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

Question

mfclose 2019-7-19

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/472545-find-first-non-nan-in-each-column-of-array-combine-into-one-vector

编辑： Adam Danz 2019-7-19

采纳的回答： Bruno Luong

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...

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Answer 1

Bruno Luong 2019-7-19

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https://ww2.mathworks.cn/matlabcentral/answers/472545-find-first-non-nan-in-each-column-of-array-combine-into-one-vector#answer_383989

编辑：Bruno Luong 2019-7-19

在 MATLAB Online 中打开

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))

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mfclose 2019-7-19

Thank you, this did exactly what I needed!

Adam Danz 2019-7-19

编辑：Adam Danz 2019-7-19

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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Answer 2

Adam Danz 2019-7-19

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https://ww2.mathworks.cn/matlabcentral/answers/472545-find-first-non-nan-in-each-column-of-array-combine-into-one-vector#answer_383994

编辑：Adam Danz 2019-7-19

在 MATLAB Online 中打开

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

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mfclose 2019-7-19

Thank you this also worked!

Adam Danz 2019-7-19

编辑：Adam Danz 2019-7-19

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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Answer 3

Mario Chiappelli 2019-7-19

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https://ww2.mathworks.cn/matlabcentral/answers/472545-find-first-non-nan-in-each-column-of-array-combine-into-one-vector#answer_383984

Check out this question asked earlier on the forum, I think it asks and answers what you want.

https://www.mathworks.com/matlabcentral/answers/243654-find-first-non-nan-element-in-a-column-of-a-matrix

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Mario Chiappelli 2019-7-19

在 MATLAB Online 中打开

Try this:

x = [2,NaN,NaN,NaN;4,6,NaN,NaN;NaN,NaN,NaN,NaN;5,4,7,8];
rows = length(x(:,1));
columns = length(x(1,:));
% This finds the first non NaN in the columns
columnVector = double(columns);
for i = 1:columns
    found = 0;
    for j = 1:rows
        if ~isnan(x(j,i)) && found == 0
            found = 1;
            columnVector(i) = x(j,i);
        end
    end
    if found == 0
        columnVector(i) = NaN;
    end
end
% This finds the first non NaN in the rows
rowVector = double(rows);
for i = 1:rows
    found = 0;
    for j = 1:columns
        if ~isnan(x(i,j)) && found == 0
            found = 1;
            rowVector(i) = x(i,j);
        end
    end
    if found == 0
        rowVector(i) = NaN;
    end
end

I could include some more functionality to delete any NaN values in the resulting columnVector and rowVector arrays. This does what you want, however it skips over the middle steps. I did not know if that was a necessary step for you or just your way of visualizing what needed to be done.

Hope this helps :)

mfclose 2019-7-19

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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