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From: mcdonald@uxe.cso.uiuc.edu
Newsgroups: comp.lang.fortran
Subject: Re: Fortran versus C for numerical anal
Message-ID: <50500075@uxe.cso.uiuc.edu>
Date: 16 Sep 88 13:53:00 GMT
References: <893@amelia.nas.nasa.gov>
Lines: 69
Nf-ID: #R:amelia.nas.nasa.gov:893:uxe.cso.uiuc.edu:50500075:000:1775
Nf-From: uxe.cso.uiuc.edu!mcdonald    Sep 16 08:53:00 1988


From article <1511@ficc.uu.net>, by peter@ficc.uu.net (Peter da Silva):
> 	(*twodarray)[10][5] = 'c';
> 
> Now you have.
Reply by J. Giles:
>Gee, that looks like and array of pointers to me.  If there's no pointers
>involved, what's that asterisk doing there?  This is certainly _not_
>an example of a 2-d array that is done without an array of pointers.

I tried to do this yesterday, but failed due to not getting the
parenthesis right. But Peter's example does indeed work, to wit
on my PC (microsoft 5.1):

main() {
char (*twodarray)[10][10];
int i;
        twodarray = malloc(sizeof(*twodarray));

       for(i = 0; i < 10; i++)
        (*twodarray)[i][3] = i;
}


_main   PROC NEAR
        push    bp
        mov     bp,sp
        mov     ax,4
        call    __chkstk
        push    si
;       twodarray = -4
;       i = -2
; Line 4
        mov     ax,100
        push    ax
        call    _malloc
        add     sp,2
        mov     WORD PTR [bp-4],ax      ;twodarray
; Line 6
        mov     WORD PTR [bp-2],0       ;i
$FC106:
; Line 7
        mov     si,WORD PTR [bp-2]      ;i
        mov     ax,si
        shl     si,1
        shl     si,1
        add     si,ax
        shl     si,1
        mov     bx,WORD PTR [bp-4]      ;twodarray
        mov     al,BYTE PTR [bp-2]      ;i
        mov     [bx+3][si],al
        inc     WORD PTR [bp-2] ;i
        cmp     WORD PTR [bp-2],10      ;i
        jl      $FC106
; Line 8
        pop     si
        mov     sp,bp
        pop     bp
        ret

_main   ENDP


Sure looks like array indexing calculations to me. Of course,
for this example pointer tricks could be faster. Now if those
two array sizes could be VARIABLES ........ Why not add this,
even at this late date?  Unnecessary but convenient.

Doug McDonald