K&R p 130

[mdh]

Hi All,
Just when I thought things were going to get easy!

Structs.

I **thought** I had copied the examples pretty closely, but am getting
a number of errors.


The code:

>>>>>>

#include <stdio.h>


int main (int argc, const char * argv[]) {

struct point {
int x;
int y;
};


struct point makepoint( int, int); /* error. previous decl of
'makepoint' was here*/



struct point p1 = makepoint(8,9);

printf("%d, %d\n", p1.x, p1.y);
}




struct point makepoint(int x, int y) {
struct point temp;
temp.x = x;
temp.y = y;
return temp;
}

The error I got in the past has meant that something has been defined
twice, but as far as I can tell, ( probably incorrectly) I have
declared a struct called point, then declared a function which accepts
two integer arguments and returns a 'point' structure. Clearly I am
missing something.

In addition, I am getting a number of errors in makepoint function
defintion, which may become obvious once my initial query is cleared.
If not, I will ask then.

Lastly, K&R say , on p 130, of the makepoint function, "notice that
there is no conflict between the argument name and the member with the
same name". I assume that this refers to the lines

temp.x = x;
temp.y = y;

Thanks as usual.

[santosh]

Re: K&amp;R p 130

mdh wrote:

Hi All,
Just when I thought things were going to get easy!
>
Structs.
>
I **thought** I had copied the examples pretty closely, but am getting
a number of errors.
>
>
The code:
>

>>>>>>>

>
#include <stdio.h>
>
>
int main (int argc, const char * argv[]) {
>
struct point {
int x;
int y;
};
>
>
struct point makepoint( int, int); /* error. previous decl of
'makepoint' was here*/
>
>
>
struct point p1 = makepoint(8,9);
>
printf("%d, %d\n", p1.x, p1.y);
}
>
>
>
>
struct point makepoint(int x, int y) {
struct point temp;
temp.x = x;
temp.y = y;
return temp;
}
>
The error I got in the past has meant that something has been defined
twice, but as far as I can tell, ( probably incorrectly) I have
declared a struct called point, then declared a function which accepts
two integer arguments and returns a 'point' structure. Clearly I am
missing something.
>
In addition, I am getting a number of errors in makepoint function
defintion, which may become obvious once my initial query is cleared.
If not, I will ask then.
>
Lastly, K&R say , on p 130, of the makepoint function, "notice that
there is no conflict between the argument name and the member with the
same name". I assume that this refers to the lines
>
temp.x = x;
temp.y = y;
>
Thanks as usual.

Here's your code corrected:

#include <stdio.h>

struct point {
int x;
int y;
};

struct point makepoint( int, int);


int main (void) {
struct point p1 = makepoint(8,9);
printf("%d, %d\n", p1.x, p1.y);
return 0;

}

struct point makepoint(int x, int y) {
struct point temp;
temp.x = x;
temp.y = y;
return temp;
}

[mdh]

Re: K&amp;R p 130

On Aug 16, 11:12 pm, santosh <santosh....@gm ail.comwrote:

>
>
Here's your code corrected:
>
#include <stdio.h>
>
struct point {
int x;
int y;
>
};
>
struct point makepoint( int, int);
>
snip

}

So I once again ran into a scope issue? In this case block scope? vs
file scope which you implemented? Santosh, could you help me
understand why I can declare 'int foo(int);' within "main" and then
call it, but I cannot do the same with the struct?

[Malcolm McLean]

Re: K&amp;R p 130


"mdh" <mdeh@comcast.n etwrote in message
On Aug 16, 11:12 pm, santosh <santosh....@gm ail.comwrote:

>
>So I once again ran into a scope issue? In this case block scope? vs
>file scope which you implemented? Santosh, could you help me
>understand why I can declare 'int foo(int);' within "main" and then
>call it, but I cannot do the same with the struct?
>

The struct is not local to main. It is used in another function.
Similarly if you called foo() from anywhere except main(), you'd have to
prototype it again.
This is why we normally place structures and prototypes at the top of the
file, usually wrapped up into a header.

--
Free games and programming goodies.

http://www.personal.leeds.ac.uk/~bgy1mm

[=?GB2312?B?s8K35dHv?=]

Re: K&amp;R p 130

On 8ÔÂ17ÈÕ, ÏÂÎç2ʱ25·Ö, mdh <m...@comcast.n etwrote:

On Aug 16, 11:12 pm, santosh <santosh....@gm ail.comwrote:
>
>
>
>
>

Here's your code corrected:

>

#include <stdio.h>

>

struct point {
int x;
int y;

>

};

>

struct point makepoint( int, int);

>

snip
}

>
So I once again ran into a scope issue? In this case block scope? vs
file scope which you implemented? Santosh, could you help me
understand why I can declare 'int foo(int);' within "main" and then
call it, but I cannot do the same with the struct?

The return type of 'int foo(int)' is "int", which is a built-in type
of the language, that means you can use it in the global
scope(wherever in your program).
But "struct point" is a type you defined yourself, and you have
defined it in function main(), so you have to use it just within the
local-scope of main. In your code, you defeined a function return type
struct point out of the local-scope of main, it breaks the rule
mentioned above.:-)

[mdh]

Re: K&amp;R p 130

On Aug 17, 1:25 am, "Malcolm McLean" <regniz...@btin ternet.comwrote :

"mdh" <m...@comcast.n etwrote in message
>

>
The struct is not local to main. It is used in another function.
Similarly if you called foo() from anywhere except main(), you'd have to
prototype it again.
This is why we normally place structures and prototypes at the top of the
file, usually wrapped up into a header.
>

Well, this is really disheartening, as I thought I had it figured
out.


If we have:


int main ( void){

int foo( void);

int i = foo;

printf("...etc ");

return 0;
}


int foo (void){
do stuff;
return stuff;
}


This seems to compile and work. I also thought declaring foo() before
main allowed it to be "seen" throughout the translation unit, but
within main, only to be seen, but still to be legally used if I called
it as above. How then, did this differ from declaring the struct
"point" and function returning "point" within main and calling it from
within main. I hope this makes sense.
Thanks in advance for clarifying this.

[pete]

Re: K&amp;R p 130

mdh wrote:

int foo( void);
>
int i = foo;

This seems to compile and work.

Initializing an int object
with a function name
does not seem to compile and work.

--
pete

[mdh]

Re: K&amp;R p 130

On Aug 17, 2:39 am, pete <pfil...@mindsp ring.comwrote:

>
>
Initializing an int object
with a function name
does not seem to compile and work.

OK...here is my tested example.

#include <stdio.h>

int main (int argc, const char * argv[]) {

double foo( double);
double d = foo(8);
printf( "%f", d); /* 14.000000 */
return 0;
}


double foo ( double d){
return 6.0 + d;
}

[Ben Bacarisse]

Re: K&amp;R p 130

mdh <mdeh@comcast.n etwrites:

On Aug 17, 2:39 am, pete <pfil...@mindsp ring.comwrote:

>>
>Initializing an int object
>with a function name
>does not seem to compile and work.

>
OK...here is my tested example.
>
#include <stdio.h>
>
int main (int argc, const char * argv[]) {
>
double foo( double);
double d = foo(8);
printf( "%f", d); /* 14.000000 */
return 0;
}
>
double foo ( double d){
return 6.0 + d;
}

Yes, a correct use of a function prototype with block scope. In your
struct example, the struct also had block scope and so was not
available at file scope when the function was defined. Nothing in the
definition of foo requires anything from the block of main so the
definition matches the prototype as required.

--
Ben.

[mdh]

Re: K&amp;R p 130

On Aug 17, 6:50 am, Ben Bacarisse <ben.use...@bsb .me.ukwrote:

mdh <m...@comcast.n etwrites:

>

#include <stdio.h>

>

int main (int argc, const char * argv[]) {

>

   double foo( double);
   double d = foo(8);
   

>snip

}

>

double foo ( double d){
   return 6.0 + d;
}

>
......... In your struct example, the struct also had block scope and sowas not
available at file scope when the function was defined.  ...... Nothing in the
definition of foo requires anything from the block of main........
>

Firstly, thank you for answering.
So Ben, what am I missing, or is this just C fatigue? I **thought**
(obviously incorrectly :-)), ( but without understanding why) that I
had, within the block of main,

declared the struct point, then declared a function called makepoint,
then outside of main, defined the function makepoint. I know what you
are saying about block scope, but on the face of it, why is this so
different from my example of foo that you say is correct. And...I
apologize in advance for belaboring this point.

[mdh]

Re: K&amp;R p 130

On Aug 17, 7:47 am, Ben Bacarisse <ben.use...@bsb .me.ukwrote:

.  Let me write it out
compressed for convenience:
>
int main(void)
{
    struct point { int x, y; };       /* The struct tag point has a */
    struct point makepoint(int, int); /* meaning all the way down */
    struct point p = makepoint(1, 2); /* to here... */
>
}
>
struct point makepoint(int a, int b) /* ... but not here where it */
{                                    /* is also needed. */
  /* stuff */
>
}
>
Outside main, "struct point" has no meaning, so the definition is
rejected.

OK...I think I am getting it.
Is this then correct.

A struct is a type ( just like int is a type) but one which I ( the
programmer...if I can call myself that :-) ) are able to declare. So,
essentially, up to now, when I have declared *anything* within main,
it has always been of a type that was ?"inherent" to C, unlike my own
type "point" which I need when defining "makepoint" . So,
theoretically , "re-declaring" it just before the definition would
work, except it would be "clunky" and in any case, that is in essence
what one is doing by moving the declarations outside of main.

[mdh]

Re: K&amp;R p 130

On Aug 17, 7:47 am, Ben Bacarisse <ben.use...@bsb .me.ukwrote:

>
Your function foo does not rely on anything with a limited scope.  Its
return type and parameter type are all keywords that have the same
meaning everywhere.  If any of these types involved identifiers with
function scope, it too would have gone wrong.
>

One last little point. The identifier "foo" as in the following.


int main(void){

sometype foo ( some argument );

use foo;


}


sometype foo ( some arguments){};
}

When I call "foo" in main, that identifier does "know" where to find
the definition of foo. The identifier "foo" though has block scope, if
I understand this correctly.
So, how then does the call to foo get handled correctly?

Thank you.

[Ben Bacarisse]

Re: K&amp;R p 130

mdh <mdeh@comcast.n etwrites:

On Aug 17, 7:47 am, Ben Bacarisse <ben.use...@bsb .me.ukwrote:
>

>>
>Your function foo does not rely on anything with a limited scope.  Its
>return type and parameter type are all keywords that have the same
>meaning everywhere.  If any of these types involved identifiers with
>function scope, it too would have gone wrong.
>>

>

>
One last little point. The identifier "foo" as in the following.
>
>
int main(void){
>
sometype foo ( some argument );
>
use foo;
>
>
}
>
>
sometype foo ( some arguments){};
}
>
When I call "foo" in main, that identifier does "know" where to find
the definition of foo. The identifier "foo" though has block scope, if
I understand this correctly.

No, there are two identifiers with the same letters and thus two
scopes. They are "linked" by the fact the declaration of foo in
main is, by default, a declaration of a function with extern linkage.

So, how then does the call to foo get handled correctly?

I don't know how to answer questions like this, because I don't see
the problem! Maybe I have answered it with the above clarification.

--
Ben.

[mdh]

Re: K&amp;R p 130

On Aug 17, 8:28 am, Ben Bacarisse <ben.use...@bsb .me.ukwrote:

The key part is "declared in separate translation units".  Two
structs, declared in the same translation unit, even in separate
scopes, can't ever be compatible -- no matter how similar they look.
>
>

OK..., this is unique only to structs, unions and enums. And if so,
is this due to the complicated nature of those objects, and perhaps
the way C stores them internally? Or...that's just the way it
is? :-)

If so, thanks again for clarifying this issue for me.

[mdh]

Re: K&amp;R p 130

On Aug 17, 8:47 am, Ben Bacarisse <ben.use...@bsb .me.ukwrote:

>
No, there are two identifiers with the same letters and thus two
scopes.  They are "linked" by the fact the declaration of foo in
main is, by default, a declaration of a function with extern linkage.
>

So, how then does the call to foo get handled correctly?

>

Thanks Ben...