MATLAB: Using polyfit and polyval functions with data

Question

I've been using the polyfit and polyval functions recently and I would like to approximate data between 2 vectors but I'm wondering if Ive made a mistake somewhere.

If

a=[1;2;3;4;5;nan;6;nan;nan;10];%prices with data missing
b=[1;2;3;4;5;6;6;7;8;10]; %original prices
days=[1;2;3;4;5;6;7;8;9;10]%number of days
NotMissing=~isnan(a(:,1));
p=a(NotMissing,1); %prices with data not missing
days=a(NotMissing,1);
pf=polyfit(days,p,1);
pv=polyval(pf,days); %polyval approximation of length 7 compared to b which has length 10
%I've want to calculate the mean square error between the approximate prices pv and the original prices using
%E = sqrt( sum( (b-pv).^2) / numel(b) );

Have I made a mistake using the polyval function, in this case I cannot calcuate E because the vectors are different lengths so I'm wondering if Im using the polyfit and polyval functions incorrectly because my length is being reduced by the number of nan values. Or am I 'calculating the error ' between the 2 vectors incorrectly

Answer 1

To do what you were after in your post, you need to fit with the NotMissing data, but then apply the fit at all days,

a=[1;2;3;4;5;nan;6;nan;nan;10];%prices with data missing
b=[1;2;3;4;5;6;6;7;8;10]; %original prices
days=[1;2;3;4;5;6;7;8;9;10];%number of days
NotMissing=~isnan(a);
pf=polyfit(days(NotMissing),a(NotMissing),1);
pv=polyval(pf,days)