The "plot" and "polyfit".
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Zhengsong Liao
on 25 May 2015
Commented: Zhengsong Liao
on 27 May 2015
I want to plot a graph of a function fitted by two list of t and fp. A dependent variable fp is gotten by a independent variable t with a fitting function using "ployfit". I don't know how to plot a graph of the function ,even if I got "fp"s from "t" given arbitrarily.The code is shown below.
function y=fp(t)
t_1=[0 0.15 0.49 2.1126];
t_2=[2.27 3.53 8.78 25.45 42.80 43.68 44.08];
p_1=9.80665*[331.2 614.3 505.4 607.8];
p_2=9.80665*[48.65 43.97 42.01 41 40.8 40.79 2.22];
y=polyfit(t_2,p_2,2)*[t.*t;t;1]*(t>=2.1126)+polyfit(t_1,p_1,2)*[t.*t;t;1]*(t>=0&&t<2.1126);
end
Actually,the "plot" seems workable, but I need help to plot;
1 Comment
John D'Errico
on 25 May 2015
Please learn to format your code so it will be readable. I've done that for you this time.
You do realize that this is not actually a spline that you are building? It is not even a continuous function across the breakpoint.
Accepted Answer
John D'Errico
on 25 May 2015
It looks like you are trying to build a VERY simple spline, but not doing a very credible job of it. (Sorry.) This is a piecewise quadratic function, but it is one that will not even be continuous across the breakpoint.
t_1=[0 0.15 0.49 2.1126];
t_2=[2.27 3.53 8.78 25.45 42.80 43.68 44.08];
p_1=9.80665*[331.2 614.3 505.4 607.8];
p_2=9.80665*[48.65 43.97 42.01 41 40.8 40.79 2.22];
poly1 = polyfit(t_1,p_1,2);
poly2 = polyfit(t_2,p_2,2);
plot(t_1,p_1,'bo',t_2,p_2,'ro')
hold on
t = linspace(0,2.1126,100);
plot(t,polyval(poly1,t),'b-')
t = linspace(2.1126,44.08,100);
plot(t,polyval(poly2,t),'r-')
plot([2.1126 2.1126],[0 607.8],'g-')
grid on
In fact, the entire modeling effort is (again, sorry) rather insane. That first piece with only 4 wildly scattered points does not even justify a quadratic polynomial for that fit.
I don't know what you are trying to do here, nor why you are trying to force a quadratic polynomial through that mess. But I'm not sure that you know why either.
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