I would like to add that pointers can only
add/subtract the number of element from it's type

example: 
int *pint=[0,2,5,3];

pint = pint + 2;//would now point to element [5]

John Chung



--- Rikard Bosnjakovic <rikard.bosnjakovic@gmail.com>
wrote:

> On 18/04/2008, James Salisbury
> <piclist@jsalisbury.clara.co.uk> wrote:
> 
> > Hi,
> >
> >  Does anyone have a good page and some exercises
> on the use of pointers?
> >  I am trying to find out why the code below won't
> compile line 244 I get
> >  invalid operand and failed to generate
> expression.
> >
> >  Thanks
> 
> I'm a bit tipsy at the moment, but I will try to do
> my best in explaining.
> 
> The reason you can't do that is because pointers
> can't, shouldn't and
> musn't be treated as integers. This is heavily
> pointed out(!) in K&R.
> Say, for example, pnd_point points at something in
> page 1 and
> dec_point points to something in page 42,
> subtracting them both would
> return something completely uncomprehendible. For
> your case this isn't
> the situation, as you get the address of two
> particular characters in
> the string, but the compiler doesn't know about
> this.
> 
> The solution for your problem is a loop where you
> compare your
> strchr()-pointer to the string-pointer, subtract a
> variable until they
> match to get the amount of bytes. And the same for
> the other pointer.
> Then you will have two integers, which you can
> substract.
> 
> Something like thids:
> 
> char *p_tmp
> int substraction;
> for (i = 0, p_tmp = pnd_point; p_tmp-- != rx_array;
> i++) {}
> for (j = 0, p_tmp = dec_point; p_tmp-- != rx_array;
> j++) {}
> substraction = i - j;
> 
> Untested, but hopefully you will get the idea.
> 
> The important part is that you have to unlearn(?)
> the fact that you
> cannot add or substract different pointers, only
> increment/decrement
> the pointers themselves.
> 
> 
> -- 
> - Rikard - http://bos.hack.org/cv/
> -- 
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
> 



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