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Re: What's wrong with cumtrapz?
Posted:
May 4, 2001 10:09 AM


""Leung, Randolph [COPE/HKG]"" <RckLeung@CopelandCorp.com> wrote in message news://9B636583813BD311BE6400508B10481F6C5274@fs83.hk.copelandsid.com... > > I have some experimental time traces to integrate. CUMTRAPZ seems > to be a simple and easy option for me. To test it, I tried to integrate a > simple SIN(X) time trace as follows, > > x = 0:pi/100:4*pi]; > y = sin( 2*pi*x). > > inty = cumtrapz( x', y' ); > > 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.
It seems you may be forgetting a bit of your calculus. You are numerically taking the INDEFINITE integral of your function. In that case, you must be prepared to add a constant to your solution. [I think it is really only shifted "upward", and not really wrong in phase.] You need to determine the appropriate constant from other conditions of your problem.
Tim



