Syntax Analysis and Parsing Questions and Answers

Q: State whether the following is true or false.Every property a programming language requires can be checked by its context free grammar.

Correct answer: FalseA context free grammar can describe the shape of a program but not conditions that depend on what has been declared elsewhere: that a variable is declared before use, that the types of an assignment agree, that a function is called with the right number of arguments. All of these are checked by semantic analysis, using the symbol table.

51.

State whether the following is true or false.
Every property a programming language requires can be checked by its context free grammar.

A.False only at small scales
B.True
C.False
D.True only at large scales

Answer: C

Correct answer: False

A context free grammar can describe the shape of a program but not conditions that depend on what has been declared elsewhere: that a variable is declared before use, that the types of an assignment agree, that a function is called with the right number of arguments. All of these are checked by semantic analysis, using the symbol table.

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52.

Checking that a function is called with the right number of arguments is

A.a syntactic check performed by the parser during reduction
B.a lexical check performed by the scanner as it reads the call
C.something the linker settles once it joins the modules together
D.a semantic check, since the count depends on the declaration

Answer: D

The grammar can describe an argument list of any length but cannot tie its length to a declaration elsewhere. Semantic analysis consults the symbol table to make the comparison. The same applies to argument types, which the grammar likewise cannot constrain.

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53.

The augmented grammar used in LR parsing adds

A.a new start production so acceptance has a unique reduction
B.an extra production for every terminal in the grammar
C.epsilon productions for each nonterminal that lacks one
D.a production removing left recursion from the start symbol

Answer: A

Adding S prime going to S means the parser accepts by reducing that one production. Without it, acceptance would be harder to detect unambiguously. The augmentation costs one extra state and removes an awkward special case.

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54.

An item in LR parsing is

A.a nonterminal together with its FIRST and FOLLOW sets
B.a production with a dot marking how much has been recognised
C.a token together with the attribute value carried with it
D.an entry of the parsing table indexed by state and symbol

Answer: B

The dot separates what the parser has already seen from what it still expects. Sets of such items become the states of the automaton that drives the parse. Constructing those sets is the mechanical heart of every LR table generator.

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55.

The closure of a set of LR items adds

A.the FOLLOW sets of each nonterminal appearing in the set
B.the terminals that may be shifted from the current state
C.items for the productions of any nonterminal following a dot
D.items for every production of the grammar without exception

Answer: C

If the parser expects a nonterminal, it may be about to begin any of that nonterminal's productions. Closure records all those possibilities in the same state. Without closure the automaton would have no way to begin a nested construct.

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56.

The number of states in an LALR table compared with a canonical LR table for the same grammar is

A.larger, since the lookaheads are all carried separately
B.always exactly the same for every grammar
C.unrelated, as the two methods work from different item sets
D.smaller, since every pair of states sharing a core is merged

Answer: D

LALR merges canonical LR states sharing a core, which cuts the table greatly. The merge can introduce reduce-reduce conflicts, though never shift-reduce ones. That guarantee is what makes LALR a safe default for a parser generator.

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57.

Consider the following statements about LALR parsing.
1. It has the same number of states as the SLR parser.
2. Merging states can introduce reduce-reduce conflicts.
3. It can introduce shift-reduce conflicts not present in canonical LR.

A.1 and 2 only
B.3 only
C.1 only
D.All of them

Answer: A

LALR has the SLR state count with better lookaheads. Merging may create a reduce-reduce conflict but provably never a shift-reduce one, so the third statement is false. That asymmetry is a standard result quoted in every treatment of the method.

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58.

Arrange these parser classes in order of increasing grammar coverage.
1. LALR(1)
2. LL(1)
3. canonical LR(1)
4. SLR(1)

A.2 - 1 - 4 - 3
B.2 - 4 - 1 - 3
C.3 - 1 - 4 - 2
D.4 - 2 - 1 - 3

Answer: B

Each class handles every grammar the previous one does and more. The practical sweet spot is LALR(1), which covers nearly every language grammar at a table size close to SLR. Moving up the scale costs table space and gains coverage that is rarely needed.

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59.

Yacc and Bison generate parsers of which class?

A.canonical LR(1) with no state merging performed
B.operator precedence parsers for expression grammars
C.LALR(1)
D.LL(1) predictive parsers driven by a table

Answer: C

Both tools produce LALR(1) parsers by default. The choice reflects the balance between grammar coverage and table size that suits nearly all programming language grammars. Canonical LR tables would be several times larger for very little extra coverage.

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60.

A parser generator resolves a shift-reduce conflict by default in favour of

A.the reduce, which is said to produce the shorter of the parse trees
B.the production listed first in the grammar specification
C.reporting an error and refusing to generate the parser
D.the shift, which yields the nearest if behaviour for dangling else

Answer: D

Preferring the shift resolves the dangling else the way languages specify. The generator still reports the conflict so the author can check the default is what was intended. Silently accepting the default without reading the report is a common mistake.

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