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Re: Can this be vectorized? [message #17455 is a reply to message #17445]

Tue, 26 October 1999 00:00

Gautam Sethi
Messages: 2
Registered: October 1999

Junior Member

In comp.soft-sys.matlab Bert Jagers <hrajagers@my-deja.com> wrote:
: Dear John,

: Two solutions in Matlab: one completely vectorized, one partially.
: Of course, you could also implement it as a MEX file.

: F=find([1 diff(I) 1]);
: XS=[0 cumsum(X)];
: Y=XS(F(2:end))-XS(F(1:(end-1)));

: There is one major drawback to this implementation, since

:> In reality, X consists of about one million elements,

: you may loose accuracy when taking the difference of two large
: cummulative values. So, I tried to find another solution.

: F=find([1 diff(I) 1]);
: Y=zeros(1,length(F)-1);
: for i=1:length(Y),
: Y(i)=sum(X(F(i):(F(i+1)-1)));
: end;

: The memory usage is probably comparable. In both cases there needs to
: be space for the matrices I,X,F and [1 diff(I) 1], or else Matlab will
: start swapping.

: Best regards,

: Bert Jagers

since memory is of issue, you may want to try this non-vectorized one as well:
------------------------------------------------------------ ------
function K = davis(I,J)

for i = min(I):max(I)
K(i) = sum(J(find(I == i)));
end
------------------------------------------------------------ ------

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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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