Syntax-directed definitions in Compiler Design - Step-by-Step Execution

Concept Flow - Syntax-directed definitions

Start: Parse Input

↓

Apply Grammar Rules

↓

Attach Semantic Rules

↓

Evaluate Attributes

↓

Compute Values or Actions

↓

Output Result or Error

The flow shows how input is parsed using grammar, semantic rules are attached, attributes evaluated, and results computed.

Execution Sample

Compiler Design

E -> E1 + T { E.val = E1.val + T.val }
T -> int { T.val = int.value }

Input: 3 + 4

This defines how to compute the value of an expression by adding integer values using syntax-directed rules.

Analysis Table

Step	Grammar Rule	Semantic Rule Applied	Attribute Values	Action/Result
1	T -> int	T.val = int.value	int=3, T.val=3	Assign T.val = 3
2	T -> int	T.val = int.value	int=4, T.val=4	Assign T.val = 4
3	E -> E1 + T	E.val = E1.val + T.val	E1.val=3, T.val=4, E.val=7	Compute E.val = 3 + 4 = 7
4	End	-	-	Final value of expression is 7

💡 All grammar rules applied and semantic attributes evaluated; expression value computed.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
int	N/A	3	4	N/A	N/A
T.val	undefined	3	4	N/A	N/A
E1.val	undefined	N/A	N/A	3	3
E.val	undefined	N/A	N/A	7	7

Key Insights - 3 Insights

Why do we assign values to attributes like T.val before computing E.val?

Because semantic rules depend on child attributes being computed first, as shown in steps 1 and 2 before step 3 in the execution_table.

What happens if the grammar rule does not match the input?

Parsing fails and semantic rules cannot be applied, so no attribute values are computed, stopping the process early.

Are semantic rules always simple assignments?

No, they can be complex computations or actions, but they must be defined clearly for each grammar rule as shown in the semantic rules column.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of T.val after Step 2?

A 3

B 4

C 7

D undefined

💡 Hint

Check the 'Attribute Values' column in row for Step 2.

At which step is the final value of the expression computed?

A Step 4

B Step 1

C Step 3

D Step 2

💡 Hint

Look for when E.val is assigned in the execution_table.

If the input was '5 + 6' instead of '3 + 4', how would the variable_tracker change after Step 1?

A int changes to 5, T.val to 5

B int changes to 6, T.val to 6

C No change in int or T.val

D int changes to 5, T.val remains undefined

💡 Hint

Variable_tracker shows int and T.val values after Step 1 for the first integer.

Concept Snapshot

Syntax-directed definitions link grammar rules with semantic rules.
Each grammar rule has semantic actions to compute attributes.
Attributes hold values or information about syntax parts.
Evaluation follows parse tree from leaves to root.
Used to compute meanings like expression values during parsing.

Full Transcript

Syntax-directed definitions combine grammar rules with semantic rules to compute attributes during parsing. The process starts by parsing input according to grammar, then semantic rules attached to grammar rules are applied to compute attribute values. For example, in an expression like '3 + 4', integer values are assigned to terminal nodes first, then combined at higher nodes using semantic rules. The execution table shows step-by-step how attributes like T.val and E.val are assigned. Variables change as parsing and evaluation proceed, tracked in the variable tracker. Key moments include understanding why child attributes must be computed before parent attributes and what happens if parsing fails. Visual quizzes test understanding of attribute values at different steps and how changes in input affect computation. Overall, syntax-directed definitions help compilers understand and compute meanings from syntax structures during parsing.

Practice

1. What is the main purpose of syntax-directed definitions in compiler design?

easy

A. To associate computations with grammar rules using attributes

B. To optimize machine code after compilation

C. To manage memory allocation during program execution

D. To handle user input and output operations

Solution

1. Step 1: Understand the role of syntax-directed definitions

   They link grammar rules to computations by associating attributes with grammar symbols.

2. Step 2: Identify the correct purpose

   This linking helps in semantic analysis and translation during compilation.

3. Final Answer:

   To associate computations with grammar rules using attributes -> Option A

4. Quick Check:

   Syntax-directed definitions = associate computations with grammar rules [OK]

Hint: Remember: syntax-directed means linking syntax to actions [OK]

Common Mistakes:

• Confusing syntax-directed definitions with code optimization
• Thinking they handle runtime memory management
• Assuming they manage input/output operations

2. Which of the following is the correct syntax to define an attribute in a syntax-directed definition?

easy

A. A.attr = B.attr + 1

B. attr A = B + 1

C. A := B + 1

D. define A.attr as B + 1

Solution

1. Step 1: Recall attribute assignment syntax

   Attributes are assigned using dot notation like A.attr = ....

2. Step 2: Match correct syntax

   A.attr = B.attr + 1 uses the correct form: A.attr = B.attr + 1.

3. Final Answer:

   A.attr = B.attr + 1 -> Option A

4. Quick Check:

   Attribute assignment uses dot and equals sign [OK]

Hint: Look for dot notation and equals sign for attributes [OK]

Common Mistakes:

• Using := instead of = for attribute assignment
• Writing attribute declarations like variables
• Using keywords like 'define' incorrectly

3. Given the syntax-directed definition:

S -> A B
A.val = 2
B.val = 3
S.val = A.val + B.val

What is the value of S.val after evaluation?

medium

A. Error

B. 6

C. 23

D. 5

Solution

1. Step 1: Identify attribute values for A and B

   A.val is 2 and B.val is 3 as given.

2. Step 2: Calculate S.val using the rule

   S.val = A.val + B.val = 2 + 3 = 5.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   S.val = 2 + 3 = 5 [OK]

Hint: Add attribute values as per rules to find result [OK]

Common Mistakes:

• Concatenating numbers as strings (23 instead of 5)
• Mixing up attribute names
• Assuming syntax error without checking values

4. Consider this syntax-directed definition snippet:

E -> E1 + T
E.val = E1.val + T.val
E1.val = 5
T.val = 7

What is the error in this definition?

medium

A. The rule causes infinite recursion

B. No error; the definition is correct

C. Attributes are assigned in the wrong order

D. Attribute E1.val is used before being defined

Solution

1. Step 1: Check attribute usage and definitions

   E1.val and T.val are both defined before use in E.val.

2. Step 2: Verify rule correctness

   The rule correctly sums E1.val and T.val to get E.val without recursion or order issues.

3. Final Answer:

   No error; the definition is correct -> Option B

4. Quick Check:

   Attributes defined before use, no recursion [OK]

Hint: Check if all attributes are defined before use [OK]

Common Mistakes:

• Assuming recursion error without context
• Thinking attribute order matters if dependencies are clear
• Confusing attribute names

5. You want to compute the value of an arithmetic expression using syntax-directed definitions. The grammar is:

Expr -> Expr1 '+' Term
Expr.val = Expr1.val + Term.val
Expr -> Term
Expr.val = Term.val
Term -> 'num'
Term.val = num.value

Given the input expression 3 + 4 + 5, how should the attributes be evaluated to get the correct total?

hard

A. Use inherited attributes to pass partial sums from left to right

B. Use semantic actions to print values without computing sum

C. Use synthesized attributes only, evaluating bottom-up

D. Use attributes to store tokens but not compute values

Solution

1. Step 1: Understand attribute types in the grammar

   The grammar uses synthesized attributes (Expr.val, Term.val) computed bottom-up.

2. Step 2: Evaluate expression bottom-up

   Each Term.val is the number's value; Expr.val sums left Expr1.val and Term.val recursively.

3. Step 3: Confirm correct evaluation method

   Using synthesized attributes bottom-up correctly computes 3 + 4 + 5 = 12.

4. Final Answer:

   Use synthesized attributes only, evaluating bottom-up -> Option C

5. Quick Check:

   Synthesized attributes compute values bottom-up [OK]

Hint: Use synthesized attributes to compute expression values bottom-up [OK]

Common Mistakes:

• Confusing inherited and synthesized attributes
• Trying to pass partial sums left to right incorrectly
• Ignoring attribute evaluation order