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Re: Efficient comparison of arrays [message #9798 is a reply to message #9707]

Wed, 20 August 1997 00:00

John Votaw
Messages: 6
Registered: June 1997

Junior Member

I very frequently have a task similar to finding common elements in
arrays as discussed in this thread. I have a large array A and an array
B which is known to be a subset of A. No elements in A or B are
repeated. This situation occurs when A is an array of indicies into an
image volume forming a region of interest and B are the indicies of some
feature you would like to remove from the region of interest. The
problem is to return an array that contains the elements of A that are
not in B.

Following the lead of J. D. Smith, I wrote the following routine:

function eliminate,a,b
c =[a,b]
cs=c(sort(c))
keepers=where(cs ne shift(cs,1) and cs ne shift(cs,-1), count)
if count ne 0 then return,cs(keepers)
return,-1
end

The brute force method:

function eliminate_bf,a,b
mn=min(a)
c=[mn-2,a,mn-1] ;remove possibility of end effects
for i=0,n_elements(b)-1 do begin
j=(where(b(i) eq c))(0)
c=[c(0:j-1),c(j+1:*)]
endfor
return,c(1:n_elements(c)-2)
end

In my applications, a has about 20000 elements and b has between 1 and
1000. If the number of elements in b is less than 35, then the brute
force method is faster, otherwise eliminate is faster -- very much so.
When the number of elements in b is 100, it is 3 times faster.

Does anyone have another algorithm or comments?
---
John R. Votaw
votaw@commander.eushc.org

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[Message index]

		Efficient comparison of arrays By: Andy Loughe on Fri, 08 August 1997 00:00
		Re: Efficient comparison of arrays By: J.D. Smith on Thu, 14 August 1997 00:00
		Re: Efficient comparison of arrays By: David Foster on Thu, 14 August 1997 00:00
		Re: Efficient comparison of arrays By: William Clodius on Thu, 14 August 1997 00:00
		Re: Efficient comparison of arrays By: William Clodius on Wed, 13 August 1997 00:00
		Re: Efficient comparison of arrays By: J.D. Smith on Wed, 13 August 1997 00:00
		Re: Efficient comparison of arrays By: David Foster on Wed, 13 August 1997 00:00
		Re: Efficient comparison of arrays By: J.D. Smith on Tue, 12 August 1997 00:00
		Re: Efficient comparison of arrays By: J.D. Smith on Mon, 11 August 1997 00:00
		Re: Efficient comparison of arrays By: davidf on Mon, 11 August 1997 00:00
		Re: Efficient comparison of arrays By: davidf on Mon, 11 August 1997 00:00
		Re: Efficient comparison of arrays By: John Votaw on Wed, 20 August 1997 00:00

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