Re: How do I replace this for-loop?

[Martin Weiser]

On Fri, 4 Jul 2008, Robert Macy wrote:

> What's the best way to replace this for-loop?
>
> Given an array, n, of datapoints, such that:
>
> > > size(n)
> rows = 27000
> columns = 2
>
> where n(:,1) are all the integer x values, and
> n(:,2) are all the integer y values,
> both integer values are in the range of 1 to 101
>
> I slowly did it this way:
>
> output=zeros(101,101);
> for i=1:27000
>  output( n(i,1),n(i,2) ) = output( n(i,1),n(i,2) ) + 1;
> endfor
>
> as you all know this is fairly slow.
> What's a simple way to replace this for-loop?
>
> Robert
Hi,
what about something like:
ni=zeros(27000,1);
ni=sub2ind(size(n),n(:,1),n(:,2));
ni=sort(ni); 
[breaks,garbage,indeces]=find(ni-shift(ni,1);
times=breaks-shift(breaks,1);
output=zeros(101,101);
output(indeces)=times;

(I would guess that people here will come with something much more 
straightforward and elegant, maybe with an one-liner.)
I have not tested that, but HTH
Martin W.

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