Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!mcsun!ukc!icdoc!syma!aarons
From: aarons@syma.sussex.ac.uk (Aaron Sloman)
Newsgroups: comp.lang.prolog
Subject: Re: incrementing values
Summary: Poplog Prolog can access Pop-11 variables.
Keywords: incrementing, global variables, prolog
Message-ID: <2099@syma.sussex.ac.uk>
Date: 4 Feb 90 16:51:22 GMT
References: <17467@megaron.cs.arizona.edu>
Organization: School of Cognitive & Computing Sciences, Sussex Univ. UK
Lines: 45

debray@cs.arizona.edu (Saumya K. Debray) writes:

> Date: 2 Feb 90 03:52:55 GMT
> Organization: U of Arizona CS Dept, Tucson
>
> This is an excerpt from an article posted recently to another newsgroup:
>
> | > the most frequently occuring construct in any programming language is
> | > equivalent to the construct x += 1
> |
> | Er, not quite *any* programming language.  I have been programming
> | professionally in Prolog for the last five years, and I need to do
> | this sort of thing maybe once or twice a year.
> |
> | BTW, here's how to do it:
    ...stuff omitted...
> | but as this makes two modifications to the database and is therefore
> | pretty expensive (not to mention poor style), you don't want to do it
> | too often.
>
> Comments?

Some readers may be interested to know that it's not a problem in
Poplog prolog because users have full access via the Poplog virtual
machine to Pop-11 (or, if required, Lisp) global variables.

For example Poplog Prolog provides two built in predicates

    prolog_setq(PopVariable, Value).    /*sets value of pop variable*/

    prolog_val(PopVariable, Value).     /*gets value of pop variable*/

So, if you have a global Pop-11 variable "count" you can do things like

    prolog_val(count, X), ...... prolog_setq(count, X + 1).

It is also possible for Poplog Prolog programs to access a full range of
Pop-11 data-structures including hash-tables, arrays, strings, etc. which
can considerably speed up some programs as well as making them clearer.

Aaron Sloman,
School of Cognitive and Computing Sciences,
Univ of Sussex, Brighton, BN1 9QH, England
    EMAIL   aarons@cogs.sussex.ac.uk
            aarons%uk.ac.sussex.cogs@nsfnet-relay.ac.uk