Important Questions on C Language(Last Year Questions with Answers)

Question :     What are pointers ? What are the uses of pointers in c ? How  a pointer variable is different from ordinary variable ? how pointer is useful in declaring 2D array. Explain with suitable example.

Answer :Pointer is a user defined data type which creates special types of variables which can hold the address of primitive data type like  char, int, float, double or user defined data type like function, pointer etc. or derived data type like array, structure, union,enum. A pointer is a variable that contains the address of another variable in memory.

A developer has to very carefully make use of pointers in order to avoid some problems that can be nightmare to debug.

Example :

In the above declaration

int * ptr;

int a=5;

ptr=&a;

Declaration :

Syntax :   <pointer_type>*<pointer_name>

1)    Pointer type : It specifies the type of pointer. It can be int,float,etc. thus the type specifies the type of variable whose address the pointer can store.

2)    Pointer name : It can be any name specified by the user.

EG. Char*ptr;

Int*ptr;

 Float*ptr;

In the above declaration char, int , float  signifies  the pointer type and ptr is the name of the pointer while the asterisk  (*) means it is a pointer variable.

Initialization :

Pointers are initialized in the following way :

1.    <pointer_declaration>=<address of variable>

2.    <pointer_declaration><name of variable>=<address of variable>

EG.     Char ch=’c’;

            Char*ptr=&ch;

#include<stdio.h>

int main()

{

int x=25;

int *ptr=&x; //statement one

int **temp=&ptr; //statement two

printf(“%d%d%d”.x.*ptr,**temp);

 return 0;

}

Output: 25 25 25

Oridinary variable different from pointer variable :

Pointers store the addresses of other variables as well as other pointers(pointer to a pointer). On the other hand, ordinary variablessimply store data values. A pointer is differenced using the * operatorto refer to its data value. For example:

 int a; // this is an ordinaryvariable

 int *p = &a; // this is a pointer to the integer variable.

a and the & symbol inthe above example is the address-of an operator. It returns the address of a variable a.

Pointer useful in declaring 2D array

A 2D array is viewed as an array of 1D arrays. That is, each row in a 2D array is a 1D array. Therefore given a 2D array A,

 int A[m][n]

 we can think of A[0] as the address of row 0, A[1] as address of row 1 etc..

Hence to find, say A[0][2] we do the following :

A[0][2] = *(A[0] + 2)

In general, A[i][j] = *(A[i] + j)

The syntax of writing 2-D array as 1-D array of pointers-

Data_type*arrayname(pointer name)[size];

Here data type refers to data type of original name whose name is array name and size is +ve integer value thet indicates the max. elements associated with that array.

#include<stdio.h>
#include<conio.h>
void main()
{
int i,j;
int (*a)[4];
clrscr();
printf("ennter elements of array:");
for(i=0;i<3;i++)
{
 for(j=0;j<3;j++)
 {
   scanf("%d",(*(a+i)+j));
 }
}
printf("output is:\n");

for(i=0;i<3;i++)
{
 for(j=0;j<3;j++)
 {
   printf("%d\t",(*(*(a+i)+j)));
 }
 printf("\n");
}
getch();
}

QUESTION 2 :DIFFERENCE B/W STRUCTURE AND UNION IN C? HOW ARE THEY REPRESENTED IN C?

Structure	Union
1.The keyword  struct is used to define a structure	1. The keyword union is used to define a union.
2.syntax struct struct_name {     structure element 1;     structure element 2; ---------- ----------     structure element n; }struct_var_nm;	2.syntax union union_name {     union element 1;     union element 2; ---------- ----------     union element n; }union_var_nm;
2. 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.	2. 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.
3. Each member within a structure is assigned unique storage area of location.	3. Memory allocated is shared by individual members of union.
4. The address of each member will be in ascending order This indicates that memory for each member will start at different offset values.	4. The address is same for all the members of a union. This indicates that every member begins at the same offset value.
5 Altering the value of a member will not affect other members of the structure.	5. Altering the value of any of the member will alter other member values.
6. Individual member can be accessed at a time	6. Only one member can be accessed at a time.
7. Several members of a structure can initialize at once.	7. Only the first member of a union can be initialized.
9.Example: struct item_mst {     int rno;     char nm[50]; }it;	9.Example: union item_mst {     int rno;     char nm[50]; }it;

Representation of union in c:

union [union tag]

{

   member definition1;

   member definition2;

   member definition3;

} [one or more union variables]; 

Example :

#include <stdio.h>

#include <string.h>

 union Data

{

   int i;

   float f;

   char  str[20];

};

 int main( )

{

  union Data data;       

 printf( "Memory size occupied by data : %d\n", sizeof(data));

return 0;

}

Representation of structure in c :

struct Point

 {

 int x;

   int y;

};

Example :

#include<stdio.h>

#include<conio.h>

Struct person

{

Char name[20];

Int day;

%

Int year;

};

Main()

{

Struct person p1, p2;

Clrscr();

Printf(“\nenter values\n\n”);

Scanf(“%s%d%d%d”,p1.name,&p1.day,&p2.manth,&p1.year);

Scanf(“%s%d%d%d”,p2.day,&p2.day,&p2.month,&p2.year);

Printf(“\nvalues you entered \n”);

Printf(“\n%s : %d %d\n”,p1.name,p1.month,p1.year);

Printf(“\n%s : %d %d”,p2.name.p2.day,p2.month,p2.year);

Getch();

}