Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!accelerator.eng.ohio-state.edu!kaa.eng.ohio-state.edu!rob
From: rob@kaa.eng.ohio-state.edu (Rob Carriere)
Newsgroups: comp.lang.c
Subject: Re: Numerical Recipes is non-portable code
Message-ID: <564@accelerator.eng.ohio-state.edu>
Date: 5 Sep 88 01:33:44 GMT
References: <557@accelerator.eng.ohio-state.edu> <15029@ism780c.isc.com>
Sender: news@accelerator.eng.ohio-state.edu
Reply-To: rob@kaa.eng.ohio-state.edu (Rob Carriere)
Organization: Ohio State Univ, College of Engineering
Lines: 54

In article <15029@ism780c.isc.com> marv@ism780.UUCP (Marvin Rubenstein) writes:
>In article <557@accelerator.eng.ohio-state.edu> rob@kaa.eng.ohio-state.edu 
>(Rob Carriere) writes:
>> [ still on non-0 base arrays ]
>Could you give a small example, perhaps I don't understand. with a
>declaration (using an extended version of C):
>
>     int x(5:49),y(0:44);
>     int i,k;
>     i=6;
>     k=x(i);        /* easy to write access to second item */
>     k=y(i-5);      /* more trouble to write access to second item */
>
>However, with a reasonable compiler, both assignments to k will generate
>identical code.  Your compiler may vary :-).  But if you are writing time
>critical code with a faulty compiler, I guess you do what you can.

You more or less have the idea (except that the array would be float,
I'm one of them there dirty numerical geeks :-).  Actually, a
reasonable (as opposed to a *very* good) compiler would probably
generate better code for k=x[i]; because it cannot do the optimization
I'm trying to manually make either: illegal pointer!  To add insult to
the poor compiler's injury, in most cases the code would actually
read:

void foo( int n )
{
    float *x;
    int i;
    float k;

    i = 6;
    x = vector( 5, n );
...
    k = x[i];
...
}
...
foo( 49 );

As a further and at least as important point, the code is generated
from some underlying math that insists that x starts at (say) 5.  So I
can fake it and confuse everyone with my constant -5 offsets in all
the indeces, or I can use a non-zero base offset.

If you are curious as to what perverse math would generate such stuff
(and it's *not* because they don't know to start counting at zero!), I
sent off a mail message with a couple of examples to Marty Fouts who
asked the same question.  It's a bit too long and too marginal to this
group to post, but I'd gladly send you a copy (be sure to give me a
*very* complete email address, our mail deamon spends far more time
earning its last than its first name!)

Rob Carriere