# Arbitrary length multi-dimensional arrays

Discussion in 'C Programming' started by kd, Jun 26, 2006.

1. ### kdGuest

Newbie question here. It's been a while since I've done C programming,
and I hit a wall last night.

Let's say I have a three dimensional array, like so:

int p[2][3][3] =
{{{0,0,0},
{1,1,1},
{0,1,0}},

{{0,1,0},
{1,1,0},
{0,1,0}}};

I also have a number of other three dimensional arrays, generated with
a code generating script. The size of each dimension varies with each
one. Some are [5][3][3], some are [2][5][5], etc...

How would I declare a variable that could hold any of these
3-dimensional arrays? I'm tripping over the pointer syntax.

I'd like to be able to do something like:
int ***val = p; //The variable p from the last example.

I'm pretty sure that ***val is the wrong way to go about it.

-kd

kd, Jun 26, 2006

2. ### Tom St DenisGuest

kd wrote:
> Newbie question here. It's been a while since I've done C programming,
> and I hit a wall last night.
>
> Let's say I have a three dimensional array, like so:
>
> int p[2][3][3] =
> {{{0,0,0},
> {1,1,1},
> {0,1,0}},
>
> {{0,1,0},
> {1,1,0},
> {0,1,0}}};
>
> I also have a number of other three dimensional arrays, generated with
> a code generating script. The size of each dimension varies with each
> one. Some are [5][3][3], some are [2][5][5], etc...
>
> How would I declare a variable that could hold any of these
> 3-dimensional arrays? I'm tripping over the pointer syntax.
>
> I'd like to be able to do something like:
> int ***val = p; //The variable p from the last example.
>
> I'm pretty sure that ***val is the wrong way to go about it.

No you'd need three levels of indirection. A smart way though is to
just have a single pointer and compute the address yourself, especially
if you're dealing with a *variable* number of dimensions.

Tom

Tom St Denis, Jun 26, 2006

3. ### Frederick GothamGuest

kd posted:

> Newbie question here. It's been a while since I've done C programming,
> and I hit a wall last night.
>
> Let's say I have a three dimensional array, like so:
>
> int p[2][3][3] =
> {{{0,0,0},
> {1,1,1},
> {0,1,0}},
>
> {{0,1,0},
> {1,1,0},
> {0,1,0}}};
>
> I also have a number of other three dimensional arrays, generated with
> a code generating script. The size of each dimension varies with each
> one. Some are [5][3][3], some are [2][5][5], etc...
>
> How would I declare a variable that could hold any of these
> 3-dimensional arrays? I'm tripping over the pointer syntax.

Do you want a pointer to the first element of the array?

int array1[2][3][3];
int array2[5][3][3];
int array3[2][5][5];

int *p;

p = ***array1;
p = ***array2;
p = ***array3;

Writing:

array[0]

is the same as writing:

*array

Therefore:

***array

(which can also be written as):

*(*(*array))

becomes:

((array[0])[0])[0]

which, because of C operator precedence rules, is simply:

array[0][0][0]

--

Frederick Gotham

Frederick Gotham, Jun 26, 2006
4. ### Andrew PoelstraGuest

On 2006-06-26, kd <> wrote:
> Newbie question here. It's been a while since I've done C programming,
> and I hit a wall last night.
>
> Let's say I have a three dimensional array, like so:
>
> int p[2][3][3] =
> {{{0,0,0},
> {1,1,1},
> {0,1,0}},
>
> {{0,1,0},
> {1,1,0},
> {0,1,0}}};
>
> I also have a number of other three dimensional arrays, generated with
> a code generating script. The size of each dimension varies with each
> one. Some are [5][3][3], some are [2][5][5], etc...
>
> How would I declare a variable that could hold any of these
> 3-dimensional arrays? I'm tripping over the pointer syntax.
>
> I'd like to be able to do something like:
> int ***val = p; //The variable p from the last example.
>
> I'm pretty sure that ***val is the wrong way to go about it.
>

No, ***p will do it. You'll be able to go through the array with p++,
(*p)++, and (**p)++, depending on which dimension you are moving
through. Indeed, it is pretty complicated.

Why are you doing this?

--
Andrew Poelstra < http://www.wpsoftware.net/blog >
To email me, use "apoelstra" at the above address.
I know that area of town like the back of my head.

Andrew Poelstra, Jun 26, 2006
5. ### Ben CGuest

On 2006-06-26, kd <> wrote:
> Newbie question here. It's been a while since I've done C programming,
> and I hit a wall last night.
>
> Let's say I have a three dimensional array, like so:
>
> int p[2][3][3] =
> {{{0,0,0},
> {1,1,1},
> {0,1,0}},
>
> {{0,1,0},
> {1,1,0},
> {0,1,0}}};
>
> I also have a number of other three dimensional arrays, generated with
> a code generating script. The size of each dimension varies with each
> one. Some are [5][3][3], some are [2][5][5], etc...
>
> How would I declare a variable that could hold any of these
> 3-dimensional arrays?

You can't really. The thing to think about is how does the compiler
interpret an expression like:

p[j][k]

Suppose p is declared int p[2][3][4]. To find its way to element i,j,k,
the compiler needs to work out the offset from the start of where p is
stored to this element. This amounts to something like:

4*3*i*n + 3*j*n + k*n

where n is sizeof (int). The point is the compiler needs to know the 3
and the 4, which it determined by looking at the type of p.

You can make a pointer to p like this:

int (*pp)[2][3][4] = &p;

or even like this:

int (*pp)[][3][4] = &p;

since the compiler needs to know the 3 and the 4, but not the 2.

But you should get an "initialization from incompatible pointer type" or
similar warning if you try to make pp point to an array that was
declared int q[2][5][5]. If you force the initialization with a cast,
you will get the wrong results when you use pp because the compiler will
be working with the wrong dimensions for q.

Ben C, Jun 26, 2006
6. ### Michael MairGuest

Frederick Gotham schrieb:
<snip>
> Do you want a pointer to the first element of the array?
>
> int array1[2][3][3];

<snip>
>
> int *p;
>
> p = ***array1;

You are assigning an int value to a pointer.

Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

Michael Mair, Jun 26, 2006
7. ### Frederick GothamGuest

Michael Mair posted:

> Frederick Gotham schrieb:
><snip>
>> Do you want a pointer to the first element of the array?
>>
>> int array1[2][3][3];

><snip>
>>
>> int *p;
>>
>> p = ***array1;

>
> You are assigning an int value to a pointer.
>
> Cheers
> Michael

Wups.

p = **array1;

--

Frederick Gotham

Frederick Gotham, Jun 26, 2006
8. ### Michael MairGuest

kd schrieb:
> Newbie question here. It's been a while since I've done C programming,
> and I hit a wall last night.
>
> Let's say I have a three dimensional array, like so:
>
> int p[2][3][3] =
> {{{0,0,0},
> {1,1,1},
> {0,1,0}},
>
> {{0,1,0},
> {1,1,0},
> {0,1,0}}};
>
> I also have a number of other three dimensional arrays, generated with
> a code generating script. The size of each dimension varies with each
> one. Some are [5][3][3], some are [2][5][5], etc...
>
> How would I declare a variable that could hold any of these
> 3-dimensional arrays? I'm tripping over the pointer syntax.
>
> I'd like to be able to do something like:
> int ***val = p; //The variable p from the last example.
>
> I'm pretty sure that ***val is the wrong way to go about it.

It is the wrong way.
int (*val)[3][3] = p;
is the right way to deal with arbitrary amounts of "3 by 3" matrices.
If you want to be able to deal with "arbitrary amounts of arbitrary
row by arbitrary column number matrices", you need three levels of
indirection. For every level but the last you need "index arrays".

Now, there are two ways of representing your "3D array" in memory
which _can_ make things easier:
1) Condensed. I.e. the last column of the first row of the
first matrix is immediately followed by the first column of the
second row of the first matrix and the last column of the last
row of the first matrix is immediately followed by the first
column of the first row of the second matrix.
This means that you could do with one array of int and could
access everything "matrixlike" via
#define INDEX(i, j, k, num_j, num_k) \
(((i) * (max_j) + (j)) * (max_k) + (k))
and
int *array = malloc(sizeof p);
if (NULL == array) {
/* error handling and abort */
}
memcpy(array, p, sizeof p);
for (mat = 0; mat < num_mat; ++mat)
for (row = 0; row < num_row; ++row)
for (col = 0; col < num_col; ++col) {
do_something(array[INDEX(mat,row,col, num_row,num_col)]);
}

If you really insist on
int ***val;
you need the following steps:
num_mat = sizeof p/sizeof p[0];
val = malloc(num_mat * sizeof *val);
if (NULL == val) {
/* error handling and abort */
}
num_row = sizeof p[0] / sizeof p[0][0];
*val = malloc(num_mat*num_row * sizeof **val);
if (NULL == *val) {
/* error handling and abort */
}
for (mat = 1; mat < num_mat; ++mat) {
val[mat] = val[0] + mat*num_row;
}
/* 1 */
for (mat = 0; mat < num_mat; ++mat)
for (row = 0; row < num_row; ++row)
val[mat][row] = p[mat][row];
/* 2 */
in order to use
for (mat = 0; mat < num_mat; ++mat)
for (row = 0; row < num_row; ++row)
for (col = 0; col < num_col; ++col) {
do_something(val[mat][row][col]);
}
Note that this operates on the original array p.
If you want to have val as a "copy of p", you have to replace
/* 1 */ to /* 2 */ by
num_col = sizeof p[0][0] / sizeof p[0][0][0];
**val = malloc(num_mat*num_row*num_col * sizeof ***val); /*3*/
if (NULL == **val) {
/* error handling and abort */
}
memcpy(val, p, sizeof p);
for (mat = 0; mat < num_mat; ++mat)
for (row = 0; row < num_row; ++row)
val[mat][row] = val[0][0] + (mat*num_row + row)*num_col;

2) Maximum array: Say you know that the largest possible array
dimensions are MAX_MAT, MAX_ROW, MAX_COL and
MAX_MAT*MAX_ROW*MAX_COL is not too large. Then declare your
"intermediate" matrix as
int val[MAX_MAT][MAX_ROW][MAX_COL];
and copy the values:
for (mat = 0; mat < num_mat; ++mat)
for (row = 0; row < num_row; ++row)
for (col = 0; col < num_col; ++col) {
val[mat][row][col] = p[mat][row][col];
}

Merits: 1) makes it possible to just memcpy() the array but
can mean resizing of "array" or "**val", "*val", and "val",
respectively. 2) means no resizing but potentially increased
cost for copying -- and much memory consumption.

If you allocate each row separately instead of at once (/*3*/),
you can "resize" the matrix in an easier manner but have
more allocations to take care of.

It depends on your application whether 1) or 2) or a mixed
form or something completely different is best for your...

Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

Michael Mair, Jun 26, 2006
9. ### Guest

Andrew Poelstra wrote:

> Why are you doing this?

Tetris! Every budding game developer's first game!

It's the sequence of rotations for different pieces!

, Jun 27, 2006