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Re: Can this be vectorized? [message #17445 is a reply to message #17442]

Tue, 26 October 1999 00:00

davis
Messages: 15
Registered: March 1995

Junior Member

On 26 Oct 1999 09:08:26 GMT, Struan Gray <struan.gray@sljus.lu.se>
wrote:
> In IDL an alternative to vectorisation is to use the fast built-in
> function HISTOGRAM. In your case you should take the histogram of 'I'
> and use the REVERSE_INDICES keyword to get a list of which elements
> are in which bin. Summing the same elements of 'X' will give you the
> answer you want. This will work even if 'I' and 'X' are not in
> ascending order. There is some memory overhead, but it is of the same
> order as creating working copies of your original data.

Let's suppose that the integer array `I' is:

I = [0, 1, 1, 2, 3, 4, 4, 4, 5]

and `X' = [a, b, c, d, e, f, g, h, i].

Then, I want `Y' to be:

Y = [a, (b+c), d, e, (f+g+h), i]

The histogram `H' is:

H = [1, 2, 1, 1, 3, 1]

and the REVERSE_INDICES array `R' is:

R = [7, 8, 10, 11, 12, 15, 16, 0, 1, 2, 3, 4, 5, 6, 7, 8]

It seems to me that I would have to loop over all terms in the
histogram via (pseudocode!):

H = H[where(H != 0)] # get rid of non-zero elements
j = 0
for i = 0 to i = length(H)
if R[i] != R[i+1]
J = [ R[i]:R[i+1]-1 ]
Y[j] = sum (X[J])
endif
next i

Of course this is useful as long as `I' contains many repeated
elements so that the histogram will be small. Unfortunately, in my
case this is unlikely.

At least I have a better understanding of the REVERSE_INDICES keyword.
Thanks,
--John

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[Message index]

		Re: Can this be vectorized? By: davis on Wed, 27 October 1999 00:00
		Re: Can this be vectorized? By: davis on Tue, 26 October 1999 00:00
		Re: Can this be vectorized? By: Struan Gray on Tue, 26 October 1999 00:00
		Re: Can this be vectorized? By: Gautam Sethi on Tue, 26 October 1999 00:00
		Re: Can this be vectorized? By: Bert Jagers on Tue, 26 October 1999 00:00

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