Re: [BLACKBOX] LibVector question

From: [at]} <Robert>
Date: Fri, 27 Mar 2009 21:13:54 +0000

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Wojtek Skulski wrote:

> Meanwhile I have a question. Your function LibVectors.Neg is allocating
> a new vector. Why? Flipping the sign of every vector element can be done
> in situ. Why are you calling NEW?

And the same comment applies to other functions, such as Abs, Mag, Map etc.

One has to make design choices, which usually have pros & cons. It is rare that
one choice is superior from every aspect.

I assume you have read the section in the Docu called "Assignments".

The issue is:

VAR
   a, b : LibVectors.Vector;
BEGIN
   a := .....;
   b := a;

   LibVectors.Neg (b);


If this was in-place it would also negate the 'a', whereas with
my design:

   b := LibVectors.Neg (b); (* Neg inclueds a NEW *)

I have negated b without the non-obvious side effect of negating a.

I think this is safer, less prone to accidental error.

It is not 'completely safe'; to do that Vectors would have to be immutable with
their components set to be Read-only (somehow). This I considered too
inconvenient and inefficient, hence the design compromise you see.


By contrast the design philosophy for the type LibComplex.Complex is 'safe'; the
type is immutable, the fields are read-only, and 'Big' sums with Complexes
generate a lot of garbage.


I am sure there are applications where these design choices are sub-optimal, if
not unacceptable. But that would be true of any choice. C'est la vie.


Cheers

Robert

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Received on Fri Mar 27 2009 - 22:13:54 UTC