SLR(1) parsing in Compiler Design - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does SLR(1) stand for in parsing?

SLR(1) stands for Simple LR parsing with 1 lookahead symbol. It is a type of bottom-up parser used in compiler design.

beginner

What is the main purpose of SLR(1) parsing?

The main purpose of SLR(1) parsing is to analyze the syntax of a programming language by reading input from left to right and producing a rightmost derivation in reverse, using a single lookahead symbol to decide parsing actions.

intermediate

How does SLR(1) parsing decide when to shift or reduce?

SLR(1) parsing uses a parsing table built from the grammar's LR(0) items and FOLLOW sets. It shifts when the next input symbol matches a transition and reduces when the lookahead symbol is in the FOLLOW set of the production's left side nonterminal.

intermediate

What is a conflict in SLR(1) parsing and why does it occur?

A conflict occurs when the parser cannot decide between shifting or reducing (shift-reduce conflict) or between two reductions (reduce-reduce conflict). It happens because the grammar is not SLR(1) compatible, meaning the parsing table has ambiguous entries.

advanced

What is the difference between SLR(1) and LR(1) parsing?

SLR(1) uses FOLLOW sets for lookahead decisions, which is simpler but less powerful. LR(1) parsing uses specific lookahead sets for each item, making it more precise and able to handle more grammars without conflicts.

What does the '1' in SLR(1) parsing represent?

A One lookahead symbol

B One production rule

C One parsing state

D One grammar symbol

The '1' means the parser looks ahead one symbol to decide parsing actions.

Which set is used in SLR(1) parsing to decide reductions?

A FIRST set

B LR(0) items

C FOLLOW set

D Nullable set

SLR(1) uses FOLLOW sets to decide when to reduce.

What type of parsing method is SLR(1)?

A Top-down parsing

B Bottom-up parsing

C Recursive descent parsing

D Predictive parsing

SLR(1) is a bottom-up parsing technique.

What causes a shift-reduce conflict in SLR(1) parsing?

A Missing production

B Too many lookaheads

C Incorrect FOLLOW set

D Ambiguous grammar

Ambiguous grammars can cause shift-reduce conflicts.

Compared to SLR(1), LR(1) parsing is:

A More powerful

B Slower but less accurate

C The same

D Less powerful

LR(1) parsing is more powerful because it uses specific lookahead sets.

Explain how SLR(1) parsing uses FOLLOW sets to decide when to reduce.

Think about how the parser checks the next input symbol against FOLLOW sets before reducing.

Describe what a shift-reduce conflict is and why it might happen in SLR(1) parsing.

Consider when the parser cannot decide between shifting or reducing.

Practice

1. What does the 1 in SLR(1) parsing represent?

easy

A. One grammar rule applied

B. One step shift operation

C. One reduction per state

D. One symbol lookahead during parsing

Solution

1. Step 1: Understand the meaning of lookahead in parsing

   Lookahead means the parser looks at upcoming input symbols to decide the next action.

2. Step 2: Identify what the number 1 specifies in SLR(1)

   The number 1 indicates the parser looks ahead by exactly one symbol to make decisions.

3. Final Answer:

   One symbol lookahead during parsing -> Option D

4. Quick Check:

   SLR(1) means 1 symbol lookahead [OK]

Hint: Remember: number in SLR(n) = lookahead symbols [OK]

Common Mistakes:

• Confusing lookahead with number of grammar rules
• Thinking it means one shift or reduction only
• Assuming it refers to parser states

2. Which of the following is a correct component of the SLR(1) parsing table?

easy

A. ACTION and GOTO tables

B. PREDICT and FOLLOW tables

C. SHIFT and REDUCE stacks

D. FIRST and LAST tables

Solution

1. Step 1: Recall the structure of SLR(1) parsing tables

   SLR(1) parsing uses two main tables: ACTION for parser actions and GOTO for state transitions.

2. Step 2: Identify the correct table names

   Only ACTION and GOTO tables are part of the SLR(1) parsing table, others are unrelated or parts of grammar analysis.

3. Final Answer:

   ACTION and GOTO tables -> Option A

4. Quick Check:

   SLR(1) uses ACTION and GOTO tables [OK]

Hint: ACTION table decides moves; GOTO table changes states [OK]

Common Mistakes:

• Mixing parsing tables with grammar sets like FIRST or FOLLOW
• Confusing stacks with tables
• Assuming PREDICT table exists in SLR(1)

3. Given the grammar:

S -> CC
C -> cC | d

What is the correct ACTION for state 2 on input c in SLR(1) parsing?

medium

A. Reduce by C -> d

B. Shift to state 3

C. Accept input

D. Error

Solution

1. Step 1: Identify state 2's items and possible moves

   State 2 likely has an item expecting c to continue parsing C -> cC, so it should shift on c.

2. Step 2: Determine the action for input c

   Since c can start a new C, the parser shifts to state 3 to parse C -> cC.

3. Final Answer:

   Shift to state 3 -> Option B

4. Quick Check:

   Input 'c' leads to shift in state 2 [OK]

Hint: Shift on terminals that start productions [OK]

Common Mistakes:

• Choosing reduce too early without checking lookahead
• Confusing accept with shift
• Assuming error when shift is valid

4. In an SLR(1) parser, a conflict occurs when:
state 4 has both shift and reduce actions on the same input symbol. What is the likely cause?

medium

A. Grammar is not SLR(1) compatible

B. Parser table was built incorrectly

C. Lookahead symbol is missing

D. Input string is invalid

Solution

1. Step 1: Understand shift-reduce conflicts in SLR(1)

   Shift-reduce conflicts happen when the parser cannot decide whether to shift or reduce on the same input symbol.

2. Step 2: Identify the cause of such conflicts

   This usually means the grammar is not suitable for SLR(1) parsing, i.e., it is not SLR(1) compatible.

3. Final Answer:

   Grammar is not SLR(1) compatible -> Option A

4. Quick Check:

   Shift-reduce conflict = grammar not SLR(1) [OK]

Hint: Conflicts mean grammar is not SLR(1) compatible [OK]

Common Mistakes:

• Blaming parser table construction errors
• Thinking input string causes conflicts
• Assuming missing lookahead causes conflict

5. You have the grammar:

E -> E + T | T
T -> T * F | F
F -> ( E ) | id

Why might SLR(1) parsing fail for this grammar, and what is a common solution?

hard

A. Grammar is left-factored; remove left recursion

B. Grammar lacks terminals; add more tokens

C. Grammar is ambiguous; use LALR(1) or LR(1) parsing

D. Grammar is too simple; add more productions

Solution

1. Step 1: Recognize the grammar type

   This grammar is classic expression grammar with ambiguity due to operator precedence and associativity.

2. Step 2: Understand SLR(1) limitations

   SLR(1) parsing often fails on ambiguous grammars because it cannot resolve conflicts with only one symbol lookahead.

3. Step 3: Identify common solutions

   More powerful parsers like LALR(1) or LR(1) handle such grammars by using more precise lookahead and states.

4. Final Answer:

   Grammar is ambiguous; use LALR(1) or LR(1) parsing -> Option C

5. Quick Check:

   Ambiguous grammar needs stronger parser than SLR(1) [OK]

Hint: Ambiguous grammar needs LR(1) or LALR(1) [OK]

Common Mistakes:

• Thinking adding tokens fixes ambiguity
• Confusing left recursion removal with ambiguity
• Assuming grammar simplicity causes failure