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Can this be vectorized? [message #17457]

Tue, 26 October 1999 00:00

davis
Messages: 15
Registered: March 1995

Junior Member

I am looking for either a matlab or IDL solution to this problem.
Suppose that I have two 1-d arrays, `I' and `X', where `I' is an integer
array and `X' is a floating point array. `I' is assumed to be sorted in
ascending order. I would like to produce a third array `Y' that is
formed from the elements of `X' as follows (pseudocode):

len = length (X); #number of elements of X

i = 0;
j = 0;

while (i < len)
{
last_I = I[i];
sum = X[i];
i = i + 1;
while ((i < len)
AND (I[i] == last_I))
{
sum = sum + X[i];
i = i + 1;
}
Y[j] = sum;
j = j + 1;
}

For example, suppose

I = [ 1 2 3 3 4 4 4 5]
X = [ a b c d e f g h]

Then, Y would be 5 element array:

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

One partially vectorized pseudocode solution would be:

jj = 0
for (i = min(I) to max(I))
{
J = WHERE (I == i);
Y[jj] = sum_elements (X[J])
jj = jj + 1
}

What is the best way to vectorize this? In reality, X consists of
about one million elements, so I would prefer a solution that is
memory efficient. I apologize for posting to both newsgroups, but I
am looking for a solution in either language.

Thanks,
--John

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[Message index]

		Can this be vectorized? By: davis on Tue, 26 October 1999 00:00
		Re: Can this be vectorized? By: Richard G. French on Tue, 02 November 1999 00:00
		Re: Can this be vectorized? By: Richard G. French on Tue, 02 November 1999 00:00
		Re: Can this be vectorized? By: Gautam Sethi on Wed, 03 November 1999 00:00

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