Interpolating NaN-s

Question

Karlito 2012-4-3

2
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/34346-interpolating-nan-s

评论： Matteo Soldini 2020-2-13

I have a time series, where there are some missing values. I've marked them as NaN. How would it be possible for me to interpolate them from the rest of the data. So my data to interpolate looks like that (just example numbers):

x=
482230405436799
0140930751890233
622880344434796
NaN
527433962776634
724991961357239
607415791144935
588366445795251
433434840033058
244172893507025
428960353778007
0101774555217771
608821449044360
957975188197358
NaN
0355905633801477
886235111325751
246941365057497
00891505625333700
814920271448039
140499436548767
879866440284003
0953767860905247
352560101248640

How to get a value for those NaN-s? I've tried Interp1 but i can't get it working properly.

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Jan 2012-4-3

It is a good idea to post your trials with INTERP1, such that the problem can be fixed. This is usually easier that creating a solution from the scratch and you could learn, what went wrong.

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Answer 1

Andrei Bobrov 2012-4-3

5
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https://ww2.mathworks.cn/matlabcentral/answers/34346-interpolating-nan-s#answer_43054

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inn = ~isnan(x);
i1 = (1:numel(x)).';
pp = interp1(i1(inn),x(inn),'linear','pp')
out = fnval(pp,linspace(i1(1),i1(end),1000));
plot(i1,x,'ko',linspace(i1(1),i1(end),1000),out,'b-')
grid on

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Answer 2

Jan 2012-4-3

14
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/34346-interpolating-nan-s#answer_43060

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nanx = isnan(x);
t    = 1:numel(x);
x(nanx) = interp1(t(~nanx), x(~nanx), t(nanx));

This does not catch NaNs at the margins, such that you have to care for them by your own, e.g. by repeating the first or last non-NaN value.

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Muhammad Usman Saleem 2016-2-3

编辑：Muhammad Usman Saleem 2016-2-3

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Thanks @Jan Simon.

Please go to my question where i have posted data set also with complete description of my question(as Stephen mentioned). Actually i am interpolating NaN values in text file these NaN values are only 3rd column of each text file. I search your way of interpolation in my code

% Read the file data:
S = dir('*Output_*.txt');
N = sort({S.name});
P = numel(N);
R = size(load(N{1},'-ascii'),1);
M = zeros(R,3,P);
for k = 1:P
  M(:,:,k) = load(N{k},'-ascii');
end
tmp = 0==diff(M(:,1:2,:),1,3);
assert(all(tmp(:)),'First columns do not match')
% Rearrange:
[NA,NA,X] = unique(M(:,1,1)); 
[NA,NA,Y] = unique(M(:,2,1));
Z = reshape(M(:,3,:),max(X),max(Y),P);
% Detect -9999
Z(Z<-9998) = NaN;
 nanx = isnan(Z);%logical vector for NaN
   t = 1:numel(Z);%t run through length of z
 Z(nanx) = interp1(t(~nanx), Z(~nanx), t(nanx));

after texting on one text file of my data, i have seen that all NaN values interpolated by this code but bottom NaN values are not interpolated? As you says, it will be linear interpolation by default then why when i replace this line with

Z(nanx) = interp1(t(~nanx), Z(~nanx), 'spline');

I am getting this error

???  In an assignment  A(I) = B, the number of elements in B and
 I must be the same.

Steven Lord 2017-8-20

Check if the fillmissing function does what you want.

Matteo Soldini 2020-2-13

If I have 50 variables and I use Jan's code to interpolate each nan in each variable (variables have different ranges of values, for example var1 = [1,2], var2 = [300,400]), will it work or should I use a for loop?

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Answer 3

John D'Errico 2012-4-3

9
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https://ww2.mathworks.cn/matlabcentral/answers/34346-interpolating-nan-s#answer_43062

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My inpaint_nans will fill them in with interpolated values, even if there are multiple consecutive nans, and it will extrapolate nicely at the ends. It works on 1-d problems.

http://www.mathworks.com/matlabcentral/fileexchange/4551-inpaintnans

x = inpaint_nans(x);