Pointers initialization syntax and example in c programming Language.


The n is an int variable, &n returns the address of the variable n .
Following are the syntax for pointer variable initialization:
1. Static initialization.
2. Dynamic initialization.

1. Static Initialization:
The process of pointer variable initialization at compile time is known as static initialization.
Once the static variable is initialized its scope remains until the program termination. The static variable holds its value even the variable goes out of scope. static variable cannot be re-declared and initialized once it is declared and initialized. The scope and lifetime of static exist till the termination of that program.
A declaration of static pointer is shown below.

Syntax:
data type variable_name=value;
static data type *ptr_variable=&variable_name;

C pragram for static variable Declaration

#include<stdio.h>
 void staticptr()
{
 int x = 100, y = 200;
 static int *ptr;  // ptr is declared as static pointer variable
 if(ptr == 0)
 {
   ptr = &x;
 }
 else
 {
   ptr = &y;
 }
 printf("value = %d\n", *ptr);
 printf("address of pointer ptr = %d\n", &ptr);
 printf("address of memory pointed by pointer ptr = %d\n", ptr);
 printf("address of x = %d\n", &x);
 printf("address of y = %d\n", &y);
}
int main(){
  staticptr();
 staticptr();
}
Output:
 value = 100
 address of pointer ptr= 4218932
 address of memory pointed by pointer ptr = 6422204
 address of x = 6422204
 address of y = 6422200
 value = 200
 address of pointer ptr= 4218932
 address of memory pointed by pointer ptr = 6422200
 address of x = 6422204
 address of y = 6422200


In the given above program a function staticptr() has been called from the main() function. In first call of the function, static variable is initialized to zero 0 by compiler , so value of 'x' and ‘y’ will be assigned and as *ptr is static variable it holds its value even after completion of function execution. So that in second iteration of staticptr() static pointer 'ptr' is assigned to y's value. We can observe the given output. Address of static pointer is constant throughout the execution of program. But where it points-to can be modified.

2. Dynamic Initialization:

The process of initializing the pointer variable at run time in c programming is known as dynamic initialization.

Syntax:
 data type *ptr-variable;
 ptr-variable=(data type *)malloc(sizeof(data type));
 For example,
 int *ptr;
 ptr = (int *)malloc(sizeof(int));  //dynamic initialization

Here, the malloc() function is used to allocate memory at run time and the ptr is a pointer variable that is initialized at run time.

The C program to create dynamic array

#include<stdio.h>
#include <stdlib.h>
int main()
{
  int* ptr;    // This pointer will hold the
       // base address of the block
 int n, q;
  printf("\nEnter The number of elements:");
  scanf("%d",&n);
  printf("\nThe number You entered is: %d", n);
 // Dynamically allocate memory using malloc()
  ptr = (int*)malloc(n * sizeof(int));
  if (ptr == NULL) {    // check if the memory is allocated
  printf("Memory has been not allocated.\n");
  exit(0);
}
 else {
    // Memory has been successfully allocated
    printf("Memory has been allocated by using malloc()for %d numbers .\n",n);
   // Assignes the elements to the array
   for (q = 0; q < n; q++) {
     ptr[q] = q * 2;
  }
   // show the elements of the array
   printf("The elements in the array are: ");
   for (q = 0; q < n; q++) {
     printf("%d, ", ptr[q]);
   }
   }
  return 0;
}

Output:
Enter number of elements: 10
Memory has been allocated by using malloc()for 10 numbers .
The elements of the array are: 0, 2, 4, 6, 8,10,12,14,16,18

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