Structures, Unions, Enums Questions and Answers

Q: What is the output of this program? #include <stdio.h> void main() { struct demo{ char * a; int n; }; struct demo p = {"hello", 2015}; struct demo q = p; printf("%d", printf("%s",q.a)); }

Correct answer: hello5The inner printf runs first, writing hello and returning 5 - the number of characters it wrote. The outer printf then prints that return value. So the output is hello5, in that order. (The assignment q = p copies the structure, including the pointer, so both point at the same string.)

Q: What will be the output of the program if it is executed like below? cmd> sample /* sample.c */ #include <stdio.h> int main(int argc, char **argv) { printf("%s\n", argv[argc-1]); return 0; }

Correct answer: sampleargv[0] is the program name as invoked, and with no further arguments argc is 1 - so argv[argc-1] is argv[0], which prints sample. The array always has argc entries plus a null pointer at argv[argc].

Q: Members of a union are accessed as ________________

Correct answer: Both a & bA union member is reached exactly as a structure member is: with the dot through a variable (u.member) and with the arrow through a pointer (p->member). The syntax is identical; only the storage layout differs.

Q: The number of distinct nodes the following struct declaration can point to is struct node { struct node *left; struct node *centre; struct node *right; };

Correct answer: All of the aboveThe structure holds three pointers, so at most it points to three nodes - but nothing requires them to be different. Two of the pointers may hold the same address, or all three may, so the number of distinct nodes reached may be 1, 2 or 3. All the counts offered are possible.

Q: #include <stdio.h> typedef enum{ Male, Female=-1 }SEX; int main() { SEX var = Male; SEX var1 = Female; printf("%d %d",var, var1); return 0; }

Correct answer: 0 -1Male takes the default first value, 0, and Female is then explicitly set to -1. Explicit values may be given in any order and may be negative - the compiler only fills in the ones you leave out, continuing from the last value given.

Q: Which of the following accesses a variable in structure b?

Correct answer: b.var;b is a structure variable, so its members are reached with the dot: b.var. The arrow is for pointers.

Q: What is the output of this program? #include <stdio.h> int main() { union demo { int x; int y; }; union demo a = 100; printf("%d %d",a.x,a.y); }

Correct answer: Compilation ErrorA union cannot be initialised from a bare value: union demo a = 100; is rejected, because the compiler must be told which member is meant. union demo a = {100}; would initialise the first member, and {.y = 100} would name it explicitly.

Q: What is the output of this program? #include <stdio.h> void main() { struct number { int no; char name[20]; }; struct number s; s.no = 50; printf("%d", s.no); }

Correct answer: 50Declaring the structure type inside a function is legal - it is simply local to that block. The member is set and printed, giving 50.

Q: Is the size of a union the size of its largest member?

Correct answer: YesA 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.

Q: It is not possible to create an array of pointer to structures.

Correct answer: FALSEAn array of pointers to structures is perfectly ordinary - struct node *arr[10]; - and it is common, because swapping pointers is far cheaper than copying whole structures. So the statement that it is impossible is false.

11.

What is the output of this program?

#include <stdio.h>         
void main()
{
struct demo{
    char * a;
    int n;
};

struct demo p = {"hello", 2015};
struct demo q = p;
printf("%d", printf("%s",q.a));
}

A.hello
B.5hello
C.hello5
D.6hello

Answer: C

Correct answer: hello5

The inner printf runs first, writing hello and returning 5 - the number of characters it wrote. The outer printf then prints that return value. So the output is hello5, in that order. (The assignment q = p copies the structure, including the pointer, so both point at the same string.)

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12.

What will be the output of the program if it is executed like below?
cmd> sample

/* sample.c */
#include <stdio.h>

int main(int argc, char **argv)
{
    printf("%s\n", argv[argc-1]);
    return 0;
}

A.sample
B.samp
C.No output

Answer: A

Correct answer: sample

argv[0] is the program name as invoked, and with no further arguments argc is 1 - so argv[argc-1] is argv[0], which prints sample. The array always has argc entries plus a null pointer at argv[argc].

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13.

Members of a union are accessed as ________________

A.union-name.member
B.union-pointer->member
C.Both a & b
D.None of the mentioned

Answer: C

Correct answer: Both a & b

A union member is reached exactly as a structure member is: with the dot through a variable (u.member) and with the arrow through a pointer (p->member). The syntax is identical; only the storage layout differs.

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14.

The number of distinct nodes the following struct declaration can point to is

struct node
{
   struct node *left;
   struct node *centre;
   struct node *right;
};

A.All of the above
B.1
C.2
D.3

Answer: A

Correct answer: All of the above

The structure holds three pointers, so at most it points to three nodes - but nothing requires them to be different. Two of the pointers may hold the same address, or all three may, so the number of distinct nodes reached may be 1, 2 or 3. All the counts offered are possible.

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15.

#include <stdio.h>

typedef enum{
    Male, Female=-1
}SEX;

int main()
{
  SEX var = Male;
  SEX var1 = Female;
  printf("%d %d",var, var1);
  return 0;
}

A.Compilation Error
B.Garbage Value 0
C.-1 -1
D.0 -1

Answer: D

Correct answer: 0 -1

Male takes the default first value, 0, and Female is then explicitly set to -1. Explicit values may be given in any order and may be negative - the compiler only fills in the ones you leave out, continuing from the last value given.

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16.

Which of the following accesses a variable in structure b?

A.b->var;
B.b.var;
C.b-var;
D.b>var;

Answer: B

Correct answer: b.var;

b is a structure variable, so its members are reached with the dot: b.var. The arrow is for pointers.

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17.

What is the output of this program?

#include <stdio.h>
int main()
{
    union demo {
        int x;
        int y;
    };
 
    union demo a = 100;
    printf("%d %d",a.x,a.y);
}

A.100 0
B.100 100
C.0 0
D.Compilation Error

Answer: D

Correct answer: Compilation Error

A union cannot be initialised from a bare value: union demo a = 100; is rejected, because the compiler must be told which member is meant. union demo a = {100}; would initialise the first member, and {.y = 100} would name it explicitly.

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18.

What is the output of this program?

#include <stdio.h>
void main()
{
  struct number
  {
    int no;
    char name[20];
  };
  struct number s;
  s.no = 50;
  printf("%d", s.no);
}

A.Nothing
B.Compile time error
C.Junk
D.50

Answer: D

Correct answer: 50

Declaring the structure type inside a function is legal - it is simply local to that block. The member is set and printed, giving 50.

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19.

Is the size of a union the size of its largest member?

A.Yes
B.No
C.May Be
D.Can't Say

Answer: A

Correct answer: Yes

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.

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20.

It is not possible to create an array of pointer to structures.

A.TRUE
B.FALSE
C.May Be
D.Can't Say

Answer: B

Correct answer: FALSE

An array of pointers to structures is perfectly ordinary - struct node *arr[10]; - and it is common, because swapping pointers is far cheaper than copying whole structures. So the statement that it is impossible is false.

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