# Conditional cumsum - how to create?

8 views (last 30 days)
Eric Sampson on 2 Apr 2013
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
the cyclist on 2 Apr 2013
It would be useful if you also described conceptually what you are trying to do.
Eric Sampson on 2 Apr 2013
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 :)

Roger Stafford on 2 Apr 2013
Try this.
outp = cumsum([inp(1);min(diff(inp),0)]);
Eric Sampson on 2 Apr 2013
Ding ding ding! Roger wins, to Matt's detriment :) Thanks!

Matt Tearle on 2 Apr 2013
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)
Sean de Wolski on 2 Apr 2013
As you know, the DWIM Toolbox still hasn't been released to the public.
Eric Sampson on 2 Apr 2013
Sean, wasn't Loren or Steve supposed to be on that? Slackers.

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