[SOLVED] pointer addition

atlantis43 · 08-30-2013, 10:50 PM

Wondering if anyone can clarify something in a text I'm using to learn C (by s. Prata).
Regarding pointer addition, it states:
"You can use the + operator to add an integer to a pointer or a pointer to an integer."

Does this mean only that ptr1 + 3 is same as 3 +
ptr1, or is the author referring to something more like adding values to dereferenced pointers?

psionl0 · 08-31-2013, 12:26 AM

No, the author is actually saying that you can add ints to pointers so ptr + 3 is the same as 3 + ptr.

It should be noted that it is not just adding 3 to a number but 3 * sizeof(data type) to ptr. So if ptr points to a data type that is 8 bytes long then ptr + 3 is really (int) ptr + 24 so that ptr + 3 is synonymous with &ptr[3].

(Thanks NevemTeve)

atlantis43 · 08-31-2013, 07:49 AM

---so he is simply affirming the associativity of the addition operator regarding addition of integers to pointers?? If so, then looks like I was confused over a trivial matter!

psionl0 · 08-31-2013, 09:00 AM

Quote:

Originally Posted by atlantis43

---so he is simply affirming the associativity of the addition operator regarding addition of integers to pointers?? If so, then looks like I was confused over a trivial matter!

I think the word is "commutativity" but yes. In fact, I have read that ptr[3] is equivalent to 3[ptr] but I don't have a reference for that now.

atlantis43 · 08-31-2013, 09:56 AM

(wasn't quite sure whether it was commutativity or associativity). Thanks again.

NevemTeve · 08-31-2013, 12:14 PM

Note: quite often I write sg like &someptr[1] instead of someptr+1 just to emphasize the fact that it doesn't mean 1 byte, but sizeof(*someptr) bytes