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Delete rows when a value repeated less than 5 times

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I have a matrix:
a b c
1 201 1
2 202 1
3 203 1
4 204 1
5 205 1
6 206 1
7 207 2
8 208 2
9 209 2
10 210 2
11 222 3
12 232 3
I would like to remove rows 7-12 which have a value in the last column (c) repeated less than 5 times. Any help is appreciated.

Accepted Answer

Sean de Wolski
Sean de Wolski on 31 Aug 2017
Edited: Sean de Wolski on 31 Aug 2017
m = [1 201 1
2 202 1
3 203 1
4 204 1
5 205 1
6 206 1
7 207 2
8 208 2
9 209 2
10 210 2
11 222 3
12 232 3];
% Unique elements and locations in third column
[uv, ~, id] = unique(m(:,3));
% How many of each?
n = histcounts(id);
% Keep ones with more than 5.
m2 = m(ismember(m(:,3), uv(n>=5)),:)
  2 Comments
Dinh-Vinh Vo
Dinh-Vinh Vo on 31 Aug 2017
Thank you very much for your help, Sean. It is definitely what I'm looking for.
Karena Weduwen
Karena Weduwen on 19 Mar 2018
Edited: Karena Weduwen on 19 Mar 2018
Hello Sean, your code works perfectly fine on a small testfile I use (190 rows).
But as soon as I add some more rows, the
n = histcounts(id);
part starts giving weird values. I cannot find the reason for that change.
Attached you find the two files:
working.txt code runs through without any trouble
not working.txt "n = histcounts(id);" gives strange output and the m2 stays empty.
Any suggestions on how the code could be adjusted to work on a bigger file?
since my file looks a little different (I am searching in the first column) here is the code I use:
if true
load working.txt
a = working(:,:);
[uv, ~, id] = unique(a(:,1));
n = histcounts(id);
m2 = a(ismember(a (:,1), uv(n==4)),:);
end

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More Answers (1)

José-Luis
José-Luis on 31 Aug 2017
Edited: José-Luis on 31 Aug 2017
idx = unique(m(:,3));
count = accumarray(c.',(1:numel(c)).',[],@numel);
to_keep = ~ismember(m(:,3),idx(count < 5));
result = m(to_keep,:)
  5 Comments
José-Luis
José-Luis on 31 Aug 2017
No, the solution is not robust. You'd need to adjust the accumarray function.

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