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How to take only few rows in a large matrix

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afrya
afrya 2013-11-11
评论： Image Analyst 2013-11-11
Dear All,
I'm working with a large amount of data in a matrix.My problem is the following one.
I have a large matrix which looks like:
  • Initial matrix
X Y Z
0.2 0 6
0.4 0 7
0.6 0 8 values of Y are repeated a lot of time
.. .. ..
.. .. ..
0.1 2 9
0.4 2 10
0.8 2 11
.. .. ..
11 4 6
12 4 8
4 4 9
  • Final marix
X Y Z
0.2 0 6
0.1 2 9
11 4 6
My goal is to take few rows of the matrix because Y values are repeated.
  1 个评论
Image Analyst
Image Analyst 2013-11-11
Which row do you want to take? The first row when that value of Y appears? Are y always integers or can they be fractional?

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回答（1 个）

A Jenkins
A Jenkins 2013-11-11
编辑：A Jenkins 2013-11-11
You can use the function unique()
[C,IA,IC]=unique(A(:,2))
A(IA,:)
Or perhaps if you already know the indicies of the rows you want, you can select them row-wise:
idxIwant=[1 3 9]
C=A(idxIwant,:);
  1 个评论
afrya
afrya 2013-11-11
Thanks for your answer but I don't know the indices of the rows, the matrix that I'm working with has 700 000 rows. I'll do it with the function unique

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