

A'('

B')'

C'id'

D'+'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same parenthesis for both blanks.

Matching an identifier instead of a closing parenthesis.

The function checks for an opening parenthesis '(' and matches a closing parenthesis ')' after parsing the expression inside.

5fill in blank

hard

Fill both blanks to complete the parse_term function that handles multiplication and factors.

Compiler Design

def parse_term():
    parse_factor()
    while lookahead == [1]:
        match([2])
        parse_factor()

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

A'*'

C'+'

D'/'

Attempts:

3 left

💡 Hint

Common Mistakes

Using different operators in the blanks.

Using addition or division instead of multiplication.

The function loops while the lookahead is the multiplication operator '*', matching it and parsing the next factor.

Practice

1. What is the main idea behind recursive descent parsing?

easy

A. Using a single loop to scan the input once

B. Converting input into machine code directly

C. Using functions that call themselves to analyze input step-by-step

D. Using tables to decide parsing actions

Solution

1. Step 1: Understand recursive descent parsing concept

   Recursive descent parsing uses functions that call themselves to process parts of the input one by one.

2. Step 2: Compare options with concept

   Only Using functions that call themselves to analyze input step-by-step describes this method correctly; others describe different parsing or compilation techniques.

3. Final Answer:

   Using functions that call themselves to analyze input step-by-step -> Option C

4. Quick Check:

   Recursive descent = functions calling themselves [OK]

Hint: Look for self-calling functions in parsing [OK]

Common Mistakes:

• Confusing recursive descent with table-driven parsing
• Thinking parsing is done in one pass without recursion
• Mixing parsing with code generation

2. Which of the following is a correct syntax feature of a recursive descent parser function?

easy

A. It calls itself to process nested grammar rules

B. It uses a while loop to parse all tokens at once

C. It directly generates machine code

D. It uses a global variable to store the entire parse tree

Solution

1. Step 1: Identify function behavior in recursive descent parsing

   Functions in recursive descent parsers call themselves to handle nested or repeated grammar parts.

2. Step 2: Evaluate options

   It calls itself to process nested grammar rules correctly states this behavior. Options B, C, and D describe unrelated or incorrect features.

3. Final Answer:

   It calls itself to process nested grammar rules -> Option A

4. Quick Check:

   Recursive descent functions call themselves [OK]

Hint: Look for self-calling function behavior [OK]

Common Mistakes:

• Assuming loops replace recursion in these functions
• Confusing parsing with code generation
• Thinking parse tree is stored globally by default

3. Given this simple grammar rule:
Expr -> 'a' Expr | 'b'
What will a recursive descent parser output when parsing the input aaab?

medium

A. Successfully parses the input

B. Syntax error at first character

C. Infinite recursion without stopping

D. Parses only the first character

Solution

1. Step 1: Understand the grammar and input

   The rule says Expr can be 'a' followed by Expr again, or just 'b'. Input 'aaab' fits this pattern: 'a' + 'a' + 'a' + 'b'.

2. Step 2: Trace parsing steps

   The parser calls Expr recursively for each 'a', finally matching 'b' and stopping successfully.

3. Final Answer:

   Successfully parses the input -> Option A

4. Quick Check:

   Input matches recursive pattern = success [OK]

Hint: Match input step-by-step with grammar rules [OK]

Common Mistakes:

• Thinking recursion never ends here
• Assuming syntax error on repeated 'a's
• Believing parser stops after first character

4. What is the main problem if a recursive descent parser function calls itself without consuming any input tokens?

medium

A. It generates an incorrect parse tree but finishes

B. It correctly parses empty input

C. It skips tokens and misses errors

D. It causes infinite recursion leading to a crash

Solution

1. Step 1: Analyze recursion without input consumption

   If a parser function calls itself but does not move forward in the input, it repeats the same call endlessly.

2. Step 2: Understand consequences

   This causes infinite recursion, which usually crashes the program or stack overflow.

3. Final Answer:

   It causes infinite recursion leading to a crash -> Option D

4. Quick Check:

   No input consumed + recursion = infinite loop [OK]

Hint: Check if input advances before recursive call [OK]

Common Mistakes:

• Assuming parser can handle infinite recursion gracefully
• Thinking empty input is always valid
• Believing parse tree is built despite infinite recursion

5. How can left recursion in grammar rules cause problems in recursive descent parsing, and what is a common solution?

hard

A. Left recursion causes syntax errors; add more tokens to grammar

B. Left recursion causes infinite recursion; rewrite grammar to remove left recursion

C. Left recursion causes slow parsing; use iterative loops instead

D. Left recursion causes missing parse trees; use global variables

Solution

1. Step 1: Understand left recursion problem

   Left recursion means a rule calls itself first without consuming input, causing infinite recursion in recursive descent parsers.

2. Step 2: Identify common solution

   To fix this, grammars are rewritten to remove left recursion, often by using right recursion or iteration.

3. Final Answer:

   Left recursion causes infinite recursion; rewrite grammar to remove left recursion -> Option B

4. Quick Check:

   Left recursion = infinite recursion; fix by rewriting [OK]

Hint: Rewrite grammar to eliminate left recursion [OK]

Common Mistakes:

• Thinking adding tokens fixes left recursion
• Assuming loops replace recursion without grammar change
• Believing global variables solve recursion issues