Arrays Questions and Answers

Q: A file is preferable to an array of structures because

Correct answer: All of the aboveAll three reasons hold. A file persists after the program that wrote it ends, while an array vanishes with the process. A file need not be sized in advance, so no space is reserved and wasted. And the operating system provides tools - copy, search, sort, back up - that work on files and not on a program’s memory.

Q: The information about an array used in program will be stored in

Correct answer: Dope VectorA dope vector is the descriptor the implementation keeps for an array: its base address, element size, number of dimensions and the bounds of each. It is what makes address computation and bounds checking possible. The symbol table holds compile-time names and types; an activation record holds a call’s local storage.

Q: What is the syntax for accessing the element at index 7 of an array a?

Correct answer: a[7]C uses square brackets for subscripting, so the element at index 7 is a[7]. Braces are for initialiser lists, parentheses would be read as a function call, and quotes make a string literal. Note that a[7] is the eighth element, since indexing starts at 0.

Q: If x is an array of integers, then the value of &x[i] is same as

Correct answer: x+iBy definition x[i] is *(x + i), so taking the address of both sides gives &x[i] == x + i. The scaling is already built into pointer arithmetic: adding 1 to an int * advances by sizeof(int) bytes. That is why the two options that multiply or add sizeof(int) explicitly are wrong - they would scale twice.

Q: A one dimensional array A has indices 1....75. Each element is a string and takes up three memory words. The array is stored at location 1120 decimal. The starting address of A[49] is

Correct answer: 1264Indices start at 1 here, not 0, so \(A[49]\) is the 49th element and 48 elements precede it. Each takes 3 words:\[ 1120 + 3 \times (49 - 1) = 1120 + 144 = 1264 \]Forgetting the 1-based start gives 1267, which is the distractor.

Q: In C, if you pass an array as an argument to a function, what actually gets passed?

Correct answer: Base address of the arrayAn array argument decays to a pointer to its first element, so what the function receives is the base address, not a copy of the elements. This is why passing a large array costs nothing extra, why the function can modify the caller’s data, and why sizeof inside the function returns the size of a pointer rather than of the array.

Q: Under which of the following conditions, the size of an one-dimensional array need not be specified?

Correct answer: All of the aboveAll three permit the size to be left out. With an initialiser the compiler counts the values. In a declaration such as extern int a[]; the size comes from the definition elsewhere. And as a formal parameter the array is adjusted to a pointer, so no bound is needed - nor at the call site, where the name is passed alone.

Q: Predict the output of the code below, if the array begins at address 65472 and each integer occupies 2 bytes. #include <stdio.h> int main() { int a[3][4] = {1, 2, 3, 4, 4, 3, 2, 1, 7, 8, 9, 0}; printf("%u, %u\n", a+1, &a+1); return 0; }

Correct answer: 65480, 65496a is a 3×4 array of 2-byte ints. a decays to a pointer to its first row, so a+1 steps over 4 ints:\[ 65472 + 4 \times 2 = 65480 \]&a is a pointer to the whole array, so &a+1 steps over all 12:\[ 65472 + 12 \times 2 = 65496 \]

Q: Choose the correct statements

Correct answer: Both (a) and (b)(a) is true: C really has only one-dimensional arrays - int a[3][4] is an array of 3 objects, each of type int[4]. (b) says the same thing from the other side: an element may itself be an array. The third statement is false; elements are always contiguous, which is exactly what makes subscripting work.

Q: If S is an array of 80 characters, then the value assigned to S through the statement scanf("%s",S) with input 12345 would be

Correct answer: "12345"%s reads a whitespace-delimited sequence of characters and stores them followed by a null, without interpreting them. The digits 12345 are just five characters here, so S holds the string "12345". Note that S is passed without &, because an array name already yields its base address.

11.

A file is preferable to an array of structures because

A.file lives even after the program that created it terminates
B.memory space will not be wasted
C.there are many system tools to manipulate files
D.All of the above

Answer: D

Correct answer: All of the above

All three reasons hold. A file persists after the program that wrote it ends, while an array vanishes with the process. A file need not be sized in advance, so no space is reserved and wasted. And the operating system provides tools - copy, search, sort, back up - that work on files and not on a program’s memory.

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12.

The information about an array used in program will be stored in

A.Symbol Table
B.Activation Record
C.Dope Vector
D.Both A and B

Answer: C

Correct answer: Dope Vector

A dope vector is the descriptor the implementation keeps for an array: its base address, element size, number of dimensions and the bounds of each. It is what makes address computation and bounds checking possible. The symbol table holds compile-time names and types; an activation record holds a call’s local storage.

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13.

What is the syntax for accessing the element at index 7 of an array a?

A.a{7}
B.a(7)
C.a"7"
D.a[7]

Answer: D

Correct answer: a[7]

C uses square brackets for subscripting, so the element at index 7 is a[7]. Braces are for initialiser lists, parentheses would be read as a function call, and quotes make a string literal. Note that a[7] is the eighth element, since indexing starts at 0.

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14.

If x is an array of integers, then the value of &x[i] is same as

A.&x[i-1] + sizeof (int)
B.x + sizeof (int) * i
C.x+i
D.none of these

Answer: A

Correct answer: x+i

By definition x[i] is *(x + i), so taking the address of both sides gives &x[i] == x + i. The scaling is already built into pointer arithmetic: adding 1 to an int * advances by sizeof(int) bytes. That is why the two options that multiply or add sizeof(int) explicitly are wrong - they would scale twice.

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15.

A one dimensional array A has indices 1....75. Each element is a string and takes up three memory words. The array is stored at location 1120 decimal. The starting address of A[49] is

A.1264
B.1164
C.1167
D.1267

Answer: A

Correct answer: 1264

Indices start at 1 here, not 0, so \(A[49]\) is the 49th element and 48 elements precede it. Each takes 3 words:

\[ 1120 + 3 \times (49 - 1) = 1120 + 144 = 1264 \]

Forgetting the 1-based start gives 1267, which is the distractor.

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16.

In C, if you pass an array as an argument to a function, what actually gets passed?

A.Value of elements in array
B.First element of the array
C.Base address of the array
D.Address of the last element of array

Answer: C

Correct answer: Base address of the array

An array argument decays to a pointer to its first element, so what the function receives is the base address, not a copy of the elements. This is why passing a large array costs nothing extra, why the function can modify the caller’s data, and why sizeof inside the function returns the size of a pointer rather than of the array.

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17.

Under which of the following conditions, the size of an one-dimensional array need not be specified?

A.when initialization is a part of definition
B.when it is a declaration
C.when it is a formal parameter and an actual argument
D.All of the above

Answer: D

Correct answer: All of the above

All three permit the size to be left out. With an initialiser the compiler counts the values. In a declaration such as extern int a[]; the size comes from the definition elsewhere. And as a formal parameter the array is adjusted to a pointer, so no bound is needed - nor at the call site, where the name is passed alone.

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18.

Predict the output of the code below, if the array begins at address 65472 and each integer occupies 2 bytes.

#include <stdio.h>

int main()
{
    int a[3][4] = {1, 2, 3, 4, 4, 3, 2, 1, 7, 8, 9, 0};
    printf("%u, %u\n", a+1, &a+1);
    return 0;
}

A.65430, 43569
B.65239, 64763
C.65480, 65496
D.61123, 74123

Answer: C

Correct answer: 65480, 65496

a is a 3×4 array of 2-byte ints. a decays to a pointer to its first row, so a+1 steps over 4 ints:

\[ 65472 + 4 \times 2 = 65480 \]

&a is a pointer to the whole array, so &a+1 steps over all 12:

\[ 65472 + 12 \times 2 = 65496 \]

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19.

Choose the correct statements

A.Strictly speaking C supports 1-dimensional arrays only
B.An array element may be an array by itself
C.Array elements need not occupy contiguous memory locations
D.Both (a) and (b)

Answer: D

Correct answer: Both (a) and (b)

(a) is true: C really has only one-dimensional arrays - int a[3][4] is an array of 3 objects, each of type int[4]. (b) says the same thing from the other side: an element may itself be an array. The third statement is false; elements are always contiguous, which is exactly what makes subscripting work.

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20.

If S is an array of 80 characters, then the value assigned to S through the statement scanf("%s",S) with input 12345 would be

A."12345"
B.nothing since 12345 is an integer
C.S is an illegal name for string
D.%s cannot be used for reading in values of S

Answer: A

Correct answer: "12345"

%s reads a whitespace-delimited sequence of characters and stores them followed by a null, without interpreting them. The digits 12345 are just five characters here, so S holds the string "12345". Note that S is passed without &, because an array name already yields its base address.

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