Re: Vectorizing loops

[marco atzeri]

On 1/12/2012 8:59 PM, andrewcd wrote:
> Bumping an old thread of mine with a new (similar) question.  I hope I'm not
> being a pest, but I'm an occasional user at best -- I mostly work in stata,
> but occasionally I need to do simulations (or huge-N stats).
>
> My problem:  I'm trying to simulate diffusion of a thing in space, and to do
> that I need to calculate the Euclidian distance between all of the nodes.  I
> have it below in loop format.  it is slow enough with 1000 units, but I want
> to bring it to 1 million once I get the rest of the code finished.  Any
> hints on how I could vectorize this particular loop?
>
> Many thanks, muchas gracias, asante sana.
>
> peeps=1000
>
>
> %Procedure
> %1) Define Space
> x = 100
> y = 100
> %2)Define units
>         folks = ones(peeps,1)
>         %-Give these folks coordinates
>         folks(:,2) = rand(peeps,1)*x;
>         folks(:,3) = rand(peeps,1)*y;
> %3) Define distances between dudes
> for i = 1:peeps
> for j = 1:peeps
>         dist(i,j) = (abs(folks(i,2)-folks(j,2)).^2 +
> abs(folks(i,3)-folks(j,3)).^2).^.5;
>         end end
>
> --

for peeps = 1e6 , I guess you will run out of memory as dist size
will be 1e12

With next 3.6.0 broadcasting

folks = [ones(peeps,1), rand(peeps,1)*x, rand(peeps,1)*y];

dist = sqrt((folks(:,2)-folks(:,2)').^2 + (folks(:,3)-folks(:,3)').^2 ));


Regards
Marco