polyfit returns NaN values in p

Question

florence briton 2015-5-18

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评论： Steven Lord 2015-12-9

I want to do a linear regression on the response Y (attached)with a predictor vector X. Therefore, I used Matlab polyfit function: [p,s,mu]=polyfit(X,Y,1) but it returns p=[NaN,Inf]

I have Inf values in vector Y. Is it the cause?

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Answer 1

John D'Errico 2015-5-18

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编辑：John D'Errico 2015-5-18

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Yes. infs in your data will surely cause NaNs, and essentially garbage results. Drop them out first if you expect something meaningful.

y = [rand(10,1);inf];
x = rand(11,1);
p = polyfit(x,y,1)
p =
   NaN   Inf

Surely you don't expect anything reasonable to come out of such a computation with INFs in it. What would the slope of such a line be with some data points at infinity? Of course, any such point at infinity would have infinite leverage.

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Answer 2

carlo lefevre 2015-12-9

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ind = 1:length(a); k = ~isnan(a); p = polyfit(ind(k),a(k),n)

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Steven Lord 2015-12-9

You probably want to use ISFINITE instead of ~ISNAN to include only finite values (excluding Inf, -Inf, and NaN) rather than including only non-NaN values (which would NOT exclude Inf or -Inf.)

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