Please excuse my nit-picking: In physicists the third and fourth derivative of position are jerk (or jolt) and jounce, respectively. Impulse is the integral of force with respect to time.
Charles Annis, P.E. charles.an...@statisticalengineering.com phone: 561-352-9699 eFax: 614-455-3265 http://www.StatisticalEngineering.com -----Original Message----- From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On Behalf Of David Winsemius Sent: Wednesday, April 22, 2009 8:47 AM To: Hans-Henning Gabriel Cc: r-help@r-project.org Subject: Re: [R] Discover significant change in sorted vector On Apr 22, 2009, at 8:06 AM, Hans-Henning Gabriel wrote: >> Hans-Henning Gabriel wrote: >>> Hi, >>> >>> suppose I have a simple sorted vector like this: >>> >>> a <- c(2,3,3,5,6,8,8,9,15, 25, 34,36,36,38,41,43,44,44,46); >>> >>> Is there a function in R, I can use to discover that from index 8 >>> to index 11 the values are changing significantly? >>> The function should return a value pointing to one of the indices >>> 8, 9, 10 or 11. Any of them would be fine. >>> The difficulty is that there may be no big gap. I mean, indices 8 >>> and 11 are somehow "connected" by indices 9 and 10. So, it's not >>> an option to just search for biggest difference between the values. >>> >>> Perfect would be a function that is able to discover multiple >>> changes if it is present in the data. >>> >> Hi Henning, >> Is max(diff(a)) any good to you? >> >> Jim >> > > Hi Jim, > > no, I'm affraid it's not. It's more like that the slope is low for > many values, then it starts getting stronger for few values, then it > gets low again for many values. But the thing is that the slope may > _not_ change immediately. > Well <something> needs to change immediately. Think of this as position, speed and acceleration, i.e. speed is the first derivative of position and acceleration is the second derivative. (In physics the third derivative is sometimes called impulse, the thing that changes when acceleration goes from zero to some positive value.) If first differences is not what you want, then look at second differences as below: > x <- c(2,3,3,5,6,8,8,9,15, 25, 34,36,36,38,41,43,44,44,46); > max(diff(diff(x))) [1] 5 > diff(diff(x)) [1] -1 2 -1 1 -2 1 5 4 -1 -7 -2 2 1 -1 -1 -1 2 > which(diff(diff(x))==max(diff(diff(x)))) [1] 7 David Winsemius, MD Heritage Laboratories West Hartford, CT ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
