| Sort a HASH [message #76701] |
Wed, 22 June 2011 05:06 |
JDS
Messages: 94
Registered: March 2009
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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?
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