C Programming - Pointers - Discussion

#include<stdio.h>

int main()
{
char str[20] = "Hello";//here given char str is hello
char *const p=str; //char * const p="hello" i.e str
char const *p="hello" so
*p='M'; //*p=M so it is changed H to M
printf("%s\n", str); // its prints Mello
return 0;
}

NOTE:

const char *p="hello";
char const *p="hello";

char * const p="hello";
const char * const u="hello";

1. const char *p : means p is pointer pointing to a constant char i.e. you can not change the content of the location where it is pointing but u can change the pointer itself to point to some other char.

2. char const *p, and char * const p : both are same & in this case p is a constant pointer poiting to some char location. you can change the contents of that location but u can't change the pointer to point to some other location.

Hope i answered the way you wanted !!

@Red.

The string str is assigned with str[0]='H', str[1]='e' and so on.
The other way of initialising that string would be
char str[]={'h','e','l','l','o','\0'};

NOTE: The name of the array itself holds the base address of the string.
So when p=str,the base address of the string str[0] is assigned to p.

*p='M' implies that the value at p which is str[0]is changed to 'M', which was previously 'H'.

Hope you got it.

char *const p

Declares a constant pointer to a character. The location stored in the pointer cannot change, whereas the value stored at that position can change. You cannot change where this pointer points.

const char * P

Declares a pointer to a constant character. You cannot use this pointer to change the value being pointed to, whereas you can change the address to which the pointer points to.

char *const p; here p is a constant pointer so here the value of pointer can change, but the address it pointing is not able to change.

*p='M' //valid
but if we change the address it will give error for example
int main()
{
char str[]="hai" ,str1[]="india";
char *const p=str;
p=str1 //error here changing the address of pointer so it gives error.
}

Guys a pointer variable whose address never changed is called pointer constant. So here we can't change the address but we can change the value at that address
int x=10;
int *const p=&x;
int y=50;
p=&y;// wrong

And constant pointer means whose value can never be changed
const int *p;
int x=10;
p=&x;
*p=20;// can't modify the existing value..

[H][e][l][l][o][w]
10 11 12 13 14 15 (Assume these are address of memory)
here,str=10

char*const p=str (here,p will store 10 that is constant)

so,now *p indicate "H" that has replaced by "M"

note: Base address also is a pointer & constant.

As given example
char*const t="Hello";
in this statement we have see pointer is constant but String t is not that why next statement
*t='M';
it is a String pointer which point base address is nothing but 'H'
so it will replace with it and give output: "Mello"

Here,

p is pointing to str, hence p and str are same.
Now str[0]=p[0]='H'
After changing the first character str becomes str[0]=p[0]='M' (since *p, *str, str[0], p[0], all are same and representing the first character of the str)

So str="Mello" is the final answer.

We are not modifying the string of str ,
Then how it can change.

char str[20] = "Hello";
char *const p=str;
*p='M';
printf("%s\n", str);

Here we have not change the str[20] = "Hello".

I didn't get,
Please Tell me the reason.