C Structure & Union

Question 1

#include‹stdio.h›
int main()
{
    struct site
    {
        char name[] = "GeeksQuiz";
        int no_of_pages = 200;
    };
    struct site *ptr;
    printf("%d ", ptr->no_of_pages);
    printf("%s", ptr->name);
    getchar();
    return 0;
}

• A. 200 GeeksQuiz
• B. 200
• C. Runtime Error
• D. Compiler Error

Answer: (D)

C Structure & Union
Discuss it

When we declare a structure or union, we actually declare a new data type suitable for our purpose. So we cannot initialize values as it is not a variable declaration but a data type declaration.

Question 2

Assume that size of an integer is 32 bit. What is the output of following program?

#include<stdio.h>
struct st
{
    int x;
    static int y;
};

int main()
{
    printf("%d", sizeof(struct st));
    return 0;
}

• A. 4
• B. 8
• C. Compiler Error
• D. Runtime Error

Answer: (C)

C Structure & Union
Discuss it

In C, struct and union types cannot have static members. In C++, struct types are allowed to have static members, but union cannot have static members in C++ also.

Question 3

C

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

The above C declaration define 's' to be (GATE CS 2000)

• 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)

C Structure & Union
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It defines an array, each element of which is a pointer to a structure of type node.

Question 4

Consider the following C declaration 

C

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

Assume that 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 considerations, is (GATE CS 2000)

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

Answer: (C)

C Structure & Union 50 C Language MCQs with Answers
Discuss it

Short array s[5] will take 10 bytes as size of short is 2 bytes. 

When we declare a union, memory allocated for the union is equal to memory needed for the largest member of it, and all members share this same memory space. Since u is a union, memory allocated to u will be max of float y(4 bytes) and long z(8 bytes). So, total size will be 18 bytes (10 + 8).

Question 5

#include<stdio.h> 
struct st 
{ 
    int x; 
    struct st next; 
}; 
  
int main() 
{ 
    struct st temp; 
    temp.x = 10; 
    temp.next = temp; 
    printf("%d", temp.next.x); 
    return 0; 
}

• A. Compiler Error
• B. 10
• C. Runtime Error
• D. Garbage Value

Answer: (A)

C Structure & Union
Discuss it

A structure cannot contain a member of its own type because if this is allowed then it becomes impossible for compiler to know size of such struct. Although a pointer of same type can be a member because pointers of all types are of same size and compiler can calculate size of struct

Question 6

Which of the following operators can be applied on structure variables?

• A. Equality comparison ( == )
  
• B. Assignment ( = )
  
• C. Both of the above
  
• D. None of the above
  

Answer: (B)

C Structure & Union 50 C Language MCQs with Answers
Discuss it

A structure variable can be assigned to other using =, but cannot be compared with other using ==

Question 7

union test
{
    int x;
    char arr[8];
    int y;
};

int main()
{
    printf("%d", sizeof(union test));
    return 0;
}

Predict the output of above program. 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.

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

Answer: (C)

C Structure & Union
Discuss it

When we declare a union, memory allocated for a union variable of the type is equal to memory needed for the largest member of it, and all members share this same memory space. In above example, "char arr[8]" is the largest member. Therefore size of union test is 8 bytes.

Question 8

union test
{
    int x;
    char arr[4];
    int y;
};

int main()
{
    union test t;
    t.x = 0;
    t.arr[1] = 'G';
    printf("%s", t.arr);
    return 0;
}

Predict the output of above program. 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.

• A. Nothing is printed
• B. G
• C. Garbage character followed by 'G'
• D. Garbage character followed by 'G', followed by more garbage characters
• E. Compiler Error

Answer: (A)

C Structure & Union
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Since x and arr[4] share the same memory, when we set x = 0, all characters of arr are set as 0. O is ASCII value of '\0'. When we do "t.arr[1] = 'G'", arr[] becomes "\0G\0\0". When we print a string using "%s", the printf function starts from the first character and keeps printing till it finds a \0. Since the first character itself is \0, nothing is printed.

Question 9

# include <iostream>
# include <string.h>
using namespace std;

struct Test
{
  char str[20];
};

int main()
{
  struct Test st1, st2;
  strcpy(st1.str, "GeeksQuiz");
  st2 = st1;
  st1.str[0] = 'S';
  cout << st2.str;
  return 0;
}

• A. Segmentation Fault
• B. SeeksQuiz
• C. GeeksQuiz
• D. Compiler Error

Answer: (C)

C Structure & Union
Discuss it

Array members are deeply copied when a struct variable is assigned to another one. See Are array members deeply copied? for more details.

Question 10

Predict the output of following C program

#include<stdio.h>
struct Point
{
  int x, y, z;
};

int main()
{
  struct Point p1 = {.y = 0, .z = 1, .x = 2};
  printf("%d %d %d", p1.x, p1.y, p1.z);
  return 0;
}

• A. Compiler Error
• B. 2 0 1
• C. 0 1 2
• D. 2 1 0

Answer: (B)

C Structure & Union
Discuss it

Refer designated Initialization discussed here.