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.


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