From: mortee <mortee.lists@...>
Date: 2007-10-26T10:49:22+09:00
Subject: Re: Intervals in Ruby

ara.t.howard wrote:
> well, frankly, that example is made up but i have tons of image
> processing code that does that in spirit - the point is that using
> #each_with_index is going to give you errors: you cannot iterate using
> map/each/each_with_index *while* also modifying a datastructure - try this:
> 
>   cfp:~ > ruby -e' (a=[42]).each_with_index{ a << 42 } '
>   ...
> 
> it will never exit.
> 
> i certainly abstract that kind of code to yield the pixel and it's
> neighbors (it doesn't always have them near the edges of an image) but
> abstracting it will require both iteration by size *and* using random
> access unless the algorithm is read only.  you can see
> http://kogs-www.informatik.uni-hamburg.de/~koethe/vigra/ for example of
> how neighborhood iterators can generally be abstracted - my point is
> just that the this *type* of problem - iterating while modifying and
> looking at neighbors - requires an external iterator if one is using the
> 'obvious' solution.

Fair enough. However, the code in question hasn't altered the array of
pixels itself, just the pixel values. It's true that you have to pay
more attention when adding and/or deleting items while iterating.

mortee