Conditional cumsum - how to create?

2013 4 2
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0 个投票

This is probably easy, but my brain isn't working today...
How can you do the following operation in a vectorized way? I'd think it should be possible with some combination of cumsum, diff & logical indexing:
input = rand(10,1);
output = zeros(size(input);
output(1) = input(1);
for ind = 2:numel(input)
dif = input(ind) - input(ind-1);
if dif < 0
output(ind) = output(ind-1) + dif;
else
output(ind) = output(ind-1);
end
end

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the cyclist
the cyclist 2013-4-2
It would be useful if you also described conceptually what you are trying to do.
Eric Sampson
Eric Sampson 2013-4-2
Basically it's a copy of the input, but anytime that the original increases from one val to the next, the output should be hold constant. Sort of like a copy that can only go down :)

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Roger Stafford
Roger Stafford 2013-4-2

1 个投票

Try this.
outp = cumsum([inp(1);min(diff(inp),0)]);

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Matt Tearle
Matt Tearle 2013-4-2

2 个投票

There may be better ways, but this works:
d = [true;diff(input)<0];
idx = find(d);
output = input(idx(cumsum(d)));
When the array is large enough, there's a pretty decent speedup (~50x)

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Eric Sampson
Eric Sampson 2013-4-2
Matt that looks promising, but I don't think it generates the same results as the algorithm?
Matt Tearle
Matt Tearle 2013-4-2
I copy/pasted your code and checked mine against it:
input = rand(100000,1);
tic
output = zeros(size(input));
output(1) = input(1);
for ind = 2:numel(input)
if (input(ind) - input(ind-1)) < 0
output(ind) = input(ind);
else
output(ind) = output(ind-1);
end
end
t1=toc;
tic
d = [true;diff(input)<0];
idx = find(d);
output2 = input(idx(cumsum(d)));
t2=toc;
max(abs(output-output2))
t1/t2
I always seem to get a difference of 0.
Eric Sampson
Eric Sampson 2013-4-2
Argh, I'm sorry Matt. See my revised problem statement. This was clearly a case of 'do what I mean, not what I say' :)
Sean de Wolski
Sean de Wolski 2013-4-2
As you know, the DWIM Toolbox still hasn't been released to the public.
Eric Sampson
Eric Sampson 2013-4-2
Sean, wasn't Loren or Steve supposed to be on that? Slackers.

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