A little help with lists please...

[reptonite]

Hi there

I am busy swatting, but have hit a speed-bump. It has to do with reversing arrays whilst not allowing use of the array brackets (doing it with pointers in other words).

Basically, I have written this.

void reverse (int *numbers, int lwb, int upb) { 

int *p1, *p2, temp;

p1 = numbers + lwb
p2 = numbers + upb

while (p1 < p2) {
temp = *p1;
*p1 = *p2;
*p2 = temp;
*p1++;
*p2--;
}

...in a bid to swap them. Swapping opposite ends until the pointers are at the same memory address.

However, I've been told that the following would work too. Can you verify?

while (p1 < p2) {
temp = *p1;
*p1++ = *p2;
*p2-- = temp;

It's just that to me, you're trying to do two things at once with the pointers. Does this not mean that the pointer increments and then assigns the value?

Cheers

M

[Salem]

> Does this not mean that the pointer increments and then assigns the value?
No, that would be *(++p1) = *p2;
Yes, the () are necessary on the pre-increment version of the expression.

As written, you do the assignment, THEN increment the pointer.

while (p1 < p2) {
  temp = *p1; 
  *p1 = *p2;
  *p2 = temp;
  p1++;
  p2--;
}

Is probably just as good as anything more condensed.

[reptonite]

Salem, thanks for the reply, but I am not clear. Are you saying that the second piece of code in my first post is actually correct? And are both versions of the while loop going to work anyway?

[reptonite]

Ok, we lost Salem, anyone? Can you tell me if the while loop in both cases is correct?

[whiteflags]

Well the while loop that Salem gave you seems most correct. The reason I say this is because:

*p1 = /* the value stored in the memory location of p1  */
p1++ = /* assigns p1 to the very next memory location */
*(++p1) = /* increment the pointer, assign the value */
*p1++ = /* assign the value of p1 and then increment the pointer */

There is quite a difference in the syntax of your statements, so Salem is most correct.

[reptonite]

OK, thanks Citizen. I just didn't understand his reply. So what you are basically saying is that both are correct, but my second while loop is slicker. Excellent.

You learn something new everyday. I was worried that dual increment/assign in one command could get messy, but thanks to the two of you I now have the rules governing both states.

many thanks.

[Salem]

> So what you are basically saying is that both are correct
No, the pre-increment / decrement versions will fail to swap the first/last elements.

I suggest you try both forms as a study exercise

[whiteflags]

Salem, even though you're right, you took his comment out of context. Both the while loops should work.

[quzah]

I will never understand people that post questions like this:

I've been told that the following would work too. Can you verify?

Why should I verfiy it? Write some test code and try it yourself.


Quzah.