# Not fully comprehending arrays and dynamic memory.

Discussion in 'C Programming' started by Mark Healey, Apr 13, 2006.

1. ### Mark HealeyGuest

I'd like to allocate some memory for a two dimensional array.

The problem is that whenever I try to use pointer arithmetic I screw up.
I just want to use conventional array type statements to get at the memory.

Is there a way to do this?

--
Mark Healey
marknews(at)healeyonline(dot)com

Mark Healey, Apr 13, 2006

2. ### Keith ThompsonGuest

Mark Healey <> writes:
> I'd like to allocate some memory for a two dimensional array.

[...]

<http://www.c-faq.com/aryptr/dynmuldimary.html>

--
Keith Thompson (The_Other_Keith) <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

Keith Thompson, Apr 13, 2006

3. ### Ben CGuest

On 2006-04-13, Mark Healey <> wrote:
> I'd like to allocate some memory for a two dimensional array.
>
> The problem is that whenever I try to use pointer arithmetic I screw up.
> I just want to use conventional array type statements to get at the memory.
>
> Is there a way to do this?

It can be done, but it's a bit tricky, and even easier to screw up I
would say.

You can allocate one like this:

int (*d)[3][3] = malloc(sizeof *d);

And use it like this:

(*d)[2][1] = 100;

...

free(d);

etc.

It might be easier to use a typedef:

typedef int matrix_t[3][3];
matrix_t *m = malloc(sizeof *m);

...

(*m)[2][1] = 100;

...

free(m);

With this way of doing it both d and m are pointers to a single block of
memory, which only has to be freed once. Other solutions involve making
arrays of pointers (one pointer for each row), and potentially calling
malloc to allocate each row. It's important to be clear which you're
doing and to call free the right number of times.

Ben C, Apr 13, 2006
4. ### Peter Shaggy HaywoodGuest

Groovy hepcat Ben C was jivin' on 13 Apr 2006 09:45:02 GMT in
comp.lang.c.
Re: Not fully comprehending arrays and dynamic memory.'s a cool scene!
Dig it!

>On 2006-04-13, Mark Healey <> wrote:
>> I'd like to allocate some memory for a two dimensional array.
>>
>> The problem is that whenever I try to use pointer arithmetic I screw up.
>> I just want to use conventional array type statements to get at the memory.
>>
>> Is there a way to do this?

>
>It can be done, but it's a bit tricky, and even easier to screw up I
>would say.
>
>You can allocate one like this:
>
> int (*d)[3][3] = malloc(sizeof *d);
>
>And use it like this:
>
> (*d)[2][1] = 100;

Or, even easier, like this:

#define ROWS 3
#define COLS 3
int (*d)[COLS] = malloc(ROWS * sizeof *d);
.... /* Checking for errors, etc. */
d[2][1] = 100;

Or, slightly trickier (because you have to handle multiple
allocations and deallocations), like so:

#define ROWS 3
#define COLS 3
int i;
int **d = malloc(ROWS * sizeof *d);
.... /* Checking for errors, etc. */
for(i = 0; i < ROWS; i++)
{
d = malloc(COLS * sizeof *d);
... /* Checking for errors, etc. */
}

d[2][1] = 100;

Or, even better still, simpler (because there are only two
allocations):

#define ROWS 3
#define COLS 3
int i;
int **d = malloc(ROWS * sizeof *d);
.... /* Checking for errors, etc. */
*d = malloc(ROWS * COLS * sizeof *d);
.... /* Checking for errors, etc. */
for(i = 1; i < ROWS; i++)
{
d = d[i - 1] + COLS;
}

d[2][1] = 100;

--

Dig the even newer still, yet more improved, sig!

"Ain't I'm a dog?" - Ronny Self, Ain't I'm a Dog, written by G. Sherry & W. Walker.
I know it's not "technically correct" English; but since when was rock & roll "technically correct"?

Peter Shaggy Haywood, Apr 16, 2006
5. ### Ben CGuest

On 2006-04-16, Peter "Shaggy" Haywood <> wrote:
> Groovy hepcat Ben C was jivin' on 13 Apr 2006 09:45:02 GMT in
> comp.lang.c.
> Re: Not fully comprehending arrays and dynamic memory.'s a cool scene!
> Dig it!

>>On 2006-04-13, Mark Healey <> wrote:
>>> I'd like to allocate some memory for a two dimensional array.
>>>
>>> The problem is that whenever I try to use pointer arithmetic I screw up.
>>> I just want to use conventional array type statements to get at the memory.
>>>
>>> Is there a way to do this?

>>
>>It can be done, but it's a bit tricky, and even easier to screw up I
>>would say.
>>
>>You can allocate one like this:
>>
>> int (*d)[3][3] = malloc(sizeof *d);
>>
>>And use it like this:
>>
>> (*d)[2][1] = 100;

> Or, even easier, like this:
>
> #define ROWS 3
> #define COLS 3
> int (*d)[COLS] = malloc(ROWS * sizeof *d);
> ... /* Checking for errors, etc. */
> d[2][1] = 100;

This is certainly nicer, and makes sense because after all when you
allocate a one-dimensional array of N ints, you usually do it with:

int *p = malloc(N * sizeof *p);

not with:

int (*p)[N] = malloc(sizeof *p);

> Or, slightly trickier (because you have to handle multiple allocations
> and deallocations), like so: [...]

I would say it's better to avoid multiple allocations except when you
need them (when the rows are different lengths, say).

Ben C, Apr 16, 2006