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Removing adjacent duplicate numbers in rows of a matrix

Asked by AA
on 21 Jun 2019
Latest activity Commented on by infinity
on 23 Jun 2019
Hi,
assume the following matrix format:
xx=[1 2 3 3 4 4; 1 1 2 2 3 3 ; 5 5 5 3 3 2]
I want to remove the adjacent duplicate numbers in each row of this matrix so I get the following output:
result=[1 2 3 4; 1 2 3 ; 5 3 2]
Help would be very much appreciated with a formula. Thanks.

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2 Answers

Answer by infinity
on 21 Jun 2019

Hello,
In your problem, you may get error since your result is not a matrix (number of colume of the first row is different with other rows).
So, you result should be a cell like this
clear
xx=[1 2 3 3 4 4; 1 1 2 2 3 3 ; 5 5 5 3 3 2]
for i = 1:size(xx,1)
yy{i} = unique(xx(i,:));
end

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I would suggest using the code below instead. With a NaN you avoid confusion if the last value happens to be a 0.
clear
xx=[1 2 3 3 4 4; 1 1 2 2 3 3 ; 5 5 5 3 3 2]
%yy = zeros(size(xx));
yy=NaN(size(xx))
for row = 1:size(xx,1)
temp = unique(xx(row,:),'stable');
yy(row,1:length(temp)) = temp;
end
Hi,
somehow I see an error when I modify the xx and increase the set of numbers. Adjacent numbers can repeat like 3 or 2 or 5 but they are not displayed in the output.
clear
xx=[1 2 3 3 4 4 3; 1 1 2 2 3 3 2; 5 5 5 3 3 2 5]
%yy = zeros(size(xx));
yy=NaN(size(xx))
for row = 1:size(xx,1)
temp = unique(xx(row,:),'stable');
yy(row,1:length(temp)) = temp;
end
The last digit 3 or 2 or 5 is not shown in the output. Instead i get
1 2 3 4 NaN NaN NaN
1 2 3 NaN NaN NaN NaN
5 3 2 NaN NaN NaN NaN
but I should get
1 2 3 4 3 NaN NaN
1 2 3 2 NaN NaN NaN
5 3 2 5 NaN NaN NaN
How can I modify the formula?
Hello,
You can modify the code like this
clear
% xx=[1 2 3 3 4 4; 1 1 2 2 3 3 ; 5 5 5 3 3 2]
% xx=[1 2 3 3 4 4; 1 1 2 2 3 3 ; 5 4 5 3 3 2]
xx=[1 2 3 3 4 4 3; 1 1 2 2 3 3 2; 5 5 5 3 3 2 5]
% xx=[1 2 3 3 4 4 3 3; 1 1 2 2 3 3 2 3; 5 5 5 3 3 2 5 5]
n = size(xx,2);
% yy = 0*xx;
yy=NaN(size(xx));
for i = 1:size(xx,1)
[temp,ia,ib] = unique(xx(i,:),'stable');
f = diff(ib)~=0;
idx = [find(f);length(ib)];
% yy(i,1:length(temp)) = temp;
yy(i,1:length(idx)) = xx(i,idx);
end
yy

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Answer by Rik
on 22 Jun 2019

You can use diff to find any repeats and remove them. The compare to eps is to avoid strange things happening once you put in decimal numbers, which can lead to rounding errors, which is why exp(log(3))==3 returns false.
xx=[1 2 3 3 4 4 3; 1 1 2 2 3 3 2; 5 5 5 3 3 2 5];
yy=NaN(size(xx));
for row = 1:size(xx,1)
temp = xx(row,:);
%keep values that are different from the one before
%L=[true diff(temp)~=0];%line below is safe for float rounding
L=[true abs(diff(temp))>=(2*eps)];
yy(row,1:sum(L)) = temp(L);
end
%remove trailing NaN cols:
while all(isnan(yy(:,end)))
yy(:,end)=[];
end
disp(yy)

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