2d Arrays and pointers

Quote:

---

Originally Posted by Adak

Seems to be at a variance with K&R "The C Programming Language", 2nd edition, isn't it? It states that array notation is immediately changed into pointer notation.

And how would the cp pointer "move three past it and fetch the character there", except by looking at the addresses involved and adding the appropriate type onto that, to move the cp?

The RAM has addresses, only, and that is the only thing that a compiler can use to do anything to move anywhere.

The array[index] is allowed only to help out us poor humans in trying to solve coding problems. RAM knows nothing about such a construct.

Nice explanation, but "Buzzzzt!".

---

Keep looking at it.

Are you expecting it to change it's wording, Dave?

If you have some reliable source to the contrary, I'd like to hear it and know who that source was.

Anyone got something besides hand waving that authoritatively disagrees?

Quote:

---

Originally Posted by Adak

Are you expecting it to change it's wording, Dave?

If you have some reliable source to the contrary, I'd like to hear it and know who that source was.

Anyone got something besides hand waving that authoritatively disagrees?

---

Question 6.2 of the C FAQ, maintained by comp.lang.c (http://c-faq.com/aryptr/aryptr2.html) is worded virtually the same way as maven's post. There is also the mov statement in assembly which moves a constant or an offset into a register; the compiler may very well generate that code, so you are being a bit too pedantic in this case.

Quote:

---

Originally Posted by maven

* An array is very simillar to constant pointer.

---

This belief is not worth subscribing to either. Pointers and arrays are different enough to treat them as seperate data structures. Arrays simply degrade into pointers to make it possible to use arrays throughout your code, but that only happens in very common, specific situations. Read http://c-faq.com/aryptr/aryptrequiv.html

Quote:

---

Originally Posted by citizen

Question 6.2 of the C FAQ, maintained by comp.lang.c (http://c-faq.com/aryptr/aryptr2.html) is worded virtually the same way as maven's post. There is also the mov statement in assembly which moves a constant or an offset into a register; the compiler may very well generate that code, so you are being a bit too pedantic in this case.



This belief is not worth subscribing to either. Pointers and arrays are different enough to treat them as seperate data structures. Arrays simply degrade into pointers to make it possible to use arrays throughout your code, but that only happens in very common, specific situations. Read http://c-faq.com/aryptr/aryptrequiv.html

---

Agree with your remark about the mov statement - but that's not the subject, at all.

I'll let K&R provide the quote, from "The C Programming Language", page 99:

"In evaluating a[i], C converts it to *(a + i) immediately; the two forms are equivalent."

and "As the other side of this coin, if pa is a pointer, expressions may use it with a subscript; pa[i] is identical to *(pa+i). In short, an array-and-index expression is equivalent to one written as a pointer and offset."

In short, there ARE no arrays in C, except for that brief instant until C changes them into pointers with a non-variable construction. Arrays are just an artificial construct to help us poor humans who can't keep hundreds or thousands of memory addresses straight, while we're coding, with our favorite C compiler.

I have no idea who wrote (or maintains), the C-FAQ you mentioned, but I'll stick with K&R, thanks. :D

Quote:

---

Originally Posted by Adak

Agree with your remark about the mov statement - but that's not the subject, at all.

I'll let K&R provide the quote, from "The C Programming Language", page 99:

"In evaluating a[i], C converts it to *(a + i) immediately; the two forms are equivalent."

and "As the other side of this coin, if pa is a pointer, expressions may use it with a subscript; pa[i] is identical to *(pa+i). In short, an array-and-index expression is equivalent to one written as a pointer and offset."

---

No one disagrees with the quoted, but this doesn't support what you go on to say here:

Quote:

---

In short, there ARE no arrays in C, except for that brief instant until C changes them into pointers with a non-variable construction. Arrays are just an artificial construct to help us poor humans who can't keep hundreds or thousands of memory addresses straight, while we're coding, with our favorite C compiler.

---

In long, yes, arrays do exist. Arrays are an lvalue, they have a location in memory. But that very location is irrelevant until you start to use pointers, and an array's location is also very different. The key difference between arrays and pointers is that, even though an array structure may be implemented with pointer registers, arrays are not reassigned, resized, or NULLed like pointers may; that is to say, pointers are modifiable lvalues according to the standard.

However, no matter how many times you point to an array, it will not modify the array's type. The array's original location is where it always was. Therefore, there is a distiction to be made and it's important enough: basically because it allows compilers to set aside registers for the array structure at compile time (because their size and location is constant) as opposed to run time. An int *, for example, may contain the address of array[10]'s zeroeth element, but the array structure is actually ten registers for use by integers, and only indexed or otherwise accessed by a pointer containing one of those addresses.

Quote:

---

I have no idea who wrote (or maintains), the C-FAQ you mentioned, but I'll stick with K&R, thanks. :D

---

I can provide chapter and verse:

Quote:

---

Originally Posted by The Standard 3.2.2.1

A modifiable lvalue is an lvalue that does not have array type, does not have an incomplete type, does not have a const-qualified type, and if it is a structure or union, does not have any member (including, recursively, any member of all contained structures or unions) with a const-qualified type.

---

Thread over?

Quote:

---

Originally Posted by Adak

Are you expecting it to change it's wording, Dave?

---

No...

Quote:

---

Originally Posted by Adak

Seems to be at a variance with K&R "The C Programming Language", 2nd edition, isn't it?

---

Not at all, which is why I urged you to keep looking at it.

Quote:

---

Originally Posted by Adak

"In evaluating a[i], C converts it to *(a + i) immediately; the two forms are equivalent."

---

This comes too close to "an array is a pointer" for me.

Quote:

---

Originally Posted by Adak

In short, there ARE no arrays in C

---

And this seems to be your shortcut conclusion as well.

There are important differences, and I feel it is worth understanding them. That FAQ highlights one aspect.