How to get a line from two points collected using ginput

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Faisal 2023-4-18

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https://ww2.mathworks.cn/matlabcentral/answers/1949053-how-to-get-a-line-from-two-points-collected-using-ginput

评论： Faisal 2023-4-26

OBJECTIVE. I have a curve, I want to select two points on the curve, fit a line between two points (overlaying on the main curve) and then find the intercept and slope of the fitted line.

MY METHODOLOGY. I can get two points using ginput function, but after, I am having trouble to find the points between the points using "find" function. also, I am unable to fit or plot the line of points from find or ginput function.

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Rik 2023-4-18

Why do you need find? You can calculate the slope and intercept from the two points directly.

Also, what exactly have you tried when you wanted to plot the second line? Did you forget to use the hold function?

Faisal 2023-4-18

I tried polyfit to fit a line after ginput.

how to find the intercept from two points?

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Answer 1

John D'Errico 2023-4-18

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编辑：John D'Errico 2023-4-18

在 MATLAB Online 中打开

You aparently know how to use polyfit. But it seems you do not understand what it returns.

x = 1:5;
y = [2 3 5 7 11];
P1 = polyfit(x,y,1)
P1 = 1×2
    2.2000   -1.0000

The result is a vector. The first element is the slope of the vector. The second element is the y-intercept.

plot(x,y,'o')

hold on

xpl = [0,5]

xpl = 1×2

0 5

ypl = polyval(P1,xpl);

plot(xpl,ypl,'r-')

As you can see, I extended the line down to x==0, where you should see it crosses the y-axis at y==-1, the y-intercept. So polyfit gives you exactly what you need. There is no need to "find" anything. You already have it.