why this is wrong as per Pointer in c

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#include<stdio.h>
#include <string.h>
int main()
{
int *p1, *p2;
int ar[2];

p1=&ar[0];
p2=&ar[1];

printf( "%d\n",p2 - p1); // this is right

printf( "%d\n",p2 + p1); // why this is wrong

    return 0;
}

Posted 7-Jun-13 3:41am

sunilrohillacma

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3 solutions

Solution 1

The first one is correct is a coincidence probably. You must use as *p2 - *p1 and *p2 + *p1

If you write p1 it gets its address

Posted 7-Jun-13 3:47am

Osman24

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Alois Kraus · Accepted Answer · 2013-06-07T04:06:00

The first one is calculating an offset by taking the difference. This value can be used to increment an existing pointer by n values. It is basically the count of items between the two pointers.

E.g.

C++

p4 =  p3+(p2-p1);

is valid as it does increment p3 by the number of items between p2 and p1.

If you add on the other hand two pointers you are adding two addresses which will point to a memory location far far away in your allocated array and is definitely an error.

CPallini · Accepted Answer · 2013-06-07T04:50:00

Solution 3

The C programming language specification forbids the addition of pointers, see, for instance: "ISO/IEC 9899:TC3" pag. 82[^].

Posted 7-Jun-13 4:50am

CPallini