comp.lang.idl-pvwave archive: archive » Efficient pattern-matching in a large array

Home » Public Forums » archive » Efficient pattern-matching in a large array

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: Efficient pattern-matching in a large array [message #77288 is a reply to message #77061]

Thu, 18 August 2011 10:00

JDS
Messages: 94
Registered: March 2009

Member

Right you are, Bob! Thanks for the catch.

I struggled to find a simple logical test with the properties:

1) any array is true
2) !NULL is false

And I didn't remember that KEYWORD_SET looks only at the first entry. I think *any array* should be true on its own, but IDL logic treats scalars and single length arrays as identical, and issues an error when IF-testing longer arrays. This is maximally confusing, since you often don't know the tested array's length in advance, and an error is not thrown when that length drops to 1. I think David has an article somewhere about the resulting confusion.

In any case, you don't have to give up on !NULL for the test. In fact you can take !NULL even further, applying it without testing for WHERE's success at all (one of the main uses of !NULL). This further simplifies the algorithm:

w=where(a eq pat[(c=0)],/NULL)
while w ne !NULL && ++c lt n_elements(pat) do w=w[where(a[w+c] eq pat[c],/NULL)]
w=w[where(w le n-n_elements(pat),/NULL)]

This will work even if the pattern occurs right away. That last test is new and is for one corner case: if the pattern contains a string of repeating values at its end, and this value matches the very last element of the array, you will otherwise get a spurious match.

By the way, this uses the curious fact that comparison to !NULL is special cased in IDL (since you can't have an array of !NULL's):

IDL> a=[1,2,4,5]
IDL> print, a ne 2
1 0 1 1
IDL> print, a ne !NULL
1

JD

Report message to a moderator

[Message index]

		Efficient pattern-matching in a large array By: Chris O'Dell on Tue, 02 August 2011 11:05
		Re: Efficient pattern-matching in a large array By: Foldy Lajos on Tue, 02 August 2011 13:04
		Re: Efficient pattern-matching in a large array By: ez569x on Fri, 12 August 2011 05:18
		Re: Efficient pattern-matching in a large array By: Brian Daniel on Fri, 12 August 2011 07:38
		Re: Efficient pattern-matching in a large array By: guillermo.castilla.ca on Tue, 16 August 2011 09:41
		Re: Efficient pattern-matching in a large array By: guillermo.castilla.ca on Wed, 17 August 2011 06:41
		Re: Efficient pattern-matching in a large array By: JDS on Tue, 16 August 2011 16:01
		Re: Efficient pattern-matching in a large array By: Bob[4] on Wed, 17 August 2011 14:14
		Re: Efficient pattern-matching in a large array By: JDS on Thu, 18 August 2011 10:00

Previous Topic:	Re: /RELAXED keyword not allowed?
Next Topic:	Confession

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Fri Oct 10 18:36:40 PDT 2025

Total time taken to generate the page: 2.32170 seconds