calculating length of an substring

[brasilino]

Hi Folks:

I'm trying to calculating a substring length directly from pointer
address, like this:

char *e = NULL, *s = NULL;
int len = 0;

s = strchr (url,'.');
e = strrchr (url,'?');
len = (int) s - e;

Since 's' and 'e' are pointers, I think that's can be make a math
over it, like 's++'.

Using gcc 4.1.2 (on glibc 2.6 and Linux 2.6.23) it points the
following error:

"error: invalid operands to binary -"

So, how can I calculate this length in a fancy way? I don't want to
loop through it.

thanks a lot in advance
Lucas Brasilino

[William Pursell]

On Feb 29, 6:43 pm, "[e-mail]" <[e-mail]:

I'm trying to calculating a substring length directly from pointer
address, like this:

char *e = NULL, *s = NULL;
int len = 0;

s = strchr (url,'.');
e = strrchr (url,'?');
len = (int) s - e;

You need to check that both s and e are notnull, and you probably
mean:

len = (int) (e - s);

Your version is equivalent to:
len = ((int)s) - e;

and the compiler is complaining about subtracting a pointer
from an int.

[Micah Cowan]

"br*******@yahoo.com" <br*******@yahoo.comwrites:

Hi Folks:

I'm trying to calculating a substring length directly from pointer
address, like this:

char *e = NULL, *s = NULL;
int len = 0;

s = strchr (url,'.');
e = strrchr (url,'?');
len = (int) s - e;

You wanted
len = (int)(s - e);
.. The way you have it, only s will be cast to an int; and you can't
subtract a pointer from an int!

The cast is completely superfluous, though; I'd recommend removing
it. If you like, you can first check that it will actually _fit_ into
an int (the type of the difference of pointers is ptrdiff_t, defined
in <stddef.h>. Or better yet, use a ptrdiff_t to store the result (if
that suits your needs otherwise).

A size_t actually makes the most sense for representing a string
length, though, at least to me.

--
Micah J. Cowan
Programmer, musician, typesetting enthusiast, gamer...
http://micah.cowan.name/

[burton.samogradNO]

"br*******@yahoo.com" <br*******@yahoo.comwrites:

I'm trying to calculating a substring length directly from pointer
address, like this:

char *e = NULL, *s = NULL;
int len = 0;

s = strchr (url,'.');
e = strrchr (url,'?');
len = (int) s - e;

try using braces:

len = (int) (s - e);

casting binds tighter than subtraction.

--
burton

[Martin Ambuhl]

br*******@yahoo.com wrote:

Hi Folks:

I'm trying to calculating a substring length directly from pointer
address, like this:

char *e = NULL, *s = NULL;
int len = 0;

s = strchr (url,'.');
e = strrchr (url,'?');
len = (int) s - e;

^^^^^
Not only is a cast unneeded, it introduces an error into your program.
You are trying to subtract a pointer from an int.

>
Since 's' and 'e' are pointers, I think that's can be make a math
over it, like 's++'.

But your subtraction is a pointer from an int, not a pointer from a
pointer. If you _must_ use an unneccesary cast, use
len = (int)(s - e);
but that means exactly the same thing as the cast-less
len = s - e;

>
Using gcc 4.1.2 (on glibc 2.6 and Linux 2.6.23) it points the
following error:

"error: invalid operands to binary -"

And that is obviously true.

[brasilino]

Hi All!

I'd like to thanks to everybody who answer. I have forgotten
the cast precedence from any math operation!!!

thanks a lot!

Lucas Brasilino

[e-mail] wrote:

Hi Folks:

I'm trying to calculating a substring length directly from pointer
address, like this:

char *e = NULL, *s = NULL;
int len = 0;

s = strchr (url,'.');
e = strrchr (url,'?');
len = (int) s - e;

^^^^^
Not only is a cast unneeded, it introduces an error into your program.
You are trying to subtract a pointer from an int.

Since 's' and 'e' are pointers, I think that's can be make a math
over it, like 's++'.

But your subtraction is a pointer from an int, not a pointer from a
pointer. If you _must_ use an unneccesary cast, use
len = (int)(s - e);
but that means exactly the same thing as the cast-less
len = s - e;

Using gcc 4.1.2 (on glibc 2.6 and Linux 2.6.23) it points the
following error:

"error: invalid operands to binary -"

And that is obviously true.

[Default User]

br*******@yahoo.com wrote:

Hi All!

Please don't top-post. Your replies belong following or interspersed
with properly trimmed quotes. See the majority of other posts in the
newsgroup, or:
<http://www.caliburn.nl/topposting.html>

[William Pursell]

On Feb 29, 7:06 pm, Martin Ambuhl <[e-mail]:

>
But your subtraction is a pointer from an int, not a pointer from a
pointer. If you _must_ use an unneccesary cast, use
len = (int)(s - e);
but that means exactly the same thing as the cast-less
len = s - e;

s - e is of type ptrdiff_t, not int.

[Peter Nilsson]

William Pursell <[e-mail]:

Martin Ambuhl <[e-mail]:

But your subtraction is a pointer from an int, not a
pointer from a pointer. *If you _must_ use an
unneccesary cast, use
* * len = (int)(s - e);
but that means exactly the same thing as the cast-less
* * len = s - e;

s - e is of type ptrdiff_t, not int.

Note that len was declared as an int in the original post,
in which case there is an implicit conversion to int anyway.

--
Peter