Conditional cumsum - how to create?

on 2 Apr 2013

Roger Stafford (view profile)

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

the cyclist (view profile)

on 2 Apr 2013

It would be useful if you also described conceptually what you are trying to do.

Eric Sampson

Eric Sampson (view profile)

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 :)

Products

Roger Stafford (view profile)

on 2 Apr 2013

Try this.

` outp = cumsum([inp(1);min(diff(inp),0)]);`

Eric Sampson

Eric Sampson (view profile)

on 2 Apr 2013

Ding ding ding! Roger wins, to Matt's detriment :) Thanks!

Matt Tearle (view profile)

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)

Eric Sampson

Eric Sampson (view profile)

on 2 Apr 2013

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 (view profile)

on 2 Apr 2013

As you know, the DWIM Toolbox still hasn't been released to the public.

Eric Sampson

Eric Sampson (view profile)

on 2 Apr 2013

Sean, wasn't Loren or Steve supposed to be on that? Slackers.

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