Polynomial fit from the equation

Hello guys, I have an equation y=a+b*lg(x),

Time values x=[ 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0]

Data values y=[39.06 41.72 43.43 47.87 49.2 52.39 56.32 64.29 69.49 70.18 75.42 79.42 95.79 102.45 110.04 119.89 125.99 130.88 151.13 157.01 174.57]

And I am trying to draw a line with the given values, using the equation above and using polyfit polyval functions. But I don't get any output line. My code is:

z = log10(data)
p = polyfit(time, z, 1)
y4 = polyval(p, time)
figure
plot(time, log10(y4), 'g-')

Maybe you see the problem? Thanks in advance

Answer 1

[dpb]

Maybe you see the problem?

Ayup...in

plot(time, log10(y4), 'g-')

you transformed the output of the transformed model input again in the same direction instead of reverse back to original space...

plot(time, y4.^10, 'g-')

instead but I'd suggest recasting somewhat and use

yHat=10.^polyval(x);

when you compute the predicted values rather than waiting.

[adrixas]

Thank you, that was the problem.

Answer 2

[James Tursa]

You appear to have both x and time meaning the same thing, and y and data meaning the same thing. Maybe it is just a variable naming issue. E.g., try

z = log10(y);
p = polyfit(x, z, 1);
y4 = polyval(p, x);
figure
plot(x, log10(y4), 'g-');