MATLAB: Find the last non-nan in large array

findfunction on dimensionlinear indexMATLABnon-nan

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.

Best Answer

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.

Related Solutions

MATLAB: Selecting non-nan sections of a data series

test = load('test.txt');
mask = ~isnan(test.');
starts = strfind([0 mask], [0 1]);
stops = strfind([mask 0], [1 0]);

Now starts(K) is the index of the first non-nan in a group and stops(K) is the index of the last non-nan in the group.

MATLAB: Is there a simpler way to find the index of the first non NaN value in a vector

X= [NaN NaN 1 2 3 4 5]; 
first_non_NaN_index_of_X = find(~isnan(X), 1);

Best Answer

Related Solutions

MATLAB: Selecting non-nan sections of a data series

MATLAB: Is there a simpler way to find the index of the first non NaN value in a vector

Related Question