Structures, Unions, Enums Questions and Answers

Q: Which of the following structure declarations will throw an error?

Correct answer: struct temp s; struct temp{}; main(){}This one is an error. Declaring struct temp s; before the structure has been defined means declaring an object of an incomplete type - the compiler does not yet know how much storage to reserve. The other two are fine: the first defines the type and a variable together, and the second defines the type before using it. (A pointer to an incomplete type would be legal, since a pointer’s size is known.)

Q: Anyone of the followings can be used to declare a node for a singly linked list. If we use the first declaration, “struct node * nodePtr;” would be used to declare pointer to a node. If we use the second declaration, “NODEPTR nodePtr;” can be used to declare pointer to a node. /* First declaration */ struct node { int data; struct node * nextPtr; }; /* Second declaration */ typedef struct node{ int data; NODEPTR nextPtr; } * NODEPTR;

Correct answer: FALSEThe second declaration is broken: NODEPTR is used inside the structure to declare nextPtr, but the typedef that defines NODEPTR is not complete until the closing brace. A name cannot be used before it exists, so the compiler rejects it. The self-reference has to be written as struct node *nextPtr;.

Q: What is the output of this program? void main() { struct bitfields { int bits_1: 2; int bits_2: 9; int bits_3: 6; int bits_4: 1; }bit; printf("%d", sizeof(bit)); }

Correct answer: 3The bit-fields total \(2 + 9 + 6 + 1 = 18\) bits. Rounded up to whole bytes that is 3 under the packing this question assumes. (In practice most compilers align bit-fields to their underlying type and would report 4, which is why bit-field layout is implementation-defined and not portable.)

Q: All enum constants are

Correct answer: unique in their scopeEnumerator names must be unique within their scope, because they share the ordinary identifier namespace - two enumerations at file scope cannot both declare a constant called c. Their values, by contrast, may repeat freely.

Q: Predict the output of the program below. Assume that the size of an integer is 4 bytes and size of character is 1 byte. Also assume that there is no alignment needed. union test { int x; char arr[8]; int y; }; int main() { printf("%d", sizeof(union test)); return 0; }

Correct answer: 8A union is as large as its largest member. Here int x and int y are 4 bytes each, and char arr[8] is 8 - so the union is 8 bytes. That the two ints are separate members makes no difference: all three overlay the same storage.

Q: What is the output of this program? #include <stdio.h> int main(){ struct leader { char *lead; int born; }; struct leader l1 = {"AbdulKalam", 1931}; struct leader l2 = l1; printf("%s %d", l2.lead, l1.born); }

Correct answer: AbdulKalam 1931l2 = l1 copies the structure member by member - the born value and the lead pointer. That is a shallow copy: both structures now point at the same string literal. Here it does no harm, since neither modifies it.

Q: What is the output of this program? #include <stdio.h> struct employee { char *empname; int salary; }; int main() { struct employee e, e1; e.empname = "Sridhar"; e1 = e; printf("%s %s", e.empname, e1.empname); return 0; }

Correct answer: Sridhar SridharAssigning one structure to another copies its members, and for a pointer member that means copying the address, not the text. So e and e1 both point at the same string literal and both print Sridhar. Copying the text would require strcpy into separately allocated storage.

Q: Which of the following is a properly defined struct?

Correct answer: struct a_struct {int a;};A structure definition is a declaration and must end with a semicolon - which is what the others omit. (The first form, without a tag, is legal if a variable is declared with it, but as written it declares nothing and is useless.)

21.

Which of the following structure declarations will throw an error?

A.struct temp{}s; main(){}
B.struct temp{}; struct temp s; main(){}
C.struct temp s; struct temp{}; main(){}
D.None of the mentioned

Answer: D

Correct answer: struct temp s; struct temp{}; main(){}

This one is an error. Declaring struct temp s; before the structure has been defined means declaring an object of an incomplete type - the compiler does not yet know how much storage to reserve. The other two are fine: the first defines the type and a variable together, and the second defines the type before using it. (A pointer to an incomplete type would be legal, since a pointer’s size is known.)

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22.

The following C declarations

struct node
{
   int i;
   float j;
};
struct node *s[10] ;

define s to be

A.An array, each element of which is a pointer to a structure of type node
B.A structure of 2 fields, each field being a pointer to an array of 10 elements
C.A structure of 3 fields: an integer, a float, and an array of 10 elements
D.An array, each element of which is a structure of type node.

Answer: A

Correct answer: An array, each element of which is a pointer to a structure of type node

Subscript binds tighter than the dereference, so struct node *s[10] groups as struct node *(s[10]) - an array of 10 pointers, each to a struct node. A pointer to an array would need parentheses in the other place: struct node (*s)[10].

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23.

Anyone of the followings can be used to declare a node for a singly linked list. If we use the first declaration, “struct node * nodePtr;” would be used to declare pointer to a node. If we use the second declaration, “NODEPTR nodePtr;” can be used to declare pointer to a node.

/* First declaration */
struct node {
int data;
struct node * nextPtr;
};
 
/* Second declaration */
typedef struct node{
int data;
NODEPTR nextPtr;
} * NODEPTR;

A.TRUE
B.FALSE

Answer: B

Correct answer: FALSE

The second declaration is broken: NODEPTR is used inside the structure to declare nextPtr, but the typedef that defines NODEPTR is not complete until the closing brace. A name cannot be used before it exists, so the compiler rejects it. The self-reference has to be written as struct node *nextPtr;.

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24.

What is the output of this program?

void main()
  {
  struct bitfields {
  int bits_1: 2;
  int bits_2: 9;
  int bits_3: 6;
  int bits_4: 1;
  }bit;
  printf("%d", sizeof(bit));
}

Answer: B

Correct answer: 3

The bit-fields total \(2 + 9 + 6 + 1 = 18\) bits. Rounded up to whole bytes that is 3 under the packing this question assumes. (In practice most compilers align bit-fields to their underlying type and would report 4, which is why bit-field layout is implementation-defined and not portable.)

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25.

All enum constants are

A.Same in their scope
B.unique in their scope
C.contain duplicate values in their scope
D.None of the above

Answer: B

Correct answer: unique in their scope

Enumerator names must be unique within their scope, because they share the ordinary identifier namespace - two enumerations at file scope cannot both declare a constant called c. Their values, by contrast, may repeat freely.

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26.

Predict the output of the program below. Assume that the size of an integer is 4 bytes and size of character is 1 byte. Also assume that there is no alignment needed.

union test
{
    int x;
    char arr[8];
    int y;
};
 
int main()
{
    printf("%d", sizeof(union test));
    return 0;
}

A.12
B.16
C.8
D.Compiler Error

Answer: C

Correct answer: 8

A union is as large as its largest member. Here int x and int y are 4 bytes each, and char arr[8] is 8 - so the union is 8 bytes. That the two ints are separate members makes no difference: all three overlay the same storage.

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27.

What is the output of this program?

#include <stdio.h>
int main(){
    struct leader
 {
 char *lead;
 int born;
 };
 struct leader l1 = {"AbdulKalam", 1931};
 struct leader l2 = l1;
 printf("%s %d", l2.lead, l1.born);
}

A.Compilation error
B.Garbage value 1931
C.AbdulKalam 1931
D.None of the above

Answer: C

Correct answer: AbdulKalam 1931

l2 = l1 copies the structure member by member - the born value and the lead pointer. That is a shallow copy: both structures now point at the same string literal. Here it does no harm, since neither modifies it.

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28.

What is the output of this program?

#include <stdio.h>
struct employee
{
  char *empname;
  int salary;
};
int main()
{
  struct employee e, e1;
  e.empname = "Sridhar";
  e1 = e;
  printf("%s %s", e.empname, e1.empname);
  return 0;
}

A.Garbage value Sridhar
B.Sridhar Garbage value
C.Sridhar Sridhar
D.Compilation Error

Answer: C

Correct answer: Sridhar Sridhar

Assigning one structure to another copies its members, and for a pointer member that means copying the address, not the text. So e and e1 both point at the same string literal and both print Sridhar. Copying the text would require strcpy into separately allocated storage.

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29.

Which of the following is a properly defined struct?

A.struct {int a;}
B.struct a_struct {int a;}
C.struct a_struct int a;
D.struct a_struct {int a;};

Answer: D

Correct answer: struct a_struct {int a;};

A structure definition is a declaration and must end with a semicolon - which is what the others omit. (The first form, without a tag, is legal if a variable is declared with it, but as written it declares nothing and is useless.)

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30.

What is the correct syntax of enum?

A.enum flag{constant1, constant2, constant3, ....... };
B.enum flag(constant1, constant2, constant3, ....... );
C.enum flag[constant1, constant2, constant3, ....... ];
D.enumflag{constant1, constant2, constant3, ....... };

Answer: A

Correct answer: enum flag{constant1, constant2, constant3, ....... };

The syntax is the keyword enum, an optional tag, the constants in braces separated by commas, and a closing semicolon. Parentheses and square brackets are not used, and the keyword must be separated from the tag.

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