how to plot a curve fitting on a semilog

Question

mohamed 2016-4-7

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/277822-how-to-plot-a-curve-fitting-on-a-semilog

评论： mohamed 2016-4-8

im trying to plot a curve fitting on a semi log graph to get its slope this is what ive tried

pws=[1150 1794 1823 1850 1876 1890 1910];
t=[0 2 4 8 16 24 48];
tp=3*24;
for i=2:length(pws)
horn(i)=((t(i)+tp)/(t(i)))
end
semilogx(horn,pws,'O-')
m=polyfit(horn,pws,1)
m(1)
k=((162.6*q*b*u)/(m(1)*h))
set(gca,'YLim',[1780 1960])
hold on
mm=polyval(m,horn)
semilogx(horn,mm,'-r');

but when i try it all i get is that

2 个评论
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Walter Roberson 2016-4-7

Your variable q is not defined in this code.

mohamed 2016-4-7

在 MATLAB Online 中打开

q=500;
pwf=1150;
tp=3*24;
h=22;
phi=0.2;
rw=0.3;
b=1.13;
ct=20e-6;
u=1;

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Answer 1

Walter Roberson 2016-4-8

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/277822-how-to-plot-a-curve-fitting-on-a-semilog#answer_216977

在 MATLAB Online 中打开

You are using horn as your base variable to fit against, but horn is not monotonic. If you

plot(horn,pws)

you can see that clearly.

semilogx(horn,pws) does not show how bad the graph is because horn(1) is 0 (because you did not assign any value to horn(1)), and log(0) is -infinity, so the first data point is not drawn on the semilogx plot.