NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part2/15356

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

Path: lrz-muenchen.de!news
From: [e-mail] (Kurt Watzka)
Newsgroups: comp.lang.c
Subject: Re: passing 2D arrays to functions
Date: 18 Apr 1996 17:22:59 GMT
Organization: Leibniz-Rechenzentrum, Muenchen (Germany)
Distribution: world
Message-ID: <4l5tpj$[e-mail]>
References: <4l58sk$[e-mail]>
NNTP-Posting-Host: sun2.lrz-muenchen.de

[e-mail] (Mr Paul Rodger) writes:

>Greets -

>I need to pass a 2-dimensional array to a function. Obviously I just want
>to pass a pointer to the first element, and in the past when I wanted to
>pass a one-dimensional array to a function I did the following which should
>work fine:

>fun(int array[])
>{
>...
>}

>But when I try to pass a 2D array using the same method, slightly different:

>fun(int array[][])
>{
>...
>}

This is more than slightly different from the above. "array" is a pointer
to int in the first example, whereas it is a pointer to an array of
unspecified size of int (i.e. an incomplete type) in the second example.

While there is nothing wrong with passing pointers to incomplete types
in C, it is impossible to use an incomplete type to access elements or
members of the complete type behind it.

>main()
>{
>        int array[10][10];
>...
>        fun(array);
>}

>I'm getting errors:

>main.c:221: arithmetic on pointer to an incomplete type
>make: *** [main.o] Error 1

>This occurs when I try to look at elements in the array when I am
>in the 'fun' function:

>array[1][1] = 1;

>Would this be because of vagueness about row-major or column-major form?

No, certainly not. It is because in C, an array of unspecified size is an
incomplete type.

>I've got around this in the past by putting the 2D array in a struct,
>and passing a pointer to a struct, but in this case I can't do that
>because the array size could vary as it is declared various times in
>iterations. (It isn't declared in main like above - this is just a
>simplication).

The FAQ explains one method for handling two-dimensional dynamic arrays.
If you need only the first dimension of the array to be dependent on
dynamic information, try

   int fun(int (* array)[10]);

You can pass an array of N arrays of 10 ints to "fun()", where N can
be different in different invocations of the function. (You will have
to pass the actual size of the array as an additional parameter).

Kurt
--
| Kurt Watzka                             Phone : [phone]
| [e-mail]