MATLAB: Fit points to curve with zero slope

MATLABmatlab functionpolyfit

I have to fit a set of coordinates (two points) to a linear slope, however, in one case I know the slope is 0. I'm using polyfit.

polyfit([x1 x2], [y1 y2], 1)

This returns a slope coefficient of -263.0023e-018.

I know the slope for this specific case is 0, but I want to make my script able to handle changes to input, which is why I don't want to manually change the slope coefficient. While e-018 might be accurate enough for the task at hand, I'd like to be able to do it correctly.

So how do I fit two points to a linear curve where the slope is 0?

Best Answer

The polynomial is given by

p = (sum_{i=1}^{n} yi) / n

where

(x1,y1), (x2,y2), ..., (xn,yn)

are the given data points.

In your case:

p = (y1+y2)/2

Best wishes

Torsten.

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MATLAB: To find intersection point of two lines

Hi,

not sure if I understand correctly what you want but is this what you are looking for?

%line1
x1  = [7.8 8.5];
y1  = [0.96 0.94];
%line2
x2 = [8.25 8.25];
y2 = [0 0.99];
%fit linear polynomial
p1 = polyfit(x1,y1,1);
p2 = polyfit(x2,y2,1);
%calculate intersection
x_intersect = fzero(@(x) polyval(p1-p2,x),3);
y_intersect = polyval(p1,x_intersect);
line(x1,y1);
hold on;
line(x2,y2);
plot(x_intersect,y_intersect,'r*')

MATLAB: How to fit a trend line in plotyy

You could use:

p = polyfit(y,data1,n); % looks like you want n=1 in your case

p contains the coefficients of the line:

fitLine = p(1)*y + p(2);

or equivalently:

fitLine = polyfit(p,y);

Best Answer

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MATLAB: How to fit a trend line in plotyy

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