Novice C question

[Gustaaf-Jan Heinhuis]

In article <31969 at usc> ajayshah at 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.

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