Finding x intercept for 2D data

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University 2023-11-24

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https://ww2.mathworks.cn/matlabcentral/answers/2051772-finding-x-intercept-for-2d-data

回答： Pavan Sahith 2023-12-12

Hi,

Please how can i find x intercept at y=0 for 2D array. For instance, I have V which is loop over xi=0:1:10 and l=0:0.1:1.

I want to find V(xi, l)=x-intercept and plot the intercepts.

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Catalytic 2023-11-26

If your V is a function of xi and l, then what is meant by the "x-intercept at y=0"? Is l supposed to be the same as y? xi the same as x?

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Answer 1

Pavan Sahith 2023-12-12

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Hello,

I understand that you are working with a 2D array and would like to identify all the x-intercepts at y=0 and plot them.

You can do that in MATLAB using the "find(diff(sign('your_data')))" and "interp1" refer to this code with the consideration of sample 2D arrays.

Considered using sine and cosine functions to generate 2D arrays,as there will be multiple intersections with the x-axis

% Creating sample Data 
t = linspace(0, 2*pi, 1000); 
ph = -rand(10,1); 
x = cos(2*pi*t*5 + zeros(size(ph))); 
y = sin(2*pi*t*5 + ph); 

reshaping the matrices x and y into column vectors

x = reshape(x.',[],1); 
y = reshape(y.',[],1); 

"y0 = find(diff(sign(y)))";: Finds the indices where the sign of y changes, which corresponds to approximate zero-crossings.
The code then uses a for loop to iterate over the identified zero-crossings (y0) and performs linear interpolation (interp1) around each zero-crossing to estimate the x-values where y is zero.
xv(k,:) = interp1(y(idxrng), x(idxrng), 0);: Interpolates the x-values at y=0 for each identified zero-crossing. The result is stored in the matrix xv.

y0 = find(diff(sign(y)));
for i = 1:size(y0) 
    idxrng = max(1, y0(i)-1) : min(numel(x),y0(i)+1); 
    xv(i,:) = interp1(y(idxrng), x(idxrng), 0); 
end 
% Plotting 
plot(x, y, 'DisplayName', 'Original Data') 
hold on 
plot(xv, zeros(size(xv)), '+r', 'DisplayName', 'X-Intersections') 
hold off 
grid 
legend('Location','eastoutside') 