How to find x value for a known Y in array?

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Zeyad Elreedy 2022-12-11

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https://ww2.mathworks.cn/matlabcentral/answers/1876027-how-to-find-x-value-for-a-known-y-in-array

评论： Zeyad Elreedy 2022-12-12

I have x and y arrays for a sine wave. I am trying to find the x value for when y is equal to 0, since the y array does not contain a value of 0. How can I do so?

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Torsten 2022-12-11

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https://ww2.mathworks.cn/matlabcentral/answers/1876027-how-to-find-x-value-for-a-known-y-in-array#answer_1125722

编辑：Torsten 2022-12-11

在 MATLAB Online 中打开

x = 0.1:0.1:4*pi-0.1;
y = sin(x);
i = find(y(1:end-1).*y(2:end) <= 0)
i = 1×3
    31    62    94
x_root = x(i)-y(i).*(x(i+1)-x(i))./(y(i+1)-y(i))
x_root = 1×3
    3.1416    6.2832    9.4248
[pi,2*pi,3*pi]
ans = 1×3
    3.1416    6.2832    9.4248

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Torsten 2022-12-11

编辑：Torsten 2022-12-11

在 MATLAB Online 中打开

I determine x(i) and x(i+1) between which y changes sign. Then I approximate the zero between x(i) and x(i+1) by linear interpolating between the points (x(i),y(i)) and (x(i+1),y(i+1)) and determining the x_root value as the x-value where this line crosses the x-axis.

So interp1 is implicitly used in the line

x_root = x(i)-y(i).*(x(i+1)-x(i))./(y(i+1)-y(i))

Zeyad Elreedy 2022-12-12