2D array of structures

[svata]

Hello,

I wonder how to resize such array of structures using realloc()?

#include <stdio.h>
#include <stdlib.h>
#define FIRST 7

typedef struct {
char *name;
int i;
int j;
} STRUCTURE;

STRUCTURE **p_structure;

int main() {

p_structure = (STRUCTURE **) malloc(FIRST * sizeof(STRUCTUR E));
if ( p_structure == NULL ) {
printf("Failed to allocate memory, exiting...");
return 1;
}
}

Thank you in advance

Svata

[Jens Thoms Toerring]

Re: 2D array of structures

svata <svatoboj@centr um.czwrote:

I wonder how to resize such array of structures using realloc()?

#include <stdio.h>
#include <stdlib.h>
#define FIRST 7

typedef struct {
char *name;
int i;
int j;
} STRUCTURE;

STRUCTURE **p_structure;

int main() {

Make that

int main( void )

since your main() takes no arguments.

p_structure = (STRUCTURE **) malloc(FIRST * sizeof(STRUCTUR E));

Don't cast the return value of malloc(), it only hides your mistake
should you have forgotten to include <stdlib.h>. But, more important,
you allocate here memory for 7 (FIRST) such structures. malloc() returns
a pointer to the start of this memory, which is of type 'STRUCTURE *'
(malloc() actually returns a void pointer but "STRUCTURE *' is the
correct type of a pointer to that memory). But you assign it instead
to a pointer that has type 'STRUCTURE **'. That, combined with your
use of the words "2D array" in the subjct line, leads to the suspicion
that you actually don't want to allocate memory for something that has
similar properties as an array of stuctures, but something more compli-
cated and similar to a 2D array of such structures. And that's what you
definitely won't get with that allocation, whatever the type of the
pointer you assign the return value of malloc() to. So, instead of
starting to guess what you might have intended I think it's better to
ask you to specify a bit more clearly what you intend to do here: do
you just want memory for a simple set of structures or do you want
something like a 2D array of such structures. In the first case you
could "repair" your program by simply defining 'p_structure' as

STTRUCTURE *p_structure;

Perhaps you thenn also might want to replace the line for the allo-
cation by

p_structure = malloc(FIRST * sizeof *p_structure);

because that way you don't have to change that line anymore if you
should decide to change the type of 'p_structure' sometime later.

You also write something about realloc() but I can't see any
use or mentioning of realloc() in the code you posted. But, of
course, you can use realloc() to resize the amount of memory
you obtained from malloc() - that's what realloc() was invented
for.

if ( p_structure == NULL ) {
printf("Failed to allocate memory, exiting...");
return 1;
}

It's good to see that you check that what malloc() returned!

}

Since main() returns an int, here's a missing line with a return
value...
Regards, Jens
--
\ Jens Thoms Toerring ___ jt@toerring.de
\______________ ____________ http://toerring.de

[Flash Gordon]

Re: 2D array of structures

Jens Thoms Toerring wrote:

svata <svatoboj@centr um.czwrote:

<snip>

>p_structure = (STRUCTURE **) malloc(FIRST * sizeof(STRUCTUR E));

<snip>

correct type of a pointer to that memory). But you assign it instead
to a pointer that has type 'STRUCTURE **'. That, combined with your
use of the words "2D array" in the subjct line, leads to the suspicion
that you actually don't want to allocate memory for something that has
similar properties as an array of stuctures, but something more compli-
cated and similar to a 2D array of such structures. And that's what you

<snip>

I agree with what you've said, but the OP should also read section 6 of
the comp.lang.c FAQ before posting back here. Section 6 includes ways of
dynamically allocating space for things that work like 2D arrays. The
FAQ can be found at http://c-faq.com/
--
Flash Gordon

[Frederick Gotham]

Re: 2D array of structures

svata:

STRUCTURE **p_structure;

This is a pointer to a pointer to a STRUCTURE.

(I'm against the use of ALL CAPS for anything other than macros)

int main() {
>
p_structure = (STRUCTURE **) malloc(FIRST * sizeof(STRUCTUR E));

This doesn't make sense.

You have a pointer to a pointer to a STRUCTURE. This would suggest to me
that you're going to store either of the following in it:

(1) The address of a pointer to a STRUCTURE.
(2) The address of the first element of an array, whereby each element
is a pointer to a STRUCTURE.

If it were either of the two above, the "malloc" invocation should look
something like:

p_structure = malloc( sizeof(STRUCTUR E*) );
or:
p_structure = malloc( 12 * sizeof(STRUCTUR E*) );

Note that I use the size of a "STRUCTURE* " rather than the size of a
"STRUCTURE" .

Of course, it's better to write the two of them as:

p_structure = malloc(sizeof*p );

p_structure = malloc(12 * sizeof*p);

--

Frederick Gotham

[CBFalconer]

Re: 2D array of structures

svata wrote:

>
I wonder how to resize such array of structures using realloc()?
>
#include <stdio.h>
#include <stdlib.h>
#define FIRST 7
>
typedef struct {
char *name;
int i;
int j;
} STRUCTURE;
>
STRUCTURE **p_structure;

Too many *s here.

>
int main() {

You

>
p_structure = (STRUCTURE **) malloc(FIRST * sizeof(STRUCTUR E));

and here. Also _never_ cast the return from malloc (it only hides
errors without fixing them). You want:

p_structure = malloc(FIRST * sizeof *p_structure);

if ( p_structure == NULL ) {
printf("Failed to allocate memory, exiting...");
return 1;
}
}

You can then change the size to, say, SECOND with:

STRUCTURE *tmp; /* in a suitable location, not here */

if (tmp = realloc(p_struc ture, SECOND) p_structure = tmp;
else {
/* handle lack of memory */
}

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

[svata]

Re: 2D array of structures

Hello Frederick,

I know it doesn't make sense. I was wrong in my assumption.
I should use p_structure = (STRUCTURE **) malloc(FIRST *
sizeof(STRUCTUR E*));

but anyway... should rather use array of structures.


typedef structure {
code goes here...
} _structure;

_structure *p_structure;
// and then malloc()

p_structure = malloc( INT * sizeof(_structu re));

if am I right?

svata

Frederick Gotham wrote:

STRUCTURE **p_structure;

>
>
This is a pointer to a pointer to a STRUCTURE.
>
(I'm against the use of ALL CAPS for anything other than macros)
>
>

int main() {

p_structure = (STRUCTURE **) malloc(FIRST * sizeof(STRUCTUR E));

>
>
This doesn't make sense.
>

--
>
Frederick Gotham

[Chris Dollin]

Re: 2D array of structures

svata wrote:

Hello Frederick,
>
I know it doesn't make sense. I was wrong in my assumption.
I should use p_structure = (STRUCTURE **) malloc(FIRST *
sizeof(STRUCTUR E*));

No, you shouldn't: it's wiser to drop the unnecessary cast an
to ensure that the thing you sizeof is appropriate for the
pointer:

pStructure = malloc( FIRST * sizeof (*pStructure) );

which will (try to) mallocate space for FIRST (horrid name)
lumps of space adequate for the kinds of thing that
pStructure points to.

but anyway... should rather use array of structures.

Yes, get rid of the unnecessary layer of indirection.

typedef structure {
code goes here...
} _structure;

There are two things wrong with this. One is that it's
syntax is broken, since you spelt "struct" "structure" .
The other is that so many names beginning with _ are
reserved to the implementation it's unwise for you to
define /any/ name that does. So don't.

If you /must/ use a typedef for a structure -- you don't
need to, and some wise people argue that you shouldn't
(although others argue that those arguments aren't
convincing) -- you should also give it a /sensible/
name. "structure" isn't.

typedef struct yourStructTagHe re
{
int x;
int y;
} Point;

--
Chris ".enable proofreading" Dollin
The "good old days" used to be much better.

[svata]

Re: 2D array of structures

Chris Dollin wrote:

I learn by doing. So I will see what results I get.

pStructure = malloc( FIRST * sizeof (*pStructure) );
>
which will (try to) mallocate space for FIRST (horrid name)
lumps of space adequate for the kinds of thing that
pStructure points to.
>

but anyway... should rather use array of structures.

>
Yes, get rid of the unnecessary layer of indirection.
>

typedef structure {
code goes here...
} _structure;

>
There are two things wrong with this. One is that it's
syntax is broken, since you spelt "struct" "structure" .
The other is that so many names beginning with _ are
reserved to the implementation it's unwise for you to
define /any/ name that does. So don't.
>
If you /must/ use a typedef for a structure -- you don't
need to, and some wise people argue that you shouldn't
(although others argue that those arguments aren't
convincing) -- you should also give it a /sensible/
name. "structure" isn't.
>
typedef struct yourStructTagHe re
{
int x;
int y;
} Point;
>
--
Chris ".enable proofreading" Dollin
The "good old days" used to be much better.

[svata]

Re: 2D array of structures

I always use google to search an answer, but often results are not
relevant.


svata

>
I agree with what you've said, but the OP should also read section 6 of
the comp.lang.c FAQ before posting back here. Section 6 includes ways of
dynamically allocating space for things that work like 2D arrays. The
FAQ can be found at http://c-faq.com/
--
Flash Gordon

[Richard Heathfield]

Re: 2D array of structures

svata said:

Chris Dollin wrote:
>
I learn by doing. So I will see what results I get.

That's fine sometimes, but there are also times when it's best to learn from
other people's knowledge and experience. If you learn C "by doing", you're
likely to end up doing lots of things that aren't correct, but which happen
to behave in a particular way on your current system. Switch systems, and
all your code breaks. Oops.

Chris is an expert on C. Listen to Chris.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999

http://www.cpax.org.uk

email: normal service will be restored as soon as possible. Please do not
adjust your email clients.

[CBFalconer]

Re: 2D array of structures

svata wrote:

>
I always use google to search an answer, but often results are not
relevant.
>

>I agree with what you've said, but the OP should also read section
>6 of the comp.lang.c FAQ before posting back here. Section 6
>includes ways of dynamically allocating space for things that work
>like 2D arrays. The FAQ can be found at http://c-faq.com/

I'm sure you've been told this before, but DON'T TOP-POST. See the
links in my sig below.

--
Some informative links:
<news:news.anno unce.newusers
<http://www.geocities.c om/nnqweb/>
<http://www.catb.org/~esr/faqs/smart-questions.html>
<http://www.caliburn.nl/topposting.html >
<http://www.netmeister. org/news/learn2quote.htm l>
<http://cfaj.freeshell. org/google/>

[Chris Dollin]

Re: 2D array of structures

Richard Heathfield wrote:

svata said:
>

>Chris Dollin wrote:
>>
>I learn by doing. So I will see what results I get.

Something happend to your quoting, Richard: it looks like it was me that
said "I learn by doing. So I will see what results I get.", but it was
svata.

(svata's quoting was misleading anyway, since he half-top-posted.)

That's fine sometimes, but there are also times when it's best to learn from
other people's knowledge and experience. If you learn C "by doing", you're
likely to end up doing lots of things that aren't correct, but which happen
to behave in a particular way on your current system. Switch systems, and
all your code breaks. Oops.
>
Chris is an expert on C.

I'd hesitate to describe myself so, since there's so much C I don't
know (most of the C99 stuff, for example!) and I've used it in so
few environments. I know /some/ stuff about C. I hope it's useful
stuff.

Listen to Chris.

But not when I'm singing.

--
Chris "for that, you want Annie, Rachel, Anne-Marie, or Tina." Dollin
"Reaching out for mirrors hidden in the web." - Renaissance, /Running Hard/

[CBFalconer]

Re: 2D array of structures

Chris Dollin wrote:

Richard Heathfield wrote:
>

.... snip ...

>

>Listen to Chris.

>
But not when I'm singing.

Like me, you appear to be a member of the 'crows in heat' chorus.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

[Chris Dollin]

Re: 2D array of structures

CBFalconer wrote:

Chris Dollin wrote:

>Richard Heathfield wrote:
>>

... snip ...

>>

>>Listen to Chris.

>>
>But not when I'm singing.

>
Like me, you appear to be a member of the 'crows in heat' chorus.

Very likely. Or hedgehogs ... snuffling. Perhaps we should
characterise Undefined Behaviour as "Chris or <your preferred
abbreviationwil l sing for/at you". /That/ will worry them
more than hyperbolic nasal demons.

--
Chris "we are BRO-KEN, without FEE-LING" Dollin
"Who do you serve, and who do you trust?" /Crusade/

[Richard Heathfield]

Re: 2D array of structures

Chris Dollin said:

Richard Heathfield wrote:
>

>svata said:
>>

>>Chris Dollin wrote:
>>>
>>I learn by doing. So I will see what results I get.

>
Something happend to your quoting, Richard:

No, just a snip slip. Sorry about that.

>Chris is an expert on C.

>
I'd hesitate to describe myself so,

Think "relative". Compared to most OPs around here, you're a towering
genius!

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999

http://www.cpax.org.uk

email: normal service will be restored as soon as possible. Please do not
adjust your email clients.