Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!hp4nl!dri500!heinhuis
From: heinhuis@dri.nl (Gustaaf-Jan Heinhuis)
Newsgroups: comp.lang.c
Subject: Re: Novice C question
Message-ID: <992@dri500.dri.nl>
Date: 16 Apr 91 08:52:52 GMT
References: <31969@usc>
Organization: DataCount Register Informatici, Wierden, The Netherlands
Lines: 46

In article <31969@usc> ajayshah@almaak.usc.edu (Ajay Shah) writes:
>Consider this fragment of C code (from Numerical Recipes):
>
>1	double *dvector(nl,nh)
>2	int nl,nh;
>3	{
>4	        double *v;
>5	
>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.  If you want to allocate an array going from 5 to 10,
>you would say p = dvector(5, 10).
>
>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?
>
>Thanks!
>
Well, I know why he substracts nl. The fact that you have an array from
5 to 10 implies you also have indexes 0 to 4. The lowest index your
going to use is 5 which is an offset to a base adress. To make this work
the base adress has to 5 units (= nl, unit depends on type of reserved 
memory; here it's double) lower as the adress of the allocated block 
of memory.

You can, however, not use p[0] to p[4] because you didn't alloc any
memory for this part of the array.

Hope this answers your question, Gustaaf-Jan.

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++++++++++++---===To all virgins, thanks for nothing.===---++++++++++++++
          
   ___              GJ Heinhuis student comp. sc. HIO Enschede (Holland)
  /  _)             Final Project at: DataCount, Wierden, Holland
 |  ____ ___        Mail: heinhuis@dri.nl
  \__/|   |         
      |___|         Everybody is entitled to his own	
  |   |   |         opinion, as long as it's mine!
   \_/    |         Not necessarely DataCount's opinion!!