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Re: Using value_locate with a non-monotonic vector! And it's working!??! [message #52062 is a reply to message #52054]

Tue, 02 January 2007 09:12

btt
Messages: 345
Registered: December 2000

Senior Member

Gianguido Cianci wrote:
> Hi all,
>
> I am using value_locate on a unsorted vector and I am a little worried
> because the IDL manual does not really say what happens in that case. I
> would greatly appreciate your opinion and suggestions...
>
> Say I have an unsorted vector of non-unique sparsely distributed
> numbers a:
>
> a=[4.3, 4.3, 2.1, 10000,2.1]
>
> and I want to create a 1 to 1 map to a list of consecutive numbers that
> indicate ranking...
> What I do is:
>
> b=a[uniq(a,sort(a))]
> c=value_locate(b,a)
> print, c
> 1 1 0 2 0
>
> the variable c is exactly what I want in this case.
>
> The arrays I am going to deal with are much longer (10x1e6 elements) so
> i can't really check them...
>

Hi,

You are right that VALUE_LOCATE requires that the first argument be
monotonic, but the second is not subject to the same requirement. What
you have shown is that your first argument (b) is monotonic... at least
it looks right to me.

That is a pretty big array that you will be searching. You might search
a recent discussion about how efficient VALUE_LOCATE is compared to
a fixed look-up table. (I think it was Greg Michael who posted the
original question.) VALUE_LOCATE repeatedly cuts the vector (in this
case b) in half as it works to narrow down the 'width' of the search
area. That is pretty quick - but it will still have to do that for every
element in your second argument (a).

Ben

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[Message index]

		Re: Using value_locate with a non-monotonic vector! And it's working!??! By: cgguido on Wed, 03 January 2007 18:23
		Re: Using value_locate with a non-monotonic vector! And it's working!??! By: btt on Tue, 02 January 2007 09:12
		Re: Using value_locate with a non-monotonic vector! And it's working!??! By: David Fanning on Sun, 31 December 2006 17:20
		Re: Using value_locate with a non-monotonic vector! And it's working!??! By: cgguido on Sun, 31 December 2006 15:07
		Re: Using value_locate with a non-monotonic vector! And it's working!??! By: David Fanning on Sun, 31 December 2006 06:52

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