Delete rows when a value repeated less than 5 times

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.

Answer 1

[Sean de Wolski]

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)),:)

[Dinh-Vinh Vo]

Thank you very much for your help, Sean. It is definitely what I'm looking for.

[Karena Weduwen]

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

Answer 2

[José-Luis]

idx = unique(m(:,3));
count = accumarray(c.',(1:numel(c)).',[],@numel);
to_keep = ~ismember(m(:,3),idx(count < 5));
result = m(to_keep,:)

[Dinh-Vinh Vo]

Hi Jose-Luis, Thanks for your answer. Could you please explain what c is in the second row?

[José-Luis]

My bad.

c = m(:,3);
idx = unique(c);
count = accumarray(c, (1:numel(c)).',[],@numel);
to_keep = ~ismember(c,idx(count < 5));
result = m(to_keep,:)

[Dinh-Vinh Vo]

If I change the matrix to: 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 5 12 232 9];

It doesn't work any longer :(.

[José-Luis]

No, the solution is not robust. You'd need to adjust the accumarray function.

[Dinh-Vinh Vo]

Thanks, I did it.