Why does 'unique' not sort the rows?

I'm trying to gather a set of data to be plotted. However, when I use unique, it does not sort the data and my plots are not correct.

 function pos = Lube(m,n,len,filename)

    [pos,nC,nT,lenC] = Graphic(m,n,lens,filename); %

    lubepos=zeros(length(pos),2);%

    for q=1:length(pos)
        lubepos(q,:)=[nC(1)*pos(q,1)+nC(2)*pos(q,2),nT(1)*pos(q,1)+nT(2)*pos(q,2)];
    end 

    radius=lenC/2/pi; %get the radius value
    x=radius*cos(lubepos(:,1)/radius); 
    y=radius*sin(lubepos(:,1)/radius);
    z=lubepos(:,2);

    pos=[x,y,z]; %Create the positions matrix
    pos=unique(pos,'rows');
    fid=fopen(filename, 'wt'); %write to the filename
    fprintf(fid, '%f %f %f \n',pos');%put in all the values 
    hEplot(filename) 
end

The data I end up with is: -0.551329 0.000000 0.000000 -0.551329 0.000000 1.000000 -0.000000 -0.551329 1.500000 0.000000 0.551329 1.500000 0.551329 -0.000000 0.000000 0.551329 -0.000000 1.000000 0.551329 0.000000 0.000000 0.551329 0.000000 1.000000

As you can see, rows are repeated. How do I prevent this?

[Stephen23]

"As you can see, rows are repeated"

No they are not. Small differences in the floating-point values mean that those rows are actually different. Floating-point numbers have been explained a thousand times on this forum:

https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero

https://www.mathworks.com/matlabcentral/answers/69-why-does-1-2-3-1-3-not-equal-zero#answer_271

https://www.mathworks.com/matlabcentral/answers/316889-why-two-equal-numbers-are-not-equal

https://www.mathworks.com/matlabcentral/answers/41536-why-is-2-24-10-22-4-not-equal-to-0

https://www.mathworks.com/matlabcentral/answers/20830-1-0010-does-not-equal-1-0010

etc, etc, etc

Always use a tolerance when comparing floating point values.

[Akhil Vasvani]

Thank you @Stephen Cobeldick.

Answer 1

[Steven Lord]

If you subtracted some of the rows that appear to be the same, you'll receive a very small but nonzero difference. If you want to use a little bit of a tolerance so two rows are considered the same if they're "close enough" to each other and you're using release R2015a or later, use uniquetol instead of unique.