From 2446057992158347986 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: f78e5,be4e234b6733532 X-Google-Attributes: gidf78e5,public From: Daniel Barker Subject: Re: why are pointers necessary? Date: 1997/08/04 Message-ID: #1/1 X-Deja-AN: 262433210 References: <33D43DA8.73D7@ix.netcom.crud.com> <5r3g4e$t5t$1@news.wco.com> X-Original-Date: Mon, 4 Aug 1997 05:44:47 +0100 Organization: Edinburgh University X-Auth: PGPMoose V1.1 PGP comp.std.c++ iQBVAwUBM+Y310y4NqrwXLNJAQFjdAIAr/qpAC7MILopKQZaGaUkQl87y3kSXxg+ voFYX3IaWqgqCVWXea8GUtQ4kFzyWURDlRMaaBDUMOGU6nFgkzU8qw== =5iSw Newsgroups: comp.std.c++ Originator: austern@isolde.mti.sgi.com On 23 Jul 1997, Dave Schreiber wrote: > In article <33D43DA8.73D7@ix.netcom.crud.com>, > Paul D. DeRocco wrote: > [...] > > while (*dest++ = *src++); > > > >which copies a null-terminated string. To do this without pointers would > >involve array indexing, and a separate local variable: > > > > int i; > > while (dest[i] = src[i]) i++; > > > >The former method spares the local variable, but provides no protection > >against scribbling in memory given bad arguments. The latter provides a > >place where bounds checking could be done, assuming the arrays are smart > >enough, but is less efficient. > > The efficiency depends on the implementation. For example, gcc on my Sun The best way to achieve efficient loops with arrays on a 'typical' modern RISC computer is to use for loops (with no conditional statements in the loop) where reasonably possible, and leave the rest to the compiler. The opportunities for efficient pipelining and, on multiprocessor architectures, automatic parallelization are greater. In this example, keep a record of the length of strings, then do for (i = 0; i < len; ++i) dest[i] = src[i]; Since there is no data dependency within the loop, all the assignments (len of them) can be done at once. The while versions gnarl things up, because the upper limit of i is unknown beforehand. On the data structures side, I prefer to implement trees using array indices rather than pointers to show connections. The fact that the array (tree) as we know it is really a pointer to an array, since it has been obtained dynamically, can be ignored for most of the program. Each element of the array might be struct node { int left; /* array index of left child */ int right; /* array index of right child */ } The biggest advantage is the temptation to allocate for each branch separately is not there, so the whole tree is in contiguous memory for efficient caching. I've written an entirely non-commercial C program which makes almost minimal use of pointers and other cryptic features of C. Basically I wanted to emulate Fortran 90, without having to learn all the new syntax. If anyone is interest the program source can be got from http://www.icmb.ed.ac.uk/sokal.html If there were dynamic true arrays and a few changes to the standard library, we could all forget about explicit pointer operations forever. Daniel Barker, Institute of Cell and Molecular Biology, University of Edinburgh, Daniel Rutherford Building, King's Buildings, Mayfield Road, Edinburgh EH9 3JR --- [ comp.std.c++ is moderated. To submit articles: Try just posting with your newsreader. If that fails, use mailto:std-c++@ncar.ucar.edu comp.std.c++ FAQ: http://reality.sgi.com/austern/std-c++/faq.html Moderation policy: http://reality.sgi.com/austern/std-c++/policy.html Comments? mailto:std-c++-request@ncar.ucar.edu ]