LL(1) parser implementation in Compiler Design - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the parsing table as an empty dictionary.

Compiler Design

parsing_table = [1]

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

A{}

B[]

Cset()

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using a list or set instead of a dictionary.

Assigning None which does not allow storing entries.

The parsing table in an LL(1) parser is typically represented as a dictionary (or map) to store productions indexed by non-terminals and terminals.

2fill in blank

medium

Complete the code to check if the current input symbol matches the expected terminal in the LL(1) parser.

Compiler Design

if input_symbol [1] top_of_stack:

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A==

B!=

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using inequality operator '!=' instead of equality.

Using comparison operators like '<' or '>'.

In LL(1) parsing, we compare the current input symbol with the top of the stack to see if they match exactly.

3fill in blank

hard

Fix the error in the code that pops the top element from the stack in the LL(1) parser.

Compiler Design

top_of_stack = stack[1]()

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Adiscard

Bremove

Cdelete

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'remove' which removes by value, not by position.

Using 'delete' or 'discard' which are not list methods.

To remove and return the last element from a list used as a stack, the 'pop()' method is used in Python.

4fill in blank

hard

Fill both blanks to correctly retrieve the production rule from the parsing table.

Compiler Design

production = parsing_table[[1]][[2]]

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Anon_terminal

Bterminal

Cinput_symbol

Dstack_top

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of keys.

Using incorrect variable names that do not represent the grammar symbols.

The parsing table is accessed first by the non-terminal symbol, then by the terminal (input symbol) to get the production rule.

5fill in blank

hard

Fill all three blanks to correctly update the stack with the right-hand side of the production in reverse order.

Compiler Design

for symbol in reversed([1]):
    if symbol != [2]:
        stack.[3](symbol)

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Aproduction

B''

Cappend

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Not reversing the production before pushing.

Pushing epsilon (empty string) onto the stack.

Using incorrect stack method instead of 'append'.

We reverse the production's right side and push each symbol onto the stack, skipping empty strings which represent epsilon.

Practice

1. What does the 1 in LL(1) parser stand for?

easy

A. One stack is used for parsing

B. One left recursion is allowed in the grammar

C. One lookahead token is used to decide parsing steps

D. One grammar rule is applied at a time

Solution

1. Step 1: Understand LL(1) parser basics

   LL(1) parser reads input Left to right (L), produces Leftmost derivation (L), and uses 1 lookahead token.

2. Step 2: Identify what '1' means

   The '1' specifically means the parser looks ahead by one token to decide which grammar rule to apply next.

3. Final Answer:

   One lookahead token is used to decide parsing steps -> Option C

4. Quick Check:

   LL(1) = 1 lookahead token [OK]

Hint: Remember LL(1) means 1 token lookahead [OK]

Common Mistakes:

• Confusing '1' with number of grammar rules
• Thinking '1' means one stack or recursion
• Assuming '1' means one parsing step

2. Which of the following is the correct syntax to represent the parsing table entry for non-terminal A and terminal a in LL(1) parser?

easy

A. parsing_table(a, A) = production_rule

B. parsing_table.a.A = production_rule

C. parsing_table[A, a] = production_rule

D. parsing_table[A][a] = production_rule

Solution

1. Step 1: Understand parsing table structure

   Parsing table is usually a two-dimensional structure indexed by non-terminal and terminal symbols.

2. Step 2: Identify correct syntax

   Using bracket notation with parsing_table[A][a] is the standard way to access or assign entries in a 2D table or dictionary of dictionaries.

3. Final Answer:

   parsing_table[A][a] = production_rule -> Option D

4. Quick Check:

   Use parsing_table[non-terminal][terminal] [OK]

Hint: Use double brackets for 2D table access [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Swapping terminal and non-terminal order
• Using dot notation which is invalid here

3. Given the grammar rules:
S -> a A
A -> b
and the input string ab, what will be the output of an LL(1) parser?

medium

A. Parsing successful

B. Syntax error at 'b'

C. Parsing successful but with warnings

D. Syntax error at 'a'

Solution

1. Step 1: Trace parsing steps with input 'ab'

   Start with stack containing S. Input is 'a b'. Match 'a' with S -> a A, pop S, push A. Next input 'b' matches A -> b.

2. Step 2: Confirm all input consumed and stack empty

   After matching 'b', stack is empty and input is fully consumed, so parsing is successful.

3. Final Answer:

   Parsing successful -> Option A

4. Quick Check:

   Input matches grammar rules fully [OK]

Hint: Match input tokens stepwise with grammar rules [OK]

Common Mistakes:

• Assuming error on matching terminal 'b'
• Confusing input order
• Not popping stack correctly

4. Identify the error in this LL(1) parser snippet:

stack = ['$', 'S']
input = list('ab$')
while stack:
    top = stack.pop()
    current = input[0]
    if top == current:
        input.pop(0)
    else:
        rule = parsing_table[top][current]
        stack.extend(reversed(rule))

medium

A. Stack is popped before checking if top is terminal

B. Input is not consumed correctly

C. Parsing table lookup uses wrong indices

D. Stack is extended without reversing the rule

Solution

1. Step 1: Analyze stack popping logic

   The code pops the stack before checking if the top is a terminal matching the input token, which can cause loss of needed symbol if mismatch occurs.

2. Step 2: Explain correct approach

   Correct approach is to peek at the top of the stack first, then decide to pop if it matches input or replace using parsing table if non-terminal.

3. Final Answer:

   Stack is popped before checking if top is terminal -> Option A

4. Quick Check:

   Pop after checking top symbol type [OK]

Hint: Peek stack top before popping [OK]

Common Mistakes:

• Popping stack too early
• Not reversing rule before pushing
• Incorrect input token consumption

5. You want to implement an LL(1) parser for a grammar with left recursion. What is the best approach to handle this?

hard

A. Use a stack to store left recursive calls

B. Eliminate left recursion from the grammar before building the parser

C. Increase lookahead tokens to LL(2) or more

D. Ignore left recursion and parse as usual

Solution

1. Step 1: Understand LL(1) parser limitations

   LL(1) parsers cannot handle left recursion because it causes infinite recursion or parsing conflicts.

2. Step 2: Apply grammar transformation

   To use LL(1) parsing, left recursion must be removed by rewriting grammar rules into right-recursive or iterative forms.

3. Final Answer:

   Eliminate left recursion from the grammar before building the parser -> Option B

4. Quick Check:

   Remove left recursion for LL(1) compatibility [OK]

Hint: Remove left recursion before LL(1) parsing [OK]

Common Mistakes:

• Trying to parse left recursion directly
• Assuming more lookahead solves left recursion
• Ignoring grammar transformation needs