dev-cpp-users

[Dev-C++] Question about pointers (beginner)

[Christian S. <sm...@xs...>]

From my book:

Given x = 123 and ptr_int = &x after that *ptr_int = 456 has been executed.
What is the value of x?

I thought it was 123 as the pointer *ptr_int is a null pointer and can't
have any value.

This is how I see it:

main ( )
{
int x, *ptr_int;

*ptr_int = 456; /*This pointer is a null pointer at this moment right? */

x =123;
ptr_int = &x; /* As x is just set to 123 and the statement *ptr_int = 456;
was executed before I would think *ptr_int would be 123 */

}

----------

But the books says the value of *ptr_int at the end is 456. How come??

Thanks for your time,

Christian

RE: [Dev-C++] Question about pointers (beginner)

[Christian S. <sm...@xs...>]

I just executed this code:

#include <stdio.h>

main ( )
{
  int x, *ptr_int;

  *ptr_int = 456;

  printf ("The value of ptr_int is %p\n",ptr_int);
  printf ("The value of *ptr_int is %d\n",*ptr_int);

  x = 123;

  ptr_int = &x;

  printf ("The value of ptr_int is %p\n",ptr_int);
  printf ("The value of *ptr_int is %d\n",*ptr_int);
  printf ("The value of x is %d\n",x);
  printf ("The value of &x is %p\n",&x);

}

And it gives me as expected 123 for x. However I would expect an error or
something like that for the line: *ptr_int = 456;
as *ptr_int is not linked to any memory address at that time. Anyway maybe
it was a error in the book when they stated that the end result of x should
be 456 instead of 123 as I expected.

Re: [Dev-C++] Question about pointers (beginner)

[<or...@vp...>]

Christian Schmidt wrote:
> Given x = 123 and ptr_int = &x after that *ptr_int = 456 has been executed.
> What is the value of x?

If I implemented this, this is how I would do it:

#include <stdio.h>

main()
{
   int x, *ptr_int;

   x = 123;         /* Given x = 123 */
   ptr_int = &x;    /* and ptr_int =&x */
   *ptr_int = 456;  /* after that *ptr_int = 456 has been executed */
   printf("%d",x);  /* What is the value of x? */
   /* The answer is: 123 */
}

> This is how I see it:
> 
> main ( )
> {
> int x, *ptr_int;
> 
> *ptr_int = 456; /*This pointer is a null pointer at this moment right? */
> 
> x =123;
> ptr_int = &x; /* As x is just set to 123 and the statement *ptr_int = 456;
> was executed before I would think *ptr_int would be 123 */
> 
> }

This is a *wrong* implementation of the example. The book does not state 
this. It states what I wrote above.

And the reason why it does not segfault is only luck. *ptr_int is not 
NULL at the beginning! Variables are NOT initialised, it is up to you to 
do that. *ptr_int just points to somewhere, and you only had luck, that 
using it didn't cause any trouble. Try setting ptr_int to NULL before 
you dereference it (*ptr_int). It will surely segfault then.
You should always keep it in mind, that variables are NEVER initialised 
to a default value! You must have to do that!

> But the books says the value of *ptr_int at the end is 456. How come??

The book is correct.

--
Greetings,
   Balázs

RE: [Dev-C++] Question about pointers (beginner)

[Smurf, P. <smu...@xs...>]

You are correct I misread it.

Thanks

> -----Original Message-----
> From: dev...@li...
> [mailto:dev...@li...]On Behalf Of OROSZI
> Bal=E1zs
> Sent: dinsdag 27 mei 2003 21:35
> To: dev...@li...
> Subject: Re: [Dev-C++] Question about pointers (beginner)
>
>
> Christian Schmidt wrote:
> > Given x =3D 123 and ptr_int =3D &x after that *ptr_int =3D 456 has
> been executed.
> > What is the value of x?
>
> If I implemented this, this is how I would do it:
>
> #include <stdio.h>
>
> main()
> {
>    int x, *ptr_int;
>
>    x =3D 123;         /* Given x =3D 123 */
>    ptr_int =3D &x;    /* and ptr_int =3D&x */
>    *ptr_int =3D 456;  /* after that *ptr_int =3D 456 has been executed =
*/
>    printf("%d",x);  /* What is the value of x? */
>    /* The answer is: 123 */
> }
>
> > This is how I see it:
> >
> > main ( )
> > {
> > int x, *ptr_int;
> >
> > *ptr_int =3D 456; /*This pointer is a null pointer at this moment
> right? */
> >
> > x =3D123;
> > ptr_int =3D &x; /* As x is just set to 123 and the statement
> *ptr_int =3D 456;
> > was executed before I would think *ptr_int would be 123 */
> >
> > }
>
> This is a *wrong* implementation of the example. The book does not stat=
e
> this. It states what I wrote above.
>
> And the reason why it does not segfault is only luck. *ptr_int is not
> NULL at the beginning! Variables are NOT initialised, it is up to you t=
o
> do that. *ptr_int just points to somewhere, and you only had luck, that
> using it didn't cause any trouble. Try setting ptr_int to NULL before
> you dereference it (*ptr_int). It will surely segfault then.
> You should always keep it in mind, that variables are NEVER initialised
> to a default value! You must have to do that!
>
> > But the books says the value of *ptr_int at the end is 456. How come?=
?
>
> The book is correct.
>
> --
> Greetings,
>    Bal=E1zs
>
>
>
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RE: [Dev-C++] Question about pointers (beginner)

[Ioannis V. <iv...@te...>]

> -----Original Message-----
> From: dev...@li... 
> [mailto:dev...@li...] On Behalf 
> Of Christian Schmidt
> Sent: Tuesday, May 27, 2003 9:18 PM
> To: dev...@li...
> Subject: [Dev-C++] Question about pointers (beginner)
> 
> 
> >From my book:
> 
> Given x = 123 and ptr_int = &x 


Now ptr_int has the address of x stored.



> after that *ptr_int = 456 has 


*ptr_int accesses the value of x, so *ptr_int=456 is equivalent to x=456.





Ioannis Vranos
 
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