Find the last non-nan in large array

Question

meghannmarie 2019-5-30

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https://ww2.mathworks.cn/matlabcentral/answers/464782-find-the-last-non-nan-in-large-array

评论： Walter Roberson 2021-2-24

I have very large 4-dimensional arrays with dimensions being time,depth, lat, lon. I want to find the linear index in the depth dimension to the last non-nan data value to I can evaluate the values at the "bottom". I am looking for the output to be 3-D (time, lat, lon) with linear indexes to the last depth with a value (non-nan).

Any idea how to do this efficiently? I have a bunch of very large files to run this on.

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Answer 1

Guillaume 2019-5-30

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https://ww2.mathworks.cn/matlabcentral/answers/464782-find-the-last-non-nan-in-large-array#answer_377211

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One way:

%m: a 4D matrix (time x depth x lat x lon
[~, idx] = max(cumsum(~isnan(m), 2), [], 2)

This relies on the fact that max returns the index of the first max value if several are identical. The first max value of cumsum(~isnan) is the last number before zero or more nan.

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Pepe Grillo 2021-2-24

Hi, I was looking exactly the same, but when I run this code I get a 4D array and when plt is the number of values before the last NaN, but I want to plot the value,

what I am doing wrong?

[~, idx] = max(cumsum(~isnan(m), 3), [], 3) %??

Thanks