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Re: HASH -- bug, or "feature"? [message #75876 is a reply to message #75875]

Wed, 20 April 2011 12:50

Paul Van Delst[1]
Messages: 1157
Registered: April 2002

Senior Member

It's not a bug or a feature. It's a property of hashes. And it's not peculiar to IDL. E.g. from the Ruby pickaxe:

<quote>
Compared with arrays, hashes have one significant advantage: they can use any object as an index. However, they also
have a significant disadvantage: their elements are not ordered.
</quote>

Similarly with Python dictionaries.

Typically the need for hashes where insertion order is important leads to a subclass of Hash being created where the
order is internally tracked.

cheers,

paulv

Gray wrote:
> Can anyone tell me why, when you create a hash, the elements are not
> in the order of the keys you give? And, even worse, when you index a
> hash with an array of keys, the resulting hash does not come out in
> the same order as the array?
>
> IDL> h = hash(['a','b','c','d'],indgen(4))
> IDL> print, h
> c: 2
> a: 0
> b: 1
> d: 3
> IDL> print, h[['a','c','d']]
> c: 2
> a: 0
> d: 3
>
> This is making my bookkeeping using hashes basically impossible.
>
> --Gray

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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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