Question

what is difference between "int * const ptr=&i" and const int *ptr;;

Answer 1

You have in fact told the answer in question.

int * const ptr = &i;

Why was initialization required here? Because the pointer ptr is a constant and any constant variable must be initialized in C. We can also declare the same as

const * int ptr = &i;

Now, for second one you did not initialize. And that is not required also as const int * ptr or int const * ptr is a pointer to a const int meaning it can point to any integer but we are not supposed to modify the pointed value using '*' operator (trying to modify causes undefined behaviour).

Answer 2

Reading backward and stop at $*$ symbol first: then continue:

First case:

This $ptr$ needs initialization because it is a constant.

---

Second case:

This $ptr$ needs no compulsory initialization.

Answer 3

Ohhhh

i got it.

first const int * ptr;

means we can not change the the value of *ptr . we can change the ptr means address can change.

Example

#include <stdio.h>

int main ()
{
 int i=8;
 const int *ptr=&i;
 *ptr=9;
 printf("%d",*ptr);
}

second int * const ptr;

means we can not change the address mean we can not change ptr value;

Example

#include <stdio.h>

int main ()
{
 int i=8;
 int * const ptr=&i;
 *ptr=10;
    int j=150;
    ptr=&j;
 printf("%d",*ptr);
}