Structures, Unions, Enums Questions and Answers

Q: Point out the error (if any) in the following C code? #include <stdio.h> enum hello { a,b,c; }; main() { enum hello m; printf("%d",m); }

Correct answer: Error in the statement: a,b,c;The enumerator list must not end with a semicolon inside the braces - a, b, c is a comma-separated list, and the only semicolon belongs after the closing brace. Writing a,b,c; is a syntax error.

Q: How much memory does a union occupy in C ?

Correct answer: As much as its largest memberAll members of a union share one region of storage, so the union must be large enough for the biggest of them, with padding for alignment. Only one member holds a meaningful value at a time, which is the essential difference from a structure.

Q: What is the output of this program? #include <stdio.h> struct student { int no = 5; char name[20]; }; void main() { struct student s; s.no = 8; printf("hello"); }

Correct answer: Compile time errorA structure declaration defines a type, so its members cannot be given initialisers - int no = 5; is rejected. Initialisation belongs to the variable: struct student s = {5, ""};.

Q: What will be the output of the following C code? #include <stdio.h> enum class { a,b,c }; enum class m; main() { printf("%d",sizeof(m)); }

Correct answer: Same as the size of an integerAn enumeration variable has the size of an int - 4 bytes on the usual platforms - because the compiler stores the constant itself, not a list of them. How many enumerators the type has makes no difference.

Q: The size of the following union, where an int occupies 4 bytes of memory is union demo { float x; int y; char z[10]; };

Correct answer: 10 byteA union’s members all occupy the same storage, so its size is that of its largest member - enough to hold whichever one is in use. A structure, by contrast, gives every member its own space and so is at least the sum of them. Here the largest member is char z[10], so the union is 10 bytes - not the 18 the members would occupy in a structure.

Q: Anyone of the following can be used to declare a node for a singly linked list and “NODEPTR nodePtr;” can be used to declare pointer to a node using any of the following /* First declaration */ typedef struct node { int data; struct node *nextPtr; }* NODEPTR; /* Second declaration */ struct node { int data; struct node * nextPtr; }; typedef struct node * NODEPTR;

Correct answer: TRUEBoth work. In the first, the member is declared as struct node *nextPtr - the tag is available inside its own definition, even though the typedef name is not - and NODEPTR is defined by the same declaration. In the second, the structure is completed first and the typedef written afterwards. Either way NODEPTR nodePtr; declares a pointer to a node.

Q: A user defined data type, which is used to assign names to integral constants is called ____________

Correct answer: EnumAn enumeration gives names to a set of integer constants: enum colour {red, green, blue}; makes red 0, green 1 and blue 2. It exists for readability - the compiler still stores an integer, but the source says what the value means.

Q: What is the similarity between a structure, union and enumeration?

Correct answer: All of them let you define new data typesStructures, unions and enumerations are all ways of defining a new type - a structure aggregating members, a union overlaying them, an enumeration naming a set of integer constants. Once defined, each can be used to declare variables like any built-in type.

Q: Which of the following is an incorrect syntax to pass by reference a member of a structure in a function? (Assume: struct temp{int a;}s;)

Correct answer: None of the aboveAll three are valid. &s.a works because . binds tighter than &; &(s.a) makes that grouping explicit; and &(s).a is the same again, since the parentheses round s change nothing. So none of the forms is incorrect.

61.

Point out the error (if any) in the following C code?

#include <stdio.h>
enum hello
{
    a,b,c;
};
main()
{
    enum hello m;
    printf("%d",m);
}

A.No error
B.Error in the statement: a,b,c;
C.Error in the statement: enum hello m;
D.Error in the statement: printf(“%d”,m);

Answer: B

Correct answer: Error in the statement: a,b,c;

The enumerator list must not end with a semicolon inside the braces - a, b, c is a comma-separated list, and the only semicolon belongs after the closing brace. Writing a,b,c; is a syntax error.

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62.

How much memory does a union occupy in C ?

A.The sum of the sizes of all its members
B.As much as its largest member
C.As much as its first member
D.It is decided at run time

Answer: A

Correct answer: As much as its largest member

All members of a union share one region of storage, so the union must be large enough for the biggest of them, with padding for alignment. Only one member holds a meaningful value at a time, which is the essential difference from a structure.

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63.

What is the output of this program?

#include <stdio.h>
struct student
{
  int no = 5;
  char name[20];
};
void main()
{
  struct student s;
  s.no = 8;
  printf("hello");
}

A.Nothing
B.Compile time error
C.hello
D.Varies

Answer: B

Correct answer: Compile time error

A structure declaration defines a type, so its members cannot be given initialisers - int no = 5; is rejected. Initialisation belongs to the variable: struct student s = {5, ""};.

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64.

What is the output of this program?

#include <stdio.h>
int main(){
   struct simp 
 {
  int i = 6;
  char city[] = "chennai";
 };
 struct simp s1;
 printf("%d",s1.city);
 printf("%d", s1.i);
    return 0;
}

A.chennai 6
B.Nothing will be displayed
C.Runtime Error
D.Compilation Error

Answer: D

Compilation Error: Cannot initialize a class member here When we declared members in struture, it just tells the compiler about their presence. There is no memory allocated for that members. So we can't intialize structure members.

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65.

What will be the output of the following C code?

#include <stdio.h>
enum class
{
    a,b,c
};
enum class m;
main()
{
    printf("%d",sizeof(m));
}

A.3
B.Same as the size of an integer
C.3 times the size of an integer
D.Error

Answer: B

Correct answer: Same as the size of an integer

An enumeration variable has the size of an int - 4 bytes on the usual platforms - because the compiler stores the constant itself, not a list of them. How many enumerators the type has makes no difference.

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66.

The size of the following union, where an int occupies 4 bytes of memory is

union demo
{
  float x;
  int y;
  char z[10];
};

A.8 byte
B.4 byte
C.10 byte
D.18 byte

Answer: C

Correct answer: 10 byte

A union’s members all occupy the same storage, so its size is that of its largest member - enough to hold whichever one is in use. A structure, by contrast, gives every member its own space and so is at least the sum of them. Here the largest member is char z[10], so the union is 10 bytes - not the 18 the members would occupy in a structure.

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67.

Anyone of the following can be used to declare a node for a singly linked list and “NODEPTR nodePtr;” can be used to declare pointer to a node using any of the following

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

A.TRUE
B.FALSE

Answer: A

Correct answer: TRUE

Both work. In the first, the member is declared as struct node *nextPtr - the tag is available inside its own definition, even though the typedef name is not - and NODEPTR is defined by the same declaration. In the second, the structure is completed first and the typedef written afterwards. Either way NODEPTR nodePtr; declares a pointer to a node.

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68.

A user defined data type, which is used to assign names to integral constants is called ____________

A.Union
B.Array
C.Structure
D.Enum

Answer: D

Correct answer: Enum

An enumeration gives names to a set of integer constants: enum colour {red, green, blue}; makes red 0, green 1 and blue 2. It exists for readability - the compiler still stores an integer, but the source says what the value means.

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69.

What is the similarity between a structure, union and enumeration?

A.All of them let you define new values
B.All of them let you define new data types
C.All of them let you define new pointers
D.All of them let you define new structures

Answer: B

Correct answer: All of them let you define new data types

Structures, unions and enumerations are all ways of defining a new type - a structure aggregating members, a union overlaying them, an enumeration naming a set of integer constants. Once defined, each can be used to declare variables like any built-in type.

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70.

Which of the following is an incorrect syntax to pass by reference a member of a structure in a function?

(Assume: struct temp{int a;}s;)

A.func(&s.a);
B.func(&(s).a);
C.func(&(s.a));
D.None of the above

Answer: D

Correct answer: None of the above

All three are valid. &s.a works because . binds tighter than &; &(s.a) makes that grouping explicit; and &(s).a is the same again, since the parentheses round s change nothing. So none of the forms is incorrect.

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