C Programming Pointers and Arrays

Arrays are closely related to pointers in C programming but the important difference between them is that, a pointer variable can take different addresses as value whereas, in case of array it is fixed. This can be demonstrated by an example:

#include <stdio.h>
int main(){
   char c[4];
   int i;
   for(i=0;i<4;++i){
      printf("Address of c[%d]=%x\n",i,&c[i]);
   }
   return 0;
}

Address of c[0]=28ff44
Address of c[1]=28ff45
Address of c[2]=28ff46
Address of c[3]=28ff47

Notice, that there is equal difference (difference of 1 byte) between any two consecutive elements of array.

Note: You may get different address of an array.

Relation between Arrays and Pointers

Consider and array:

int arr[4];

In arrays of C programming, name of the array always points to the first element of an array. Here, address of first element of an array is &arr[0]. Also, arr represents the address of the pointer where it is pointing. Hence, &arr[0] is equivalent to arr.

Also, value inside the address &arr[0] and address arr are equal. Value in address &arr[0] isarr[0] and value in address arr is *arr. Hence, arr[0] is equivalent to *arr.

Similarly,

&a[1] is equivalent to (a+1)  AND, a[1] is equivalent to *(a+1).
&a[2] is equivalent to (a+2)  AND, a[2] is equivalent to *(a+2).
&a[3] is equivalent to (a+1)  AND, a[3] is equivalent to *(a+3).
.
.
&a[i] is equivalent to (a+i)  AND, a[i] is equivalent to *(a+i).

In C, you can declare an array and can use pointer to alter the data of an array.

//Program to find the sum of six numbers with arrays and pointers.
#include <stdio.h>
int main(){
  int i,class[6],sum=0;
  printf("Enter 6 numbers:\n");
  for(i=0;i<6;++i){
      scanf("%d",(class+i)); // (class+i) is equivalent to &class[i]
      sum += *(class+i); // *(class+i) is equivalent to class[i]
  }
  printf("Sum=%d",sum);
  return 0;
}

Output

Enter 6 numbers:
2
3
4
5
3
4
Sum=21

Example To Demonstrate Working of Pointers

#include <stdio.h>
int main(){
   int* pc;
   int c;
   c=22;
   printf("Address of c:%d\n",&c);
   printf("Value of c:%d\n\n",c);
   pc=&c;
   printf("Address of pointer pc:%d\n",pc);
   printf("Content of pointer pc:%d\n\n",*pc);
   c=11;
   printf("Address of pointer pc:%d\n",pc);
   printf("Content of pointer pc:%d\n\n",*pc);
   *pc=2;
   printf("Address of c:%d\n",&c);
   printf("Value of c:%d\n\n",c);
   return 0;
}

Output

Address of c: 2686784
Value of c: 22

Address of pointer pc: 2686784
Content of pointer pc: 22

Address of pointer pc: 2686784
Content of pointer pc: 11

Address of c: 2686784
Value of c: 2

Explanation of program and figure

1. Code int* pc; creates a pointer pc and a code int c; creates normal variable c. Pointer pc points to some address and that address has garbage value. Similarly, variable c also has garbage value at this point.
2. Code c=22; makes the value of c equal to 22, i.e.,22 is stored in the memory location of variable c.
3. Code pc=&c; makes pointer, point to address of c. Note that, &c is the address of variable c (because c is normal variable) and pc is the address of pc (because pc is the pointer variable). Since the address of pc and address of c is same, *pc will be equal to the value of c.
4. Code c=11; makes the value of c, 11. Since, pointer pc is pointing to address of c. Value inside address pc will also be 11.
5. Code *pc=2; change the contents of the memory location pointed by pointer pc to change to 2. Since address of pointer pc is same as address of c, value of c also changes to 2.

Commonly done mistakes in pointers

Suppose, the programmer want pointer pc to point to the address of c. Then,

int c, *pc;
pc=c;  /* pc is address whereas, c is not an address. */
*pc=&c; /* &c is address whereas, *pc is not an address. */

In both cases, pointer pc is not pointing to the address of c.

How to use Pointers?

There are few important operations, which we will do with the help of pointers very frequently. (a) we define a pointer variable (b) assign the address of a variable to a pointer and (c) finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. Following example makes use of these operations:

#include <stdio.h>

int main ()
{
   int  var = 20;   /* actual variable declaration */
   int  *ip;        /* pointer variable declaration */

   ip = &var;  /* store address of var in pointer variable*/

   printf("Address of var variable: %x\n", &var  );

   /* address stored in pointer variable */
   printf("Address stored in ip variable: %x\n", ip );

   /* access the value using the pointer */
   printf("Value of *ip variable: %d\n", *ip );

   return 0;
}

When the above code is compiled and executed, it produces result something as follows:

Address of var variable: bffd8b3c
Address stored in ip variable: bffd8b3c
Value of *ip variable: 20

What Are Pointers?

A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before you can use it to store any variable address. The general form of a pointer variable declaration is:

type *var-name;

Here, type is the pointer's base type; it must be a valid C data type and var-name is the name of the pointer variable. The asterisk * you used to declare a pointer is the same asterisk that you use for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Following are the valid pointer declaration:

int    *ip;    /* pointer to an integer */
double *dp;    /* pointer to a double */
float  *fp;    /* pointer to a float */
char   *ch     /* pointer to a character */

The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.

C Program to Convert Binary Number to Decimal and vice-versa

#include <stdio.h>

#include<conio.h>

#include <math.h>

int binary_decimal(int n);

int decimal_binary(int n);

void main()

{

   int n;

   char c;

   clrscr();

   printf("Instructions:\n");

   printf("1. Enter alphabet 'd' to convert binary to decimal.\n");

   printf("2. Enter alphabet 'b' to convert decimal to binary.\n");

   scanf("%c",&c);

   if (c =='d' || c == 'D')

   {

       printf("Enter a binary number: ");

       scanf("%d", &n);

       printf("%d in binary = %d in decimal", n, binary_decimal(n));

   }

   if (c =='b' || c == 'B')

   {

       printf("Enter a decimal number: ");

       scanf("%d", &n);

       printf("%d in decimal = %d in binary", n, decimal_binary(n));

   }

   getch();

}

int decimal_binary(int n)  /* Function to convert decimal to binary.*/

{

    int rem, i=1, binary=0;

    while (n!=0)

    {

        rem=n%2;

        n/=2;

        binary+=rem*i;

        i*=10;

    }

    return binary;

}

int binary_decimal(int n) /* Function to convert binary to decimal.*/

{

    int decimal=0, i=0, rem;

    while (n!=0)

    {

        rem = n%10;

        n/=10;

        decimal += rem*pow(2,i);

        ++i;

    }

    return decimal;

}

Prime Numbers Between two Intervals using Function

#include<stdio.h>

#include<conio.h>

int check_prime(int num);

void main(){

   int n1,n2,i,flag;

   printf("Enter two numbers(intervals): ");

   scanf("%d %d",&n1, &n2);

   printf("Prime numbers between %d and %d are: ", n1, n2);

   for(i=n1+1;i<n2;++i)

   {

      flag=check_prime(i);

      if(flag==0)

         printf("%d ",i);

   }

   getch();

}

int check_prime(int num) /* User-defined function to check prime number*/

{

   int j,flag=0;

   for(j=2;j<=num/2;++j){

        if(num%j==0){

            flag=1;

            break;

        }

   }

   return flag;

}