Calculating displacement many times with respect to different elements

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I have the following data (or similar):
154240 1 7
154240 2 2
154240 5 6
154240 2 5
154240 6 7
154240 2 7
154240 5 6
154240 6 7
154240 4 6
154240 4 6
154240 4 6
641224 2 3
641224 2 1
641224 2 3
641224 1 2
641311 5 5
641311 7 5
It represents data of the form [tag number of rat, x coord, y coord] (as im studying a rat population). I would like to calculate the displacement of x & y with respect to the first x & y entries for each of the same value of the tag, i.e. I want an nx3 matrix (where n is size of my data) which gives me [tag number again,(0,x2-x1,x3-x1,x4-x1 etc),(0,y2-y1,y3-y1,y4-y1 etc)] but I am totally inept at programming and am flummoxed right now.
Any help is greatly appreciated!
  1 Comment
Sean de Wolski
Sean de Wolski on 28 Jan 2013
So what output do you expect for you above input? Please describe to us how you get that. Results are worth a 1000 words!

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Answers (1)

Azzi Abdelmalek
Azzi Abdelmalek on 28 Jan 2013
X your array
out=bsxfun(@minus,X,[0 X(1,2) X(1,3)])
  1 Comment
Timothy Russell
Timothy Russell on 29 Jan 2013
This is great, but I the first x & y displacement for each different tag number needs to be (0,0) (as the displacement for the first time a rat is captured is (0,0)), and this isn't doing that. Is there an easy way to split up a matrix into seperate matricies by using the element in the first column (i.e. the tag number in this case)? Thanks though, it does what I wanted, I just haven't sort my data out properly yet!

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