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.

11.

The above C declaration define 's' to be

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

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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12.

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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13.

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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14.

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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15.

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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16.

#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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17.

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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18.

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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19.

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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20.

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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