NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part2/12729

Internet Message Format | 1996-08-05 | 3.3 KB

Path: mail2news.demon.co.uk!genesis.demon.co.uk
From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: [Q] Experts: Multi-dim dynamic arrays
Date: Tue, 02 Apr 96 11:29:11 GMT
Organization: none
Distribution: usa
Message-ID: <[e-mail]>
References: <4jqc3d$[e-mail]>
Reply-To: [e-mail]
X-NNTP-Posting-Host: genesis.demon.co.uk
X-Newsreader: Demon Internet Simple News v1.27
X-Mail2News-Path: genesis.demon.co.uk

In article <4jqc3d$[e-mail]>
           [e-mail] "qwertyuiop" writes:

>Here is a synopsis of the code I tried:  I get no compilation errors, but
>during execution, I get floating point exception at the line marked XX.
>(Necessary file pointers, header files etc. are included but not shown here!.)
>
>int **imatrix(int nrl,int nrh,int ncl,int nch);
>main()
>{
>
>int i, j; /* loop variables */
>int R, C; /* number of rows, columns */
>int **Table; /* table of data */
>int X; /* scratch variable */
>
>fscanf(fileptr, "%d %d\n", &R, &C);
>Table = imatrix(1,R,1,C);

You are attempting (whether successfully or not) to create an effective 2D
array indexed from 1 to R, 1 to C.

>for(i = 0; i < R; i++)
>   {
>   for(j = 0; j < C; j++)

And now you try to use it indexed from 0 to R-1, 0 to C-1. Clearly you
are going out of bounds. Allocate with:

 Table = imatrix(0,R-1,0,C-1);

>      {
>        fscanf(fileptr, "%d", &X);   <------There is no need for X here, still!
>        *(*(A+i)+j) = X;     XX<------floating point exception occurs here!

More commonly and clearly written as:

         A[i][j] = X;

You haven't declared A here, did you mean Table?

>      }
>   fscanf(fileptr, "\n");

While it probably works here remember that "\n" matches while-space in general,
it doesn't simply read until the end of the line. You don't need it before %d
since that skips leading white-space.

>   }
>....
>}
>
>int **imatrix(int nrl, int nrh, int ncl, int nch) /* see numerical recipes! */
>{
>int i, **m;
>m = (int **)malloc((unsigned)(nrh-nrl+1)*sizeof(int*));
>if(!m) nerror("alloc failure, exiting!..\n");
>m -= nrl;
>
>for(i=nrl;i<=nrh;i++)
>   {
>   m[i] = (int*)malloc((unsigned)(nch-ncl+1)*sizeof(int));
>   if(!m[i]) nerror("alloc failure etc...\n");
>   m[i] -= ncl;
>   }
>return m;
>}

This is a classic example of illegal code and a well known problem with
Numerical Recipies. The problem is that m -= nrl and m[i] -= ncl typically
generate pointers pointing outside the array which results in undefined
behaviour. It is OK in this case since you should be passing nrl and ncl as
0.

>----------------------------------------------------------------------------
>Questions: 1. On my machine, size of int, int * are both 4 bytes. I'm using
>a gnu compiler on a Unix machine.  Why do I get a floating point exception
>at the line mentioned above?

You invoked undefined behaviour - anything can happen.

>2. The imatrix function returns a pointer to an array of pointers to rows.
>I ran a quick address and content check to see if my dereferencing of pointers
>is faulty.  There is  a difference of 32 bytes between the address of Table[i]
>and Table[i+1], how come?

Do you mean a difference in the values?

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
-----------------------------------------