NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part2/12058

Internet Message Format | 1996-08-05 | 2.5 KB

Path: news1.intercall.com!usenet
From: [e-mail] (Steven Ovits)
Newsgroups: comp.lang.c
Subject: Re: Dynamic 2-D arrays
Date: Fri, 29 Mar 1996 02:56:55 GMT
Organization: Intercall Inc.
Message-ID: <4jf9t1$[e-mail]>
References: <[e-mail]>
NNTP-Posting-Host: ts2-112.intercall.com
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[e-mail] (Dspse Bedford) wrote:

>I want to define a 2-D array through a pointer-to-pointer structure member
>as shown below:

>typedef struct
>{ float **matrix;}Obj;
>main()
>{Obj *m;
> int x=5;y=10;
> m=(Obj *)malloc(sizeof(Obj));
> *(m->matrix)=(float *)malloc(x*y*sizeof(float *));
>  ...
> ...
> return;}

>if I index in the following way: m->matrix[i][j], how does it increment the
>pointer to determine the memory location?  If I have
>m->matrix[1][2] and m->matrix[2][1] what is the difference?

First, I hope you have some real memory at that location, you
allocated the pointers but not the floats.  You also have to point
the float* to the memory allocated for the floats.

The compiler treats a[][j] as if it were a type definition, and
calculates a[i][j] as:   *(a + i * sizeof(a[][j]) + j).
You should notice that the compiler must know the number of
elements in a row--in all rows except the first. That takes some
getting used to, but consider how you would get to a[1][0][0][0]
in a[][x][y][z] without knowing x, y, and z.

Now you can try calculating m[1][2] and m[2][1].

More formally, given float matrix[i][j],
matrix[p][q] is evaluated as *( * (matrix+p) + q)
but this hides the details.

To answer your question, write a macro.
#define idx2d(Array, i, j, dim1, type) \
(type) ( (Array) + (((i) * (dim1)) + (j)) * sizeof(type) )

>In the past, I would use something like whats below, but the difference is
>I do now know ahead of time the Y_dimension or the X_dimension.

>#define Y_DIM 10
>#define X_DIM  5
>typedef struct
>{float *matrix[Y_DIM];}Obj;
>main()
>{
>  Obj *m;
>  m=(Obj *)malloc(sizeof(Obj));
>  for (i=0;i<Y_DIM;++i)
>  { m->matrix[i]=(float *)malloc(X_DIM*sizeof(float));}
> ..
> return;}

>The above ofcourse indexes fine since I create an array of pointers of the
>proper Y-dimension in my structure definition.

>Any help would be appreciated.

>Thank you,
>Anastasios Maurudis
>[e-mail]
>DSP Software Engineering, Inc.
>175 Middlesex Turnpike
>Bedford, MA 01730
>[phone]
>[phone] Fax
>

>---
> * WR 1.32 # 331 * Blue Wave - what Smurfs do at a football game..