pointer airthmatic

Question

we can subtract 2 pointer address then why we can't perform operations like addition, multiplication or division on pointer address???

any specific reason?

Answer 1

Why Subtraction is allowed? Two addresses can be subtracted because the memory between the two addresses will be valid memory. For example - Pointer ptr_1 is pointing to 0x1cb0010 memory location and ptr_2 is pointing to 0x1cb0030 memory location. If we subract ptr_1 from ptr_2, then the Memory region will lie in between these two location which is obviously a valid memory location.

 

Why addition, Multiplication or division is not allowed??
If we perform addition, multiplication or division on ptr_1 and ptr_2, then the resultant address may or may not be a valid address. That can be out of range or invalid address. This is the reason compiler doesn’t allow these operations on valid addresses.

Answer 2

Pointers are variables used to hold the address of other variables.

What is the result of subtraction of two pointers?

When two pointers of same type are subtracted then it returns number of elements between them. Similarly we can add or subtract integers to make a pointer to point to next or previous elements of the currently pointed elements. Mostly these operations are performed in context of arrays.

For  example, consider this example

int a [ ] = { 1, 2 ,3 , 4, 5 };
int *p=&a[0];
then a[i]= *(p+i)

 

Similarly if

int *q= &a[4];

then q-p will return number of elements between a[0] and a[4] which is 4.

There are no meaning to expressions such as p+q, p*q, p/q

 

 

Comments

Can we assign a Integer value to Pointers directly?

As far as I know the though the values of pointers are integers, you cannot initialize them directly with an integer number as we don't have access to the memory directly. The only direct initialization possible is with null that is zero. If you try to assign some value directly the program will compile, but if try to store or read using that pointer  it will give error if the memory pointed by that integer value is not in the logical memory space of the program.

 

What is exactly %d and %p ?

Both of them are format specifiers. Basically everything that you store in memory is in binary. When you wanted to print it on screen you specify the location of the variable to printf, but looking at just zeroes and ones printf cannot figure out how to interpret these binary values for that purpose we provide format specifiers. 

In your program you are storing the difference between &a and &b in variables c and p, where c is an integer and p is a pointer. When the difference is calculated it will be some integer whose respective binary value will be stored at both locations c and p. Hence basically both are equal. When %d is used the value is properly interpreted and both prints same answer. When you use %p the same binary value will now be interpreted as a pointer value and printed in hexadecimal. In fact if you use %p with c which is an integer not a pointer it will also give same hexadecimal number. 

When you are printing p in printf actually the location pointed by p is not accessed, so this is not giving any error but p is still an invalid pointer. 

Hope it helps.

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Thanks @ravee_rocks. 

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Welcome @ayushsomani 👍