pls help i don't get strings

[solidgriever]

it's about index from strchr()

Code

GeSHi (c):
#include <stdio.h>
#include <string.h>
 
int main ()
{
 char string1[] = "This is a test string";
 char * character_pointer;
 printf ("Looking for the 's' character in \"%s\"...\n",string1);
/* initialize character_pointer to the location of the first 's' in string1 */
 character_pointer=strchr(string1,'s');
 while (character_pointer!=NULL)
 {
/* convert the pointer location to the position within string1 so it can be displayed */
         printf ("found at %d\n",character_pointer - string1 + 1);
/* increment the pointer and search for the next 's' */
         character_pointer = strchr(character_pointer + 1,'s');
}
return 0;
}
Created by GeSHI 1.0.7.18

i don't uderstand the "character_pointer - string1" part
if character_pointer is "s is a test string" and string1 "This is a test string" why (character_pointer - string1) is 4?
Edit1: i mean i know that strchr returns a pointer to the first 's' and the string continues till NULL, so (string1 - character_pointer) is -4 or ?  pointer arithmetic that i don't get?

Edit2: When you subtract two pointers, as long as they point into the same array, the result is the number of elements separating them. Is this it? so you can do this

Code:

if((string1 - character_pointer)<0) {
     (string1 - character_pointer) *= -1;}

instead of that counterintuitiv character_pointer - string1?
Edit3: i think i get it
  
  |00|01|02|03|04|05|06|07|08|09|10|11|12|13|14|15|16|17|18|19|20|          
  |T | h | i | s |   | i  | s|    |a  |   |t  |e  |s  |t  |   |s |t  |r   |i  |n  |g |\0
   ^            ^
|string1|  |character_pointer|
so ((character_pointer - string1) + 1) is ((3 - 0) + 1) Is that right?

[FrozenKnight]

I think you have the hang of it.
the expression you have in your last edit would be similar to using sizeof on the string segment.
The code you gave in your edit2 should produce a run-time error because you are trying to multiply the address 4 by -1, ( I believe that address 4 is reserved on most systems so this isn't a good thing)

[solidgriever]

thx for the reply
i hit a wall on that code and was stuck... i just started learning C/C++ from a book
thx again FrozenKnight i can now move on

The code you gave in your edit2 should produce a run-time error because you are trying to multiply the address 4 by -1, ( I believe that address 4 is reserved on most systems so this isn't a good thing)

yeah, you're right that was stupid  
but if ai rewrite it like that:

Code

GeSHi (c):
#include <stdio.h>
#include <string.h>
int main (){
 char string1[] = "This is a test string";
 char * character_pointer;
 int index;
 printf ("Looking for the 's' character in \"%s\"...\n",string1);
 character_pointer=strchr(string1,'s');
 while ((character_pointer!=NULL) && (index = string1 - character_pointer))  {
      if (index < 0){
          index *= -1;}
          printf ("found at %d\n", index + 1);
          character_pointer = strchr((character_pointer) + 1,'s');
}
}
Created by GeSHI 1.0.7.18

if if multiply an int(i mean index) by -1 it's ok? altough string1 is string1[0] and character_pointer is string1[3] and inside if they are like int's

[FrozenKnight]

Ok, a few things.
the most obvious thing is

Code

GeSHi (c):
while ((character_pointer!=NULL) && (index = string1 - character_pointer))  {
Created by GeSHI 1.0.7.18

the "index = string1 - character_pointer" part isn't a Boolean expression and shouldn't be used with "&&"
I'm hoping you meant "index == string1 - character_pointer" but if you use that then you haven't initialized your index.

Second from reading your code i gather that you haven't realized that a string is the same thing as a character pointer
for example

Code

GeSHi (c):
char string[] = "This is a test string";
Created by GeSHI 1.0.7.18

and

Code

GeSHi (c):
char *string = "This is a test string";
Created by GeSHI 1.0.7.18

do exactly the same thing and on most compilers you can interchange the notation
example

Code

GeSHi (c):
return string[5];
Created by GeSHI 1.0.7.18

and

Code

GeSHi (c):
return *(string + 5);
Created by GeSHI 1.0.7.18

should be the same.

So this means that if your "string1 - character_pointer" returns a negative result then you have to handle an error condition not multiply it by -1.

Also as a general rule of thumb avoid using multiplication or division directly on pointers, the results probably won't be what you expect.
about the only time you should be using multiplication in/on/with a pointer is for cases like the following

Code

GeSHi (c):
int array[] = {0,1,2,3,4,5,6,7,8,9};
return *(array + (sizeof(int) * 5));
Created by GeSHI 1.0.7.18

Also most compilers handle bool like int's. The rules for a bool are simple if the bool (int) results to 0 then it's false if it's anything else it's true
so

Code

GeSHi (c):
if (-4356)
Created by GeSHI 1.0.7.18

is always true
if your above code works this is why, because "string1 - character_pointer" evaluates to a non-zero value then passes it to index. Because of a property of assignment where you can use "a = b = c;" this will pass the value that "string1 - character_pointer" returned to your "&&". I think you should test your code and see what happens when you have an 's' as the first character of string1, I'll bet it will fail by exiting the loop immediately without doing anything.

Also pointer sizes depend on the architecture your working on for example
if your working with an 8 bit compiler then a pointer will be 1 byte wide probably be the same size as a char (you probably wont ever work with one of these)
if it's a 16 bit compiler your pointers will be 2 bytes wide
if it's a 32 bit compiler it your pointers will be 4 bytes wide the same size as an int
This means in general you want to avoid on relining on a pointer being the same as an int, granted you can work with them that way, but I don't think anyone recommends it.
As for your index value i would consider it like a local pointer for use inside your string, so the rule for using multiplication on it should apply.

[solidgriever]

yeah i see what you mean
the original code was alright, but i didn't understood pointers and strings so in my attempt to make it easier to read.
i repaired my code below. now i get that the second condition wasn't a bool. i reviewed a few other codes i wrote and found the same mistake.
if i had put s as the first letter in my string then my second condition would fail because index would be 0.
about handling an error condition... i still havn't got to that part but i get why i don't need to multiply by -1 at least in this case. the character_pointer is incremented in every cycle so it will be greater then string1.

Code

GeSHi (c):
while (character_pointer)
{
           index =  character_pointer - string1;
              printf ("found at %d\n", index + 1);
          character_pointer = strchr((character_pointer) + 1,'s');
}
Created by GeSHI 1.0.7.18

[FrozenKnight]

That code looks much better.

What i meant by handling an error condition, is simply that you should never have

Code

GeSHi (c):
index =  character_pointer - string1;
Created by GeSHI 1.0.7.18

result to a negative number, that would merely indicate that something went wrong. and in that case you should report it accordingly
So handling the error would look something like

Code

GeSHi (c):
while (character_pointer)
{
           index =  character_pointer - string1;
           if (index < 0)
           {
              printf ("ERROR: Charicter search failed\n"); /* print error */
              break; /* break from loop because, if we hit this then the loop may continue infinitely or cause an error later on */
           }
           printf ("found at %d\n", index + 1);
           character_pointer = strchr((character_pointer) + 1,'s');
}
Created by GeSHI 1.0.7.18

Handling errors using methods similar to this is good practice in general. Because, you never know when your going to run into an error, and reporting it helps you track them down.