Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!snorkelwacker!bloom-beacon!daemon
From: scs@adam.mit.edu (Steve Summit)
Newsgroups: comp.lang.c
Subject: Re: Answers to Frequently Asked Questions (FAQ) on comp.lang.c
Summary: buglet in 2D array allocation examples
Message-ID: <1990Aug2.221734.10191@athena.mit.edu>
Date: 2 Aug 90 22:17:34 GMT
Expires: 1 Aug 90 05:00:00 GMT
References: <1990Aug1.042116.20244@athena.mit.edu>
Sender: daemon@athena.mit.edu (Mr Background)
Reply-To: scs@adam.mit.edu (Steve Summit)
Organization: Thermal Technologies, Inc.
Lines: 21

In article <1990Aug1.042116.20244@athena.mit.edu> I wrote:
>A:  It is usually best to allocate an array of pointers, and then
>    initialize each pointer to a dynamically-allocated "row."
>
>         int **array = (int **)malloc(nrows * ncolumns * sizeof(int *));
>         for(i = 0; i < nrows; i++)
>                 array[i] = (int *)malloc(ncolumns * sizeof(int));

This code wastes space and is misleading.  The initial "dope
vector" need only contain a pointer for each row of the intended
array, not each element:

          int **array = (int **)malloc(nrows * sizeof(int *));

The same problem afflicts the second posted 2D allocation
example, as well.

Thanks to Freek Wiedijk for pointing this out.

                                            Steve Summit
                                            scs@adam.mit.edu