Last non nan observation

65 views (last 30 days)
joseph Frank on 18 Feb 2012
Commented: Sven on 18 Feb 2023
Hi,
I have a matrix and I want to find in each column the last non nan observation. Is there a neat way of doing it quickly? Regards

Jiro Doke on 18 Feb 2012
I'm not sure if you're looking for the indices (row number) or the actual value of the last non nan observation. So here are both:
B = ~isnan(A);
% indices
Indices = arrayfun(@(x) find(B(:, x), 1, 'last'), 1:size(A, 2));
% values
Values = arrayfun(@(x,y) A(x,y), Indices, 1:size(A, 2));
3 CommentsShow 1 older commentHide 1 older comment
Poulomi on 18 Feb 2023
how to overcome this snag @Sven ?
Sven on 18 Feb 2023
See my answer below for a file exchange entry

Sven on 18 Feb 2012
Here I can plug a file exchange entry:
Im = rand(50);
Im(Im>.7) = nan;
lastNans = find_ndim(~isnan(Im),1,'last')
To quote: I = FIND_NDIM(BW,DIM,'last') returns the subscripts of the last nonzero elements of BW along the dimension DIM.
The solution hijacks the max() function... it's only a few lines but it packages up things nicely (such as returning 0 if no NaNs were found in a column) - hope you like it.
And if you don't want to use the function itself you can get your answer with just the relevant lines as follows:
BW = ~isnan(Im);
[~, foundPx] = max(BW,[],1);
foundPx = size(BW,1)+1 - foundPx; % Need to count backwards to get last non-zero
foundPx(~any(BW,dim)) = 0; % Account for all-zero entries by setting output to 0