polyfit returns NaN values in p

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?

Answer 1

[John D'Errico]

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.

Answer 2

[carlo lefevre]

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

[Steven Lord]

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.)