A symbol table entry for an array type records
Answer: A
The element type and bounds together determine the width and the address computation. Values exist only at run time and are of no concern to the table. Bounds are also what a compiler needs in order to emit range checks.
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Computing the width of an array from its symbol table entry requires
Answer: B
Correct answer: the number of elements multiplied by the element width
The total is the product of the dimensions multiplied by the width of one element, all of which the symbol table holds from the declaration. The address is not needed - it is what the width helps to determine, since storage is allocated by advancing a running total.
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An array is declared with dimensions 4 by 5 of 8 byte elements. How many bytes does it occupy?
Answer: C
Correct answer: 160
\[ 4 \times 5 \times 8 = 160\ \mathrm{bytes}. \] The dimensions multiply to give 20 elements, and each occupies 8 bytes. Adding the dimensions instead of multiplying them, or forgetting the element width, gives 72 and 20 respectively - both offered here.
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A compiler that permits a name to be declared in one scope and used in a nested one relies on
Answer: D
Failing to find the name locally, the compiler searches each enclosing scope in turn. Only when the outermost is exhausted is the name reported as undeclared. The search cost is proportional to the depth of nesting at the point of use.
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Access to a variable of an enclosing procedure at run time requires
Answer: A
Correct answer: following access links or consulting a display
A nested procedure’s variables live in the activation record of whichever invocation of the enclosing procedure is current, and that is found at run time. An access link in each frame points to the enclosing frame, so the chain is followed as far as the nesting depth requires; a display holds those pointers in an array so the frame is reached in one step.
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The nesting depth recorded in a symbol table entry is used to
Answer: B
Correct answer: determine how many access links to follow at run time
The difference between the nesting depth of the use and that of the declaration is exactly the number of links to traverse, and the compiler computes it from the two symbol table entries. That is why the depth is recorded: it turns a run-time search into a fixed number of pointer dereferences known at compile time.
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Which of the following is NOT normally recorded in a symbol table entry?
Answer: C
The first three are standard attributes consulted by later phases. Which instruction reads a value is decided by the code generator and has no place in an entry. Confusing what the table records with what the back end decides is a common slip.
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A symbol table is described as spanning all phases of the compiler because
Answer: D
The scanner installs, semantic analysis annotates, code generation reads. Its position in the classical diagram reflects that shared use rather than any special size. The error handler is drawn the same way and for the same reason.
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The chief cost of an over-large hash table in a compiler is
Answer: A
Lookup stays fast and collisions become rarer, so the only real penalty is space. Compilers usually start small and grow the table as the program turns out to require it. A very small program should not pay for a table sized for a very large one.
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A compiler that reports every use of an undeclared name separately is
Answer: B
Installing a placeholder entry after the first report suppresses the repeats. The quality of diagnostics is one of the more visible consequences of symbol table design. Users judge a compiler heavily on how its error messages read.
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