# how i can delete useless data from matrix

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Giuseppe Antonacci il 5 Feb 2019
hi guys,
i have a matrin m lines 11 columns.
i wish a new matrix where are deleted the lines when:
there are three lines row the same of the 8th columns and how the second condition the same but with 6 column. Like the the example in the insert.
from the 715 lines there is the problem that i would deley in the new matrix.
thanks
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Giuseppe Antonacci il 5 Feb 2019
sorry but the matrix is too big. i would remove the duplicates with the idea. when i have the same number for three lines. i think that i use a cycle for/if.
Guillaume il 5 Feb 2019
It's very unlikely that you need loops and if statement. If the matrix is too big, attached a smaller one which still has enough data to show what you want.
I'm with Bob, I'm really unclear on what you want. So attach a simple example of input and the corresponding desired output.

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### Risposte (2)

Bob Thompson il 5 Feb 2019
Modificato: Bob Thompson il 5 Feb 2019
Here is something to try. Probably not the most efficient, but it should do what you're asking.
vals = unique(data(:,8));
data2 = [];
for i = 1:length(vals);
if size(data(data(:,8)==vals(i),:),1)<=3
data2 = [data2; data(data(:,8)==vals(i),:)];
else
tmp = data(data(:,8)==vals(i),:);
data2 = [data2; tmp(1:3,:)];
end
end
I would agree with Guillaume that extra explanation would be helpful. If this loop combo does what you're looking for we can probably come up with some way of doing this without the loops, just let us know.
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Stephen23 il 6 Feb 2019
Modificato: Stephen23 il 6 Feb 2019
Do NOT use a loop for this.
MATLAB code should be neat and simple:
[~,idx] = unique(data(:,[6,8]),'stable','rows');
out = data(idx,:)
Because you have floating point numbers, I recommend either using uniquetol, or scale the values and round them before unique.
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Stephen23 il 7 Feb 2019
Modificato: Stephen23 il 7 Feb 2019
and use it like this:
>> M = [21,42,38,29;29,39,41,29;23,3,2,18;15,3,2,31;24,3,2,48;51,35,24,22;21,42,38,29;29,39,41,29;23,3,2,18;15,3,2,31;51,35,24,22]
M =
21 42 38 29
29 39 41 29
23 3 2 18
15 3 2 31
24 3 2 48
51 35 24 22
21 42 38 29
29 39 41 29
23 3 2 18
15 3 2 31
51 35 24 22
>> R = 3; % >=R identical rows will be deleted.
>> X = all(diff(M(:,[2,3]),1,1)==0,2); % check columns 2 & 3.
>> [B,N] = RunLength([false;X]|[X;false]);
>> B(N(:)<R & B) = false;
>> Y = repelem(B,N)
>> M(Y,:) = [] % remove rows
M =
21 42 38 29
29 39 41 29
51 35 24 22
21 42 38 29
29 39 41 29
23 3 2 18
15 3 2 31
51 35 24 22
Giuseppe Antonacci il 7 Feb 2019
(undefined function or variable "RunLength"). why?

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