Finding If a sub-array (1d) is there in a 2d array

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musi
musi 2011-6-12
Hi,
Lets suppose we have
array2D = [1 2 3; 4 5 6; 6 7 8] how to find out cheaply that it contain "4 5 6" in a position?
I have a main dataset, which is like 10M samples and I have a test set conatining like 2M samples. The test set is a sub set of main set (infact randomly selected elements of main array). I need to find the index of each test sample into the main data file.
Pls give me a solution, as I am not able to do it cheaply yet.
Thanks in advance

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Paulo Silva
Paulo Silva 2011-6-12
For the example provided
array2D = [1 2 3; 4 5 6; 6 7 8]
v1= [4 5 6]
[tf,loc]=intersect(array2D,v1,'rows')
loc gives you the row number where you have v1 inside array2D, you might also take a look at the ismember function for similar purposes
  2 个评论
musi
musi 2011-6-12
Many thanks, it is v good.
Just one comment, I only need indexes, which methid wld be fast?
I think is member can solve problem for all test samples in one go, and might be fast.
cheer
Paulo Silva
Paulo Silva 2011-6-12
after you get the row number you can get all the index values, this is just for your example:
as=size(array2D)
idx=loc:as(1):numel(array2D) %all the index values
The code only works if there's just one row equal to v, if there are more use arrayfun

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