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Top-Down and Predictive Parsing Short Notes for GATE CS

Top-Down and Predictive Parsing short notes for GATE CS: 3 study cards covering concepts, formulas, shortcuts and exam traps, plus solved practice questions.

top down and predictive parsing short notes

Last-minute summary — FIRST and FOLLOW in one screen

.summary-grid { display: grid; grid-template-columns: 1fr; gap: 16px; margin-top: 16px; } @media (min-width: 500px) { .summary-grid { grid-template-columns: 1fr 1fr; } } .sum-box { padding: 16px; border-radius: 12px; border: 1px solid var(--color-border); background: color-mix(in srgb, var(--color-muted) 8%, transparent); } .sum-box h4 { margin: 0 0 10px 0; font-size: 0.95rem; font-weight: 600; color: var(--color-primary); } .sum-box ul { margin: 0; padding-left: 18px; } .sum-box li { font-size: 0.85rem; line-height: 1.5; margin-bottom: 4px; } .target-box { margin-top: 20px; padding: 16px; border-radius: 12px; background: color-mix(in srgb, var(--color-primary) 10%, transparent); border: 1px solid color-mix(in srgb, var(--color-primary) 25%, transparent); text-align: center; font-size: 0.95rem; font-weight: 600; } .target-box span { display: block; font-size: 0.8rem; font-weight: 400; opacity: 0.7; margin-bottom: 4px; text-transform: uppercase; letter-spacing: 0.05em; } FIRST — the rules FIRST (a) = {a} for a terminal a . FIRST (ε) = {ε} . FIRST (X) = ⋃_{X \to γ} FIRST (γ) . FIRST (X_{1} X_{2} \dots X_{k}) : walk left to right, add each FIRST (X_{i}) ∖ {ε}, stop at the first non-nullable X_{i} . If every X_{i} is nullable, ε is in the result. FOLLOW — the rules <

Summary: The LL(1) Checklist

.checklist { list-style: none; padding: 0; margin: 16px 0; } .checklist li { padding: 12px 16px; margin-bottom: 10px; border-radius: 10px; background: color-mix(in srgb, var(--color-muted) 8%, transparent); border: 1px solid var(--color-border); font-size: 0.9rem; line-height: 1.5; } .checklist li strong { color: var(--color-primary); display: block; margin-bottom: 4px; font-size: 0.85rem; text-transform: uppercase; letter-spacing: 0.05em; } .checklist li ul { margin: 4px 0 0 0; padding-left: 18px; } .checklist li li { padding: 2px 0; margin: 0; border: none; background: none; font-size: 0.85rem; } The LL(1) Checklist Disjointness For every non-terminal A, the Predict sets of its productions must be pairwise disjoint. Predict Set If α \neq \Rightarrow^{*} ε : Predict (A \to α) = FIRST (α) If α \Rightarrow^{*} ε : Predict (A \to α) = (FIRST (α) ∖ {ε}) \cup FOLLOW (A) Fatal Flaws Left recursion \Rightarrow Never LL(1). Ambiguity \Rightarrow Never LL(1). Unfactored common prefixes \Rightarrow Never LL(1). Predictive Parser Uses 1 lookahead token, no backtracking, procedures for each non-terminal.

Quick Reference: Table Construction Checklist

Quick Reference: Table Construction Checklist Pre-requisites All FIRST sets computed correctly All FOLLOW sets computed correctly Terminals identified (including \$) Construction Steps Initialize table: rows = non-terminals, columns = terminals + \$ For each : Add to for If : Add to for Mark empty cells as error Validation No cell contains multiple productions (grammar is LL(1)) Epsilon productions properly handled via FOLLOW sets Common Pitfalls to Avoid: - Do not create an column - Do not forget FOLLOW for epsilon productions - Do not miss terminals in FIRST of sequences - Do not ignore \$ in FOLLOW of start symbol

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Top-Down and Predictive Parsing Short Notes for GATE CS

Top-Down and Predictive Parsing short notes for GATE CS: 3 study cards covering concepts, formulas, shortcuts and exam traps, plus solved practice questions.

Last-minute summary — FIRST and FOLLOW in one screen

FIRST — the rules

$FIRST (a) = {a}$ for a terminal $a$ .
$FIRST (ε) = {ε}$ .
$FIRST (X) = ⋃_{X \to γ} FIRST (γ)$ .
$FIRST (X_{1} X_{2} \dots X_{k})$ : walk left to right, add each $FIRST (X_{i}) ∖ {ε}$ , stop at the first non-nullable $X_{i}$ . If every $X_{i}$ is nullable, $ε$ is in the result.

FOLLOW — the rules

Summary: The LL(1) Checklist

The LL(1) Checklist

Disjointness For every non-terminal $A$ , the Predict sets of its productions must be pairwise disjoint.
Predict Set
- If $α \neq \Rightarrow^{*} ε$ : $Predict (A \to α) = FIRST (α)$
- If $α \Rightarrow^{*} ε$ : $Predict (A \to α) = (FIRST (α) ∖ {ε}) \cup FOLLOW (A)$
Fatal Flaws
- Left recursion $\Rightarrow$ Never LL(1).
- Ambiguity $\Rightarrow$ Never LL(1).
- Unfactored common prefixes $\Rightarrow$ Never LL(1).
Predictive Parser Uses 1 lookahead token, no backtracking, procedures for each non-terminal.

Quick Reference: Table Construction Checklist

Pre-requisites

All FIRST sets computed correctly

All FOLLOW sets computed correctly

Terminals identified (including

\

Construction Steps

Initialize table: rows = non-terminals, columns = terminals +

\

For each

A \to α

: Add to

M [A, a]

for

a \in FIRST (α) ∖ {ϵ}

ϵ \in FIRST (α)

: Add to

M [A, b]

for

b \in FOLLOW (A)

Mark empty cells as error

Validation

No cell contains multiple productions (grammar is LL(1))

Epsilon productions properly handled via FOLLOW sets

Common Pitfalls to Avoid:
- Do not create an

ϵ

column
- Do not forget FOLLOW for epsilon productions
- Do not miss terminals in FIRST of sequences
- Do not ignore

\

$ in FOLLOW of start symbol

top down and predictive parsing short notes

Last-minute summary — FIRST and FOLLOW in one screen

FIRST — the rules

FOLLOW — the rules

Summary: The LL(1) Checklist

The LL(1) Checklist

Quick Reference: Table Construction Checklist

Quick Reference: Table Construction Checklist

Pre-requisites

Construction Steps

Validation

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Personalised first. High quality throughout.

Built around you, not around a syllabus PDF

Revision that hits your weak spots

Questions calibrated to the real exam

Notes written for recall, not for volume

One place for everything

Honest progress

Unlock the whole course

Top-Down and Predictive Parsing Short Notes for GATE CS

Last-minute summary — FIRST and FOLLOW in one screen

FIRST — the rules

FOLLOW — the rules

Summary: The LL(1) Checklist

The LL(1) Checklist

Quick Reference: Table Construction Checklist

Quick Reference: Table Construction Checklist

Pre-requisites

Construction Steps

Validation

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