Structures, Unions, Enums Questions and Answers

Q: Pick the best statement for the below program: #include "stdio.h" int main() { struct {int i; char c;} myVar = {.c ='A',.i = 100}; printf("%d %c",myVar.i, myVar.c); return 0; }

Correct answer: No compile error and it’ll print 100 A.Defining a structure type inline with the variable is legal, and designated initialisers may be given in any order, since each names its target. So .c = 'A', .i = 100 is fine and prints 100 A.

Q: Pick the best statement for the below program: #include "stdio.h" int main() { union {int i1; int i2;} myVar = {.i2 =100}; printf("%d %d",myVar.i1, myVar.i2); return 0; }

Correct answer: No compile error and it’ll print “100 100”The two members of a union share the same storage, so writing 100 through i2 makes it visible through i1 as well - and since both are int, the bytes are interpreted the same way. The output is 100 100. (Reading a union through a member other than the one last written is only well defined when the types match like this.)

Q: Consider the following C declaration struct { short s[5]; union { float y; long z; }u; }t; Assume that the objects of the type short, float and long occupy 2 bytes, 4 bytes and 8 bytes, respectively. The memory requirement for variable t, ignoring alignment consideration, is

Correct answer: 18 bytesThe array contributes \(5 \times 2 = 10\) bytes, and the union takes the size of its largest member, \(\max(4, 8) = 8\). The structure sums them, giving 18 bytes. The union’s members overlap, which is why the float costs nothing extra beyond the long.

Q: Consider the following declaration : struct addr { char city[10]; char street[30]; int pin ; }; struct { char name[30]; int gender; struct addr locate; } person , *kd = &person ; Then *(kd -> name +2) can be used instead of

Correct answer: *((*kd).name + 2 )kd->name is by definition (*kd).name, so the two forms are the same expression written differently, and dereferencing +2 gives the third character. The others fail: person.name + 2 is a pointer, not the character it points to, and kd->(name+2) is not valid syntax at all - the right operand of -> must be a member name.

Q: Which of the following are themselves a collection of different data types?

Correct answer: StructuresA structure groups members of different types under one name - an int, a float and a character array together. An array requires every element to be the same type, and a string is an array of characters.

Q: Which operator connects the structure name to its member name?

Correct answer: .The dot operator selects a member of a structure variable: s.member. The arrow -> does the same through a pointer, and is exactly equivalent to (*p).member.

Q: Which of the following cannot be a structure member?

Correct answer: FunctionA structure member must be an object with storage, and a function is not - so a function cannot be a member of a C structure. What can be a member is a pointer to a function, which is how C programs implement dispatch tables and something resembling methods. Arrays and nested structures are ordinary members.

Q: What is the correct syntax to declare a function foo() which receives an array of structure in function?

Correct answer: void foo(struct *var);An array argument decays to a pointer to its first element, so a function receiving an array of structures takes a pointer to structure. (In real code the tag must be named - void foo(struct tagname *var); - since struct alone is not a type.) Passing an array never copies it, so the length must be passed separately.

21.

Pick the best statement for the below program:

#include "stdio.h"
 
int main()
{
 struct {int a[2], b;} arr[] = {[0].a = {1}, [1].a = {2}, [0].b = 1, [1].b = 2};
 
 printf("%d %d %d and ",arr[0].a[0],arr[0].a[1],arr[0].b);
 printf("%d %d %d\n",arr[1].a[0],arr[1].a[1],arr[1].b);
 
 return 0;
}

A.Compile error because struct type (containing two fields i.e. an array of int and an int) has been specified along with the definition of array arr[] of this struct type.
B.Compile error because of incorrect syntax for initialization of array arr[].
C.No compile error and two elements of arr[] would be defined and initialized. Output would be “1 0 1 and 2 0 2”.
D.No compile error and two elements of arr[] would be defined and initialized. Output would be “1 X 1 and 2 X 2” where X is some garbage random number.

Answer: C

Correct answer: No compile error and two elements of arr[] would be defined and initialized. Output would be “1 0 1 and 2 0 2”.

The initialiser uses designated initialisers with array indices: [0].a = {1}, [1].a = {2} and so on. This is valid C99, and it sizes arr at two elements, since index 1 is the highest named. Everything not explicitly initialised is zero-filled, so a[1] is 0 in both - giving 1 0 1 and 2 0 2.

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22.

Pick the best statement for the below program:

#include "stdio.h"
 
int main()
{
 struct {int i; char c;} myVar = {.c ='A',.i = 100};
 printf("%d %c",myVar.i, myVar.c);
 return 0;
}

A.Compile error because struct type (containing two fields of dissimilar type i.e. an int and a char) has been mentioned along with definition of myVar of that struct type.
B.Compile error because of incorrect syntax of initialization of myVar. Basically, member of operator (i.e. dot .) has been used without myVar.
C.Compile error for not only B but for incorrect order of fields in myVar i.e. field c has been initialized first and then field i has been initialized.
D.No compile error and it’ll print 100 A.

Answer: D

Correct answer: No compile error and it’ll print 100 A.

Defining a structure type inline with the variable is legal, and designated initialisers may be given in any order, since each names its target. So .c = 'A', .i = 100 is fine and prints 100 A.

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23.

Pick the best statement for the below program:

#include "stdio.h"
 
int main()
{
 union {int i1; int i2;} myVar = {.i2 =100};
 printf("%d %d",myVar.i1, myVar.i2);
 return 0;
}

A.Compile error due to incorrect syntax of initialization.
B.No compile error and it’ll print “0 100”
C.No compile error and it’ll print “100 100”

Answer: C

Correct answer: No compile error and it’ll print “100 100”

The two members of a union share the same storage, so writing 100 through i2 makes it visible through i1 as well - and since both are int, the bytes are interpreted the same way. The output is 100 100. (Reading a union through a member other than the one last written is only well defined when the types match like this.)

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24.

Consider the following C declaration

struct {
short s[5];
union {
float y;
long z;
}u;
}t;

Assume that the objects of the type short, float and long occupy 2 bytes, 4 bytes and 8 bytes, respectively. The memory requirement for variable t, ignoring alignment consideration, is

A.22 bytes
B.18 bytes
C.14 bytes
D.10 bytes

Answer: B

Correct answer: 18 bytes

The array contributes \(5 \times 2 = 10\) bytes, and the union takes the size of its largest member, \(\max(4, 8) = 8\). The structure sums them, giving 18 bytes. The union’s members overlap, which is why the float costs nothing extra beyond the long.

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25.

Consider the following declaration :

struct addr {
     char city[10];
     char street[30];
     int pin ;
};

struct {
char name[30];
int gender;
struct addr locate;
} person , *kd = &person ;

Then *(kd -> name +2) can be used instead of

A.person.name +2
B.kd -> (name +2 )
C.*((*kd).name + 2 )
D.either (A) or (B), but not (C)

Answer: C

Correct answer: *((*kd).name + 2 )

kd->name is by definition (*kd).name, so the two forms are the same expression written differently, and dereferencing +2 gives the third character. The others fail: person.name + 2 is a pointer, not the character it points to, and kd->(name+2) is not valid syntax at all - the right operand of -> must be a member name.

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26.

Which of the following are themselves a collection of different data types?

A.String
B.Structures
C.Char
D.None of the above

Answer: B

Correct answer: Structures

A structure groups members of different types under one name - an int, a float and a character array together. An array requires every element to be the same type, and a string is an array of characters.

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27.

Which operator connects the structure name to its member name?

A.-
B.->
C..
D.both . and ->

Answer: C

Correct answer: .

The dot operator selects a member of a structure variable: s.member. The arrow -> does the same through a pointer, and is exactly equivalent to (*p).member.

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28.

Which of the following cannot be a structure member?

A.Function
B.Array
C.Structure
D.None of the above

Answer: A

Correct answer: Function

A structure member must be an object with storage, and a function is not - so a function cannot be a member of a C structure. What can be a member is a pointer to a function, which is how C programs implement dispatch tables and something resembling methods. Arrays and nested structures are ordinary members.

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29.

What is the correct syntax to declare a function foo() which receives an array of structure in function?

A.void foo(struct *var);
B.void foo(struct *var[]);
C.void foo(struct var);
D.none of the mentioned

Answer: A

Correct answer: void foo(struct *var);

An array argument decays to a pointer to its first element, so a function receiving an array of structures takes a pointer to structure. (In real code the tag must be named - void foo(struct tagname *var); - since struct alone is not a type.) Passing an array never copies it, so the length must be passed separately.

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30.

Union differs from structure in the following way

A.All members are used at a time
B.Only one member can be used at a time
C.Union cannot have more members
D.Union initialized all members as structure

Answer: B

When a variable is associated with a structure, the compiler allocates the memory for each member. The size of structure is greater than or equal to the sum of sizes of its members. The smaller members may end with unused slack bytes. While in case of Union when a variable is associated with a union, the compiler allocates the memory by considering the size of the largest memory. So, size of union is equal to the size of largest member.

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