| Re: setintersection assumes sets have no repetitions? [message #83310] |
Sat, 23 February 2013 22:23  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Paulo Penteado writes:
> I am using Coyote's set functions, and I noticed that when I use some
> (not any) sets with repetitions as input, it crashes:
>
> IDL>
> a=[1,2,3,4,5]
> IDL>
> b=[1,2,2,2]
> IDL>
> print,setintersection(a,b,indices_a=ia,indices_b=ib,position s=pos)
> % Compiled module: SETINTERSECTION.
> % Compiled module: REVERSEINDICES.
> % Compiled module: ERROR_MESSAGE.
>
> Traceback Report from SETINTERSECTION:
>
> % Out of range subscript encountered: BINDICES.
> % Execution halted at: SETINTERSECTION 192 /software/idl/
> others/idl-coyote-read-only/coyote/setintersection.pro
> % $MAIN$
> -1
>
> Is this the expected behavior? Are the input sets supposed not to have
> repetitions? The documentation suggests they may have repeated
> elements, thus making the positions array different from indices_a.
Well, I don't know. That code was added at the request of Mr. Stockwell.
Let's see if he has any ideas about this. :-)
For the moment, I would consider commenting that section of the code
out.
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.idlcoyote.com/
Sepore ma de ni thue. ("Perhaps thou speakest truth.")
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| Re: setintersection assumes sets have no repetitions? [message #83311 is a reply to message #83310] |
Sat, 23 February 2013 13:15  |
Jeremy Bailin
Messages: 618
Registered: April 2008
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Senior Member |
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On 2/23/13 2:46 PM, Paulo Penteado wrote:
>
> I am using Coyote's set functions, and I noticed that when I use some
> (not any) sets with repetitions as input, it crashes:
>
> IDL>
> a=[1,2,3,4,5]
> IDL>
> b=[1,2,2,2]
> IDL>
> print,setintersection(a,b,indices_a=ia,indices_b=ib,position s=pos)
> % Compiled module: SETINTERSECTION.
> % Compiled module: REVERSEINDICES.
> % Compiled module: ERROR_MESSAGE.
>
> Traceback Report from SETINTERSECTION:
>
> % Out of range subscript encountered: BINDICES.
> % Execution halted at: SETINTERSECTION 192 /software/idl/
> others/idl-coyote-read-only/coyote/setintersection.pro
> % $MAIN$
> -1
>
> Is this the expected behavior? Are the input sets supposed not to have
> repetitions? The documentation suggests they may have repeated
> elements, thus making the positions array different from indices_a.
>
By definition, a set only has one copy of each element, so I wouldn't be
surprised if things fail if it's not a true set.
-Jeremy.
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| Re: setintersection assumes sets have no repetitions? [message #83457 is a reply to message #83310] |
Tue, 05 March 2013 10:44 |
penteado
Messages: 866
Registered: February 2018
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Senior Member
Administrator |
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I just noticed that the documentation changed, to say that indices_a
and indices_b require the intersection elements to be unique, while
positions does not.
I understand the reasons to assume the sets have unique elements (both
from the mathematical concept and from practical applications like
identifiers from databases). But since the positions keyword was there
to handle repetitions, I got confused when the code crashed on arrays
with repetitions.
The use I had with repeated elements happened when I was working with
matches of my list of sources in the sky (each with its unique
identifier) with other such lists (with their own unique identifiers),
by finding which objects in one list were within a certain distance of
each object in the other list. Occasionally, one object in my list
would have more than one match, and, as such, its identifier would
appear more than once in the list of results.
I eventually resorted to using histogram, through my wrapper that puts
the reverse indices into hashes or lists:
a=[1,2,3,4,5]
b=[1,2,2,2]
h=histogram_pp(a,binsize=1,reverse_hash=rh)
print,rh
;5: 4
;1: 0
;3: 2
;2: 1
;4: 3
ib=lonarr(n_elements(b))
foreach el,b,i do ib[i]=rh[el]
print,ib
; 0 1 1 1
print,b[ib]
; 1 2 2 2
histogram_pp is at
http://www.ppenteado.net/idl/pp_lib/doc/histogram_pp.html
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