AW: [Caml-list] paralell assignment problem

AW: [Caml-list] paralell assignment problem

[Beck01, Wolfgang]

> The solution is a sequence of assignments involving
> only xi, ei, and ti, where ti are temporaries introduced
> to save the values of the expressions. For example,
> a solution to
> 
> 	x,y = y,x
> 
> is
> 
> 	t = x; x = y; y = t
> 
> I seek a solution which minimises the number of assignments. 
> Note that the relation 
you probably know the 'xor trick' to swap value without a temporary:
// x = A; y = B
x = x lxor y;
// x = A XOR B; y = B
y = x lxor y;
// x = A XOR B; y = A XOR B XOR B
// x = A XOR B; y = A
x = x lxor y;
// x = A XOR B XOR A; y = A
// x = B; y = A

Certainly not what you were looking for, but always nice :-)


Wolfgang

Re: AW: [Caml-list] paralell assignment problem

[skaller]

On Wed, 2005-02-09 at 01:24, Beck01, Wolfgang wrote:

> > I seek a solution which minimises the number of assignments. 
[]
> you probably know the 'xor trick' to swap value without a temporary:

I've heard of it :)

> Certainly not what you were looking for, but always nice :-)

You are right, it isn't useful here for three reasons:

(1) the values being assigned aren't necessarily bitwise
  copyable, in fact they're (more or less) arbitrary C++ values

(2) The intent is to reduce time not storage

(3) The RHS can be arbitrary expressions. To apply XOR,
we'd first assign each RHS expression to a temporary ..
and having done that we no longer need XOR, we can just
assign variables to those temporaries.

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