Thread: pointer subtraction

hi, all

I read this programm from one ebook, but i can not under stand how
(a_ptr - a ) return 5 value.

Code:

#include <stdlib.h>

int main()
{
        int a[]={4,8,2,6,1,0,5,2};
        int *a_ptr;
        a_ptr=a;
        while((*a_ptr)!=0) ++a_ptr; // iterate until 0.
        printf("%d",a_ptr - a); //this gives the resulte 5.
        return 0;
}

andybody, help me to understand this.

Thanks and regards,

Bhrugu Mehta

Originally Posted by laserlight

Do you understand what the while loop does? Try printing *a_ptr. What is the value? To which element of the array does not value correspond? How many elements is it away from the first element of a?

I analize the while loop and unstand but my question is ,

"a_ptr" is return address and "a" also return address. than both substract than
how it retuns 5 value.Actually it returns the substraction value of address...

This only works because array elements are all kept sequentially in memory.

Originally Posted by bhrugu

I analize the while loop and unstand but my question is ,

"a_ptr" is return address and "a" also return address. than both substract than
how it retuns 5 value.Actually it returns the substraction value of address...

What in your opinion then should it return, if not 5?

Originally Posted by itCbitC

What in your opinion then should it return, if not 5?

actually it may returns the subtraction of address,

suppose, a_ptr address 2000
and a address 1000

then a_ptr-a=1000

When you increment or do pointer arithmetic on a pointer, the pointer is already *sized* to the correct size of the data it's pointing to.

So in this example, if you change the data type of the array and pointer to type char or type double, the output will still be 5, even though the size of a char is much different than the size of a double.

Re-read Grumpy's post for a more detailed description of it.

Originally Posted by grumpy

It might help understanding to realise that the difference between two pointers is related to the size of what those pointers point at.

In your example, the result of (a_ptr - a) is equivalent to (actual_memory_address_in_a_ptr - actual_memory_address_in_a)/sizeof(int) since both a and a_ptr are treated as pointers to int.

That is how pointer arithmetic works in C and C++.

If the pointers are of different types (eg one is a pointer to char, the other a pointer to int) then subtracting them will cause a compiler error, because of a mismatch.

ok, thanks, I got your point.