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Topic: What's wrong with cumtrapz?
Replies: 7   Last Post: Jun 17, 2013 10:52 AM

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Yuji

Posts: 28
Registered: 6/27/11
Re: What's wrong with cumtrapz?
Posted: Jun 16, 2013 1:35 AM
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Hi~ I've a question - maybe trivial.

why you wanna use cumtrapz(x',y') instead of cumtrapz(x,y) ? they're the same right?

Let me know. Thank you~


Nabeel <nabeel@mathworks.com> wrote in message <3AF23058.E8745FEB@mathworks.com>...
> Hi,
>
> Running this code:
>
> x = 0:pi/100:4*pi;
> y = sin( 2*pi*x);
> inty = cumtrapz(x',y');
> plot(x,y,x,inty)
>
> produces a plot that looks right to me. You wrote:
>

> > I would expect a COS(X) time trace after CUMTRAPZ. The integrated
> > time trace gave a cos pattern of variation, correct amplitude BUT was
> > wrong in phase and shifted upwards, i.e. it is greater than zero for all
> > x and gives 0, rather than 1, at x = 0. I am very confused with the
> > results.

>
> CUMTRAPZ is the cumulative integral, or the running sum of the area
> under the curve. From the graph above, you can see that the running sum
> of the area is never negative, at the lowest it's zero. As for the
> value at x=0, there's no area under the curve yet, so the value of 0 is
> correct.
>
> If you work it out by hand, you'll should be able to verify the results.
>
> -- Nabeel




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