Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!cs.utexas.edu!uunet!odi!jack
From: jack@odi.com (Jack Orenstein)
Newsgroups: comp.lang.c++
Subject: Re: puzzle/problem/yet another new feature
Message-ID: <1989Oct26.135247.20151@odi.com>
Date: 26 Oct 89 13:52:47 GMT
References: <32164@ucbvax.BERKELEY.EDU>
Reply-To: jack@odi.com (Jack Orenstein)
Organization: Object Design Inc., Burlington, MA
Lines: 33

In article <32164@ucbvax.BERKELEY.EDU> othar@ernie.Berkeley.EDU (Othar Hansson) writes:
>Imagine that I had a really nice matrix class definition, and wrote
>the following code:
>
>#include "reallynicematrixclass.h" 
>matrix A,B,C,D,E;
>
>...  // give the matrices bounds, and fill them with values
>
>matrix ANSWER = A * B * C * D * E;   // use matrix::operator*(matrix&, matrix&)
>
>Now, if I remember, C++ and C will parenthesize this expression from
>left to right, evaluating A*B first, etc.  However, one of the first
>things we learn in a class on algorithms is the dynamic programming
>example of optimizing the order of matrix multiplications.  Depending
>on the matrix bounds, this might be the parenthesization which runs
>the fastest:
>
>ANSWER = (A * B) * ((C * D) * E)
>...
>In this case, where the bounds are run-time defined, the programmer
>can't parenthesize the expression in advance.  In C++ version 8.0, how
>are we going to allow the code to decide order of evaluation at
>run-time?

Probably the same as in 1.2 or 2.0. Define matrix::operator+,
operator*, etc. to construct a tree representing the expression. Then,
when the value of the expression is needed, e.g. in assignment to
ANSWER above, or in a comparison, optimize and evaluate.


Jack Orenstein
Object Design, Inc.