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Re: HASH -- bug, or "feature"? [message #75874 is a reply to message #75873]

Wed, 20 April 2011 14:26

Gray
Messages: 253
Registered: February 2010

Senior Member

On Apr 20, 4:33 pm, Mark Piper <mpi...@ittvis.com> wrote:
> Here's an example of the technique Mike suggests:
>
> IDL> names = list('red', 'green', 'blue')
> IDL> colors = hash()
> IDL> colors[names[0]] = [255,0,0]
> IDL> colors[names[1]] = [0,255,0]
> IDL> colors[names[2]] = [0,0,255]
>
> IDL> print, colors ; order is mangled
> green: 0 255 0
> blue: 0 0 255
> red: 255 0 0
>
> IDL> foreach n, names do print, n, colors[n] ; ordered
> red 255 0 0
> green 0 255 0
> blue 0 0 255
>
> This also works with an array (and a FOR loop) instead of a list.
>
> mp

This all makes perfect sense... except that it isn't really useful for
me. I had been using a hash so that I could retrieve and store
information (in the form of structures) about particular stars by
indexing with star IDs and not having to search over arrays or lists
for individual members. When I had information for a set of stars
where some but not all were already in the hash, I would do something
like this:

tmp = replicate({star},n_new)
old = where(star_hash.haskey(new_ids),n_old)
if (n_old gt 0) then tmp[old] =
(star_hash[new_ids[old]].values()).toarray()
tmp.info = new_info & tmp.id = new_ids
star_hash[new_ids] = tmp

But that doesn't work, because there's no guarantee that the values I
retrieve from the hash are in the same order as the array elements I'm
storing them in. What I end up having to do is

tmp = replicate({star},n_new)
old = where(star_hash.haskey(new_ids),n_old)
foreach i,old do tmp[i] = star_hash[new_ids[i]]

...and the rest is the same. I'm not sure whether this is more
inefficient or not... maybe it isn't, thanks to toarray() being slow.

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[Message index]

		Re: HASH -- bug, or "feature"? By: Gray on Thu, 21 April 2011 04:20
		Re: HASH -- bug, or "feature"? By: penteado on Wed, 20 April 2011 15:09
		Re: HASH -- bug, or "feature"? By: Gray on Wed, 20 April 2011 14:26
		Re: HASH -- bug, or "feature"? By: Mark Piper on Wed, 20 April 2011 13:33
		Re: HASH -- bug, or "feature"? By: Paul Van Delst[1] on Wed, 20 April 2011 12:50
		Re: HASH -- bug, or "feature"? By: Michael Galloy on Wed, 20 April 2011 12:19

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