already know function: find x value given y AND plot (x,y) on the same graph

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Matlab_Student 2016-2-6

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/266868-already-know-function-find-x-value-given-y-and-plot-x-y-on-the-same-graph

评论： Star Strider 2018-2-6

Can someone help me with this simple question? I already know the function expression. I would like to find the x value corresponding to y=0.37 and plot the point (x,y) with dash lines connecting to x/y axis in the same graph on top of the line for y?

Here is my current code:

 x = 0:100000;
 y = exp(-x/38190.7);
 figure
 semilogy(x,y)
 grid on

Thank you!

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采纳的回答

Star Strider 2016-2-6

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/266868-already-know-function-find-x-value-given-y-and-plot-x-y-on-the-same-graph#answer_208757

在 MATLAB Online 中打开

This works:

x = 0:100000;
ofst = 0.37;
y = @(x) exp(-x/38190.7);                           % Create Anonymous Function
xval = fzero(@(x) y(x)-ofst, 1)                     % Solve For ‘ofst’ Value
figure(1)
plot(x, y(x))
hold on
plot([xval xval], [0 ofst], '--')
plot([0 xval], [ofst ofst], '--')
plot(xval, ofst, 'bp')
hold off
grid

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Kalyan Dash 2018-2-6

在 MATLAB Online 中打开

The code works fine. Can you please elaborate the code

plot([xval xval], [min(ylim) ofst], '--')
plot([0 xval], [ofst ofst], '--')
plot(xval, ofst, 'bp')

Star Strider 2018-2-6

MATLAB requires two values for the x and two values for y in order to plot the line. It will plot a constant line at a given x or y if both values of either the x or y coordinates are the same value.

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