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From: gwyn@smoke.brl.mil (Doug Gwyn)
Newsgroups: comp.lang.c
Subject: Re: Novice C question
Message-ID: <15862@smoke.brl.mil>
Date: 16 Apr 91 20:04:35 GMT
References: <31969@usc>
Organization: U.S. Army Ballistic Research Laboratory, APG, MD.
Lines: 40

In article <31969@usc> ajayshah@almaak.usc.edu (Ajay Shah) writes:
>Consider this fragment of C code (from Numerical Recipes):

Ugh, but okay.

>1	double *dvector(nl,nh)
>2	int nl,nh;
>3	{
>4	        double *v;
>6	        v=(double *)malloc((unsigned) (nh-nl+1)*sizeof(double));
>7	        if (!v) nrerror("allocation failure in dvector()");
>8	        return v-nl;
>9	}
>It's supposed to be a function which allocates a vector of doubles.
>My interpretation of nl and nh is: they're array indexes.

They're supposed to be the minimum and maximum indices that will be
used to access the vector after it is allocated.

>Question: what is happening on line 8?  Why is he not just
>returning v (a pointer)?  What is the meaning of subtracting nl
>(an int) from v without any casting?

The intention is to be able to use dvector() like this:
	double *v = dvector(5,10);
	v[5] = 42.0;
	/* ... */
	v[10] = 3.1416;
The returned pointer is supposed to be offset so that v[5] will
access the first location in the object allocated by malloc().
Pointer_to_thing plus or minus some integer is an expression that
has type pointer_to_thing and a value that points to that integer
number of "things" away from the original pointer.

Unfortunately, v-nl does not necessarily point to anything valid
(who knows what lurks below the address returned by malloc()?),
so this function may fail (most likely catastrophically) on many
C implementations.

The Numerical Recipes C code is terrible; this is just one example.