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Re: Can this be vectorized? [message #17456 is a reply to message #17445]

Tue, 26 October 1999 00:00

Bert Jagers
Messages: 1
Registered: October 1999

Junior Member

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

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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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