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Re: Efficient comparison of arrays [message #9761 is a reply to message #9707]

Tue, 12 August 1997 00:00

J.D. Smith
Messages: 214
Registered: August 1996

Senior Member

David Fanning wrote:
>
> Augh, it's too late for this:
>
> I wrote this:
>
>> Given a and b:
>>
>> a = [1,2,3,4,5]
>> b = [3,4,5,6,7]
>>
>> Let,
>>
>> array1 = BYTARR((MAX(a) > MAX(b)) - (MIN(a) < MIN(b)))
>> array2 = array1
>>
>> Then, let,
>>
>> ind1[a] = 1L
>> ind2[b] = 1L
>>
>> Finally, let,
>>
>> commonIndex = ind1 * ind2
>>
>> The vector commonIndex now has 1s at the locations where there are
>> common values in the two sets. In other words,
>>
>> Print, commonIndex
>> 0 0 0 1 1 1
>
> When I meant to write this:
>
> Given a and b:
>
> a = [1,2,3,4,5]
> b = [3,4,5,6,7]
>
> Let,
>
> array1 = BYTARR((MAX(a) > MAX(b)) - (MIN(a) < MIN(b)))
> array2 = array1
>
> Then, let,
>
> array1[a] = 1L
> array2[b] = 1L
>
> Finally, let,
>
> commonIndex = array1 * array2
>
> The vector commonIndex now has 1s at the locations where there are
> common values in the two sets. In other words,
>
> Print, commonIndex
> 0 0 0 1 1 1
>
> --

There is an error in this code. Alex Schuster presents a similar
solution, but without the error. The problem is you should be
subtracting (min(a) < min(b)) from a and b as such:

array1[a- (min(a) < min(b))]=1L

and then add the minimum of the two vectors to the location in the
commonIndex vector to get the final common values.

Otherwise, there will not, in general, be enough room in the index
arrays to mark all the data values. It is just an accident that it
works for [1,2,3,4,5],[3,4,5,6,7] ... try [1,2,3,4,5,6,7],[3,4,5,6,7,8]
and you'll see the problem.

Another question with the process... what happens when you don't have a
well grouped set of integers... e.g [1,2,3,4,5] and [3,4,10000,900,2]
... lots of wasted zeroes in those index arrays to determine this one.

JD

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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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