Single dim array as a multidim array?

[Sona]

Please could someone tell me how to use a single-dimensional array as a
two-dimensional array? Is the following correct:

int ROW_SIZE = 5;
int COL_SIZE = 3;

int *arr = (int*) malloc(sizeof(i nt) *ROW_SIZE*COL_S IZE);

arr[i *COLUMN_SIZE + j];

will this only work for square arrays or all two-dim arrays ? Thanks :)
Sona

[Irrwahn Grausewitz]

Sona <so**********@n ospam.net> wrote in
<3f********@cla rion.carno.net. au>:

Please could someone tell me how to use a single-dimensional array as a
two-dimensional array? Is the following correct:

int ROW_SIZE = 5;
int COL_SIZE = 3;

int *arr = (int*) malloc(sizeof(i nt) *ROW_SIZE*COL_S IZE); Nit1: arr isn't an array, albeit you can apply the [] operator
to an pointer to int (which it is).

Nit2: the cast to int* is superfluous in C.

arr[i * COLUMN_SIZE + j]; This will work as long as i is in [0 ; ROW_SIZE-1]
and j is in [0 ; COL_SIZE-1].

Much like in a RealArray(tm):

int realArr[YDIM][XDIM];
realArr[ y ][ x ] ...

will this only work for square arrays or all two-dim arrays ? Thanks :)

Well, for square arrays i and j are interchangeable , as long as
you use them in a consistently, but it works for all n-dimensional
arrays, e.g.:

int *arr = (int*) malloc( ZDIM * YDIM * XDIM * sizeof *arr );

arr[ z * XDIM * YDIM + y * XDIM + x ] = 42;

Irrwahn

--
If it's not on fire, it's a software problem.

[Irrwahn Grausewitz]

Irrwahn Grausewitz <ir*****@freene t.de> wrote in
<l9************ *************** *****@4ax.com>:

Well, for square arrays i and j are interchangeable , as long as
you use them in a consistently, but it works for all n-dimensional ^^^^<del>
int *arr = (int*) malloc( ZDIM * YDIM * XDIM * sizeof *arr );

^^^^^^
Damn, now I've copied this stupid cast!

--
Learn how to splel, dmanit!

[llewelly]

Irrwahn Grausewitz <ir*****@freene t.de> writes:

Irrwahn Grausewitz <ir*****@freene t.de> wrote in
<l9************ *************** *****@4ax.com>:

Well, for square arrays i and j are interchangeable , as long as
you use them in a consistently, but it works for all n-dimensional

^^^^<del>

int *arr = (int*) malloc( ZDIM * YDIM * XDIM * sizeof *arr );

^^^^^^
Damn, now I've copied this stupid cast!

That's ok. Weirdos who compile your code on boxen where int and int*
are not the same size deserve undefined behavior. Especially if
you forgot to include a header. Remember, all the world is a
wintel32.

(Note to c.l.c++ readers: that cast isn't necessary or safe in C++
either; the alternatives are:
vector<int> arr(ZDIM * YDIM * XDIM);
int *arr= new [ZDIM * YDIM * XDIM * sizeof *arr];
int *arr= static_cast<int *> malloc( ZDIM * YDIM * XDIM * sizeof *arr );
)

[Jim Fischer]

Sona wrote:

Please could someone tell me how to use a single-dimensional array as a
two-dimensional array? Is the following correct:

int ROW_SIZE = 5;
int COL_SIZE = 3;

int *arr = (int*) malloc(sizeof(i nt) *ROW_SIZE*COL_S IZE);

arr[i *COLUMN_SIZE + j];

will this only work for square arrays or all two-dim arrays ? Thanks :)

Consider the following C++ program

<example>
void foo(int **b, int rows, int cols)
{
for (int r = 0; r < rows; ++r) {
for (int c = 0; c < cols; ++c) {
b[r][c] = r*c + c;
}
}
}

int main()
{
int rows = 5;
int cols = 3;
int **a = 0;

try {
a = new int*[rows];
a[0] = 0;
a[0] = new int[rows * cols];
for (int r = 1; r < rows; ++r)
a[r] = a[r-1] + cols;

// use the "2D array" 'a' here
foo(a, rows, cols);
}
catch (...) {
// whatever
}

// house keeping
if ( a ) {
delete[] a[0];
delete[] a;
}

return 0;
}
</example>

--
Jim

To reply by email, remove "link" and change "now.here" to "yahoo"
jfischer_link58 09{at}now.here. com

[Kevin Goodsell]

Irrwahn Grausewitz wrote:

Sona <so**********@n ospam.net> wrote in
<3f********@cla rion.carno.net. au>:

Please could someone tell me how to use a single-dimensional array as a
two-dimensional array? Is the following correct:

int ROW_SIZE = 5;
int COL_SIZE = 3;

int *arr = (int*) malloc(sizeof(i nt) *ROW_SIZE*COL_S IZE);

Nit1: arr isn't an array, albeit you can apply the [] operator
to an pointer to int (which it is).

Nit2: the cast to int* is superfluous in C.

It's a bit worse than superfluous. It can be dangerous (as can any cast).

It's necessary in C++, but in C++ you normally shouldn't use 'malloc' at
all. In nearly all cases you should use 'new' instead.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

[Irrwahn Grausewitz]

Kevin Goodsell <us************ *********@never box.com> wrote in
<3f539667@shkne ws01>:

It's necessary in C++, but in C++ you normally shouldn't use 'malloc' at
all. In nearly all cases you should use 'new' instead.

From the use of malloc() in the OP's example I derived that this was a
C-question - though it was cross-posted to c.l.c and c.l.c++, like your
reply, and mine too...

--
I wish life had a scroll-back buffer.

[Al Bowers]

Sona wrote:

Please could someone tell me how to use a single-dimensional array as a
two-dimensional array? Is the following correct:

int ROW_SIZE = 5;
int COL_SIZE = 3;

int *arr = (int*) malloc(sizeof(i nt) *ROW_SIZE*COL_S IZE);

arr[i *COLUMN_SIZE + j];

will this only work for square arrays or all two-dim arrays ? Thanks :)

I like:
int i, **arr;
arr = malloc(sizeof(* arr)*ROW_SIZE);
for(i = 0;i < ROW_SIZE; i++)
arr[i] = malloc(sizeof(* *arr)*COL_SIZE) ;
arr[4][2] = 6; /* assuming successful allocations */

This is only one of various ways you can do this. The faq
demonstrates this technique and others at:
http://www.eskimo.com/~scs/C-faq/q6.16.html

--
Al Bowers
Tampa, Fl USA
mailto: xa*@abowers.com base.com (remove the x)
http://www.geocities.com/abowers822/

[foo]

llewelly <ll*********@xm ission.dot.com> wrote in message news:<86******* *****@Zorthluth ik.local.bar>.. .

Irrwahn Grausewitz <ir*****@freene t.de> writes:

Irrwahn Grausewitz <ir*****@freene t.de> wrote in
<l9************ *************** *****@4ax.com>:

Well, for square arrays i and j are interchangeable , as long as
you use them in a consistently, but it works for all n-dimensional ^^^^<del>
int *arr = (int*) malloc( ZDIM * YDIM * XDIM * sizeof *arr );

^^^^^^
Damn, now I've copied this stupid cast!

That's ok. Weirdos who compile your code on boxen where int and int*
are not the same size deserve undefined behavior. Especially if
you forgot to include a header. Remember, all the world is a
wintel32.

(Note to c.l.c++ readers: that cast isn't necessary or safe in C++
either; the alternatives are:
vector<int> arr(ZDIM * YDIM * XDIM);
int *arr= new [ZDIM * YDIM * XDIM * sizeof *arr];
int *arr= static_cast<int *> malloc( ZDIM * YDIM * XDIM * sizeof *arr );
)

IMHO, a better C++ alterantive would be to use either a
vector<vector<i nt> > object or even better yet, the following dynamic
2 dimensional array object:
template < class T, int ROW_T = 0, int COL_T = 0 >
class dynamic_2d_arra y
{
public:
dynamic_2d_arra y(int row, int col):m_row(row) ,m_col(col),
m_data((row!=0& &col!=0)?new T[row*col]:NULL){}
dynamic_2d_arra y():m_row(ROW_T ),m_col(COL_T), m_data(new
T[ROW_T*COL_T])
{if (!COL_T || !ROW_T) {int x[ROW_T] = {{ROW_T}};int y[COL_T] =
{{x[0]}};}}
~dynamic_2d_arr ay(){if(m_data) delete []m_data;}
T* operator[](int i) {return (m_data + (m_col*i));}
T const*const operator[](int i) const {return (m_data +
(m_col*i));}
private:
const int m_row;
const int m_col;
T* m_data;
};
See following link for usuage example and for vector<vector<t ype> >
example:

http://axter.com/faq/topic.asp?TOPIC..._ID=4&CAT_ID=9

[Irrwahn Grausewitz]

ma*******@axter .com (foo) wrote in
<c1************ **************@ posting.google. com>
a lot of semi-cryptical C++ code. Please do not cross-post
C++ stuff to c.l.c.
--
When things look dark,
hold your head high so it can rain up your nose.