Find first non-NaN in each column of array & combine into one vector

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mfclose
mfclose on 19 Jul 2019
Edited: Adam Danz on 19 Jul 2019
I am trying to:
1) Identify the first non-NaN value of each column in an array, and convert the rest to NaN
2) Identify the first non-NaN value of each row in the array, and convert the rest to NaN
3) Combine these numbers back into one column vector
I have created something that looks roughly like this (although much larger):
X =
2 NaN NaN NaN
4 6 NaN NaN
NaN NaN NaN NaN
5 4 7 8
After the first step (keeping only the first number in each column), I would hope to have this:
Xa =
2 NaN NaN NaN
NaN 6 NaN NaN
NaN NaN NaN NaN
NaN NaN 7 8
After the second step (keeping only the first number in each row), I would hope to have this:
Xb =
2 NaN NaN NaN
NaN 6 NaN NaN
NaN NaN NaN NaN
NaN NaN 7 NaN
Lastly, I would want to turn this into one column (there should only be NaN's or 1 non-NaN value in each row - I would want it to keep whichever is there)
Xc =
2
6
NaN
7
I apologize for not having any attempted code for this. I appear to have gotten stuck in the doorway of this one...

Accepted Answer

Bruno Luong
Bruno Luong on 19 Jul 2019
Edited: Bruno Luong on 19 Jul 2019
X = [2 NaN NaN NaN;
4 6 NaN NaN;
NaN NaN NaN NaN;
5 4 7 8 ]
[m,n] = size(X);
[I,J] = ndgrid(1:m,1:n);
Xa = X;
[~,r]=max(isfinite(Xa),[],1);
Xa(I>r) = NaN
Xb = Xa;
[~,c]=max(isfinite(Xb),[],2);
Xb(J>c) = NaN
[~,c]=max(isfinite(Xb),[],2);
Xc = Xb(sub2ind([m,n],(1:m)',c))
  2 Comments
Adam Danz
Adam Danz on 19 Jul 2019
Be sure to add semicolons to the end of 3 lines that are missing them.
Also, use ~isnan() rather than isfinite().

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More Answers (2)

Adam Danz
Adam Danz on 19 Jul 2019
Edited: Adam Danz on 19 Jul 2019
X = [2 NaN NaN NaN
4 6 NaN NaN
NaN NaN NaN NaN
5 4 7 8];
X(cumsum(cumsum(~isnan(X)))~=1) = NaN; % your Xa
X(cumsum(cumsum(~isnan(X),2),2) ~=1) = NaN; % your Xb
Xc = NaN(size(X,1),1);
Xc(any(~isnan(X),2)) = X(~isnan(X)); % your Xc
  2 Comments
Adam Danz
Adam Danz on 19 Jul 2019
Glad I could help! Just FYI, this solution and the max(infinite()) solution are both quite fast but this one is 4.5x faster (10,000 iterations, p<0.0001 Wicoxon signed rank) with less overhead. Also, instead of isfinite(), you should use ~isnan().

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Mario Chiappelli
Mario Chiappelli on 19 Jul 2019
Check out this question asked earlier on the forum, I think it asks and answers what you want.
  3 Comments
mfclose
mfclose on 19 Jul 2019
This was very close, but I did need the middle step getting rid of the other non-NaN values in a column. The code below fixed this. Thank you so much for helping!

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