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Data filtering(Give a constraint to the length of each index )

Asked by Jaehwi Bong on 19 Aug 2019 at 9:30
Latest activity Edited by Jan
on 19 Aug 2019 at 11:03
Accepted Answer by Jan
I have this kind of data. First column and second column refer to an index and x value relatively.
data = [ 1 201; 1 202; 2 301; 2 313; 2 311; 3 401; 3 452; 3 433; 3 405; 4 504; 4 303; 4 604; 4 703; 5 600; 5 700; 5 606; 5 703; 5 905; 5 444;];
For example, I want to delete from after 4th data in each index if the number of index is over 4.
The length of 3rd, 4th and 5th index is 4, 4, 6 relatively in this example data. I'd like to keep their data only from 1st to 3rd values.
Every index has less number than 4.
data_filtered = [1 201; 1 202; 2 301; 2 313; 2 311; 3 401; 3 452; 3 433; 4 504; 4 303; 4 604; 5 600; 5 700; 5 606;];
If anyone can help, it would be greatly appreciated.
Thank you!

  1 Comment

Jan
on 19 Aug 2019 at 9:46
In other words: You want to keep only the first n-1 rows for each value in the first column.

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1 Answer

Answer by Jan
on 19 Aug 2019 at 10:48
Edited by Jan
on 19 Aug 2019 at 11:03
 Accepted Answer

There are more efficient ways, but starting with a simple loop is a good apporach:
data = [ 1 201; 1 202; 2 301; 2 313; 2 311; 3 401; 3 452; 3 433; ...
3 405; 4 504; 4 303; 4 604; 4 703; 5 600; 5 700; 5 606; 5 703; ...
5 905; 5 444];
nRow = size(data, 1);
keep = false(1, nRow);
index = -1; % any not occurring index
n = 4; % keep n-1 indices
for k = 1:nRow
if data(k, 1) ~= index
index = data(k, 1);
count = 0;
end
count = count + 1;
keep(k) = count < n;
end
data_filtered = data(keep, :)
A vectorized method:
index = data(:, 1);
D = [false ; diff(index(:)) ~= 0] ;
Start = [1; find(D)]; % Start indices of next run
N = ones(size(D));
N(D) = 1 - diff(Start);
N = cumsum(N);
data_filtered = data(N < 4, :)
See also Jos' FileExchange submission https://www.mathworks.com/matlabcentral/fileexchange/56131-runindex . Then:
N = runindex(data(:, 1));
data_filtered = data(N < 4, :)

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