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From: debray@cs.arizona.edu (Saumya K. Debray)
Newsgroups: comp.lang.prolog
Subject: incrementing values
Message-ID: <17467@megaron.cs.arizona.edu>
Date: 2 Feb 90 03:52:55 GMT
Organization: U of Arizona CS Dept, Tucson
Lines: 29


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:
| 
| 	increment(Variable) :-
| 		retract(value(Variable, OldValue)),
| 		NewValue is OldValue + 1,
| 		assert(value(Variable, NewValue)).
| 
| 	..., increment(x), ...
| 
| 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?
-- 
Saumya Debray		CS Department, University of Arizona, Tucson

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