already know function: find x value given y AND plot (x,y) on the same graph
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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!
采纳的回答
Star Strider
2016-2-6
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
5 个评论
Matlab_Student
2016-2-7
Hi, thank you very much for the answer! When I changed plot(x, y(x)) to semilogy(x, y(x)) the dash line connecting to x-axis disappeared. Do you know how to fix this?
Star Strider
2016-2-7
My pleasure!
The reason is the log of zero is -Inf, so the zero didn’t plot. The fix was to change the zero to min(ylim) in the first plot call in the hold block.
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)
semilogy(x, y(x))
hold on
plot([xval xval], [min(ylim) ofst], '--')
plot([0 xval], [ofst ofst], '--')
plot(xval, ofst, 'bp')
hold off
grid
Walter Roberson
2016-2-7
When you have
plot([0 xval], [ofst ofst], '--')
then that first one has x = 0. You have a log plot so log(0) is the left coordinate. And log(0) is -infinity . MATLAB never draws lines when the coordinate is infinite. You need to change that 0 to some positive value. Likewise in the other plot() line.
Kalyan Dash
2018-2-6
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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