# Extract upper triangle of matrix (using vector different from diagonal)

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Philipp Brunnbauer le 13 Mar 2023
Hello, I am trying to extract some data from a matrixt, wherein the upper "triangle" contains the signal I am interested in. I can extract a big chunk of ut using triu(), the issue is the lower bound is not a diagonal, but steeper than the diagnonal (for every row there is about 1.5 columns if that makes sense), causing the upper section to be cut using triu(). Is there a way of doing it with a nested for loop or the like? I am banging my head against a wall here. See pictures for reference.
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Cameron le 13 Mar 2023
If you have the data in a form of X and Y data points, you can do something like this
x = randi([220,800],100,1); %100 random x data points between 220 and 800
y = randi([200,600],100,1); %100 random y data points betwen 200 and 600
myindx = y - 0.5*x >= 0; %index which is above a line from x1 = 400, y1 = 200,
%x2 = 800, y2 = 400. This gives a slope of 0.5 and y intercept of 0.
NewX = x(myindx); %x values only above your myindx line
NewY = y(myindx); %y values only above your myindx line
plot(x,y,'o',...
NewX,NewY,'o',...
[400,800],[200,400],'-k')
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Philipp Brunnbauer le 13 Mar 2023
this is great, will try it! Thanks Cameron
Philipp Brunnbauer le 15 Mar 2023
Hey Cameron, I had to modify the script a bit and now have a working version of it - thanks a lot!

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