| Re: Sort a HASH [message #76664] |
Fri, 24 June 2011 03:52  |
Gray
Messages: 253
Registered: February 2010
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Senior Member |
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On Jun 24, 4:37 am, JDS <jdtsmith.nos...@yahoo.com> wrote:
> You appear to be asking why a HASH is not ordered. Having been familiar with HASH's from other languages, I did not expect them to be ordered. What is peculiar here, is that, without converting hashes into a more primitive data type, there is no way to order them yourselves (or rearrange them in any way, other than a loop)! That is in stark contrast to other languages. For example, a common idiom in Perl is:
>
> foreach key (sort keys %hash) {
> % do something
>
> }
>
> Is there really no equivalent in IDL?
Well, I was asking why when I indexed a hash were the elements not
returned in the order I indexed them.
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| Re: Sort a HASH [message #76696 is a reply to message #76664] |
Wed, 22 June 2011 08:30  |
Gray
Messages: 253
Registered: February 2010
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Senior Member |
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On Jun 22, 8:06 am, JDS <jdtsmith.nos...@yahoo.com> wrote:
> How can you sort a HASH() in IDL?
>
> IDL> h=hash(['a','c','d','b'],[7,1,5,8])
> IDL> print,h
> c: 1
> a: 7
> b: 8
> d: 5
>
> OK, they are randomly sorted. That's cool, it's a hash. I'll just sort on the hash key...
>
> IDL> s=sort(h->keys())
> IDL> print,s
> 0
>
> Hmmm, it seems this isn't sorting correctly...
>
> IDL> help, h->keys()
> <Expression> LIST <ID=3175 NELEMENTS=4>
>
> Ahah, the return of Keys() is a list! (Why does SORT pretend it can sort a LIST?). We need an array. Continuing onwards...
>
> IDL> s=sort((h->keys())->toarray())
> IDL> print,(h.keys())[s]
> a
> b
> c
> d
>
> Now we're getting there. Let's just index...
>
> IDL> print,h[(h.keys())[s]]
> c: 1
> a: 7
> b: 8
> d: 5
>
> Uhhh... what? Maybe it has something to do with indexing via a LIST. Let's make it an array first:
>
> IDL> print,((h.keys()).toArray())[s]
> a b c d
>
> Should be perfect...
>
> IDL> print,h[((h.keys()).toArray())[s]]
> c: 1
> a: 7
> b: 8
> d: 5
>
> But it's not!
>
> Is there any way to index a hash and have the order of the keys be maintained, for example for this sorting problem? Loss of order is a well known feature of hash storage, but when you explicitly specify the order, it should be respected. Am I missing something?
http://groups.google.com/group/comp.lang.idl-pvwave/browse_t hread/thread/fa55eae4c4be5880/012750823bc9dce2
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| Re: Sort a HASH [message #76697 is a reply to message #76696] |
Wed, 22 June 2011 08:33 |
Gray
Messages: 253
Registered: February 2010
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Senior Member |
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On Jun 22, 8:06 am, JDS <jdtsmith.nos...@yahoo.com> wrote:
> How can you sort a HASH() in IDL?
>
> IDL> h=hash(['a','c','d','b'],[7,1,5,8])
> IDL> print,h
> c: 1
> a: 7
> b: 8
> d: 5
>
> OK, they are randomly sorted. That's cool, it's a hash. I'll just sort on the hash key...
>
> IDL> s=sort(h->keys())
> IDL> print,s
> 0
>
> Hmmm, it seems this isn't sorting correctly...
>
> IDL> help, h->keys()
> <Expression> LIST <ID=3175 NELEMENTS=4>
>
> Ahah, the return of Keys() is a list! (Why does SORT pretend it can sort a LIST?). We need an array. Continuing onwards...
>
> IDL> s=sort((h->keys())->toarray())
> IDL> print,(h.keys())[s]
> a
> b
> c
> d
>
> Now we're getting there. Let's just index...
>
> IDL> print,h[(h.keys())[s]]
> c: 1
> a: 7
> b: 8
> d: 5
>
> Uhhh... what? Maybe it has something to do with indexing via a LIST. Let's make it an array first:
>
> IDL> print,((h.keys()).toArray())[s]
> a b c d
>
> Should be perfect...
>
> IDL> print,h[((h.keys()).toArray())[s]]
> c: 1
> a: 7
> b: 8
> d: 5
>
> But it's not!
>
> Is there any way to index a hash and have the order of the keys be maintained, for example for this sorting problem? Loss of order is a well known feature of hash storage, but when you explicitly specify the order, it should be respected. Am I missing something?
I don't see my first post of this... so apologies if it appears
twice. I had the same question a little while ago:
http://groups.google.com/group/comp.lang.idl-pvwave/browse_t hread/thread/fa55eae4c4be5880/012750823bc9dce2
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| Re: Sort a HASH [message #76754 is a reply to message #76664] |
Fri, 24 June 2011 15:28 |
chris_torrence@NOSPAM
Messages: 528
Registered: March 2007
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Senior Member |
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Hi all,
The reason this isn't working is because when you index a hash, you
get back another hash. It doesn't matter the order of the keys when
you do the indexing, IDL will create a new hash with the desired key/
value pairs, and they will be in whatever order the hash determines.
A way around this is to just get both the keys & values out, then sort
on those:
h=hash(['a','c','d','b'],[7,1,5,8])
keys = (h.KEYS()).toArray()
val = (h.VALUES()).toArray()
s = SORT(keys)
print, keys[s], val[s]
a b c d
7 8 1 5
If you were worried about making an extra copy of the values, you
could use a foreach loop instead:
foreach key, (h.KEYS())[s] do print, key, h[key]
a 7
b 8
c 1
d 5
This will work in IDL 8.1.
Perhaps a better approach would be for us to add a SORT keyword to the
KEYS() method. So you could do something like:
foreach key, h.KEYS(/SORT) do print, key, h[key]
What do you all think about that?
-Chris
ITTVIS
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