MATLAB polyfit, palyval not working correctly

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Christina Kersten
评论： Christina Kersten 2019-3-8
Basically I'm just trying to curve fit some data but the line of best fit is not correct. Here is the code I have so far
clear all; close all
c = xlsread('Elastic Modulus.xlsx','Sheet1');
temp = c(:,1);
carbonSteel = c(:,2);
plot(temp,carbonSteel,'*')
hold on
p = polyfit(temp,carbonSteel,3);
y = polyval(p,carbonSteel);
plot(temp,y)
The temp and carbonsteel columns contain the values:
temp =
-200
-129
-73
21
93
149
204
260
316
371
427
482
538
593
carbonSteel =
31.4000
30.8000
30.2000
29.5000
28.8000
28.3000
27.7000
27.3000
26.7000
25.5000
24.2000
22.4000
20.4000
18.0000
When I plot it just shows a straight line. How do I fix this? Thank you!

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采纳的回答

John D'Errico
John D'Errico 2019-3-8
p = polyfit(temp,carbonSteel,3);
y = polyval(p,carbonSteel);
What did you build? A model, that predicts carbonsteel, as a function of temp. So temp is the INDEPENDENT variable.
Now, you want to predict the dependent variable, as as a function of temp. So you need to do this:
y = polyval(p,temp);
plot(temp,carbonsteel,'bo',temp,y,'r-')
untitled.jpg
What you did, as you did it, makes no sense in context of the model you built..

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