Indexing question for member variables of opticalFlow collected into an MxN cell array

I am attempting to find the maximum value over alll member variables (PxQ) for a multidimensional set of MxN opticalFlow system objects

Flow_PlaneA{TestNum,frame} loads up a single opticalFlow system object from a mat file

I would like to find max value of all something like:

VxMax = max(Flow_PlaneA{:,:}.Vx

VyMax = max(Flow_PlaneA{:,:}.Vy

Magnitude Max = max(Flow_PlaneA{:,:}.Magnitude

OrientationMax = max(Flow_PlaneA{:,:}.Orientation

This gives me an error for the VxMax equation for example:

"Expected one output from a curly brace or dot indexing expression, but there were " N "results." (where N = 802 for this data)

The only way I can seem to index is to do 2 nested loops for each member variable for example

for TestNum = 1:M

for frame = 1:N

VxMaxNew = max(max(Flow_PlaneA{TestNum,frame}.Vx )); % max(max( covers PxQ)

VxMax = max([VxMaxNew VxMax])

end

end

Is there an array type indexing based method to do this I am just missing? How do I handle the mix of opticalFlow objects (which look like structs to me) and the multidimensional cellarray indexing at the same time? Is that possible?

Answer 1

[Ameer Hamza]

Try this

VxMax = [flow{:}];
VxMax = max([VxMax.Vx]);

or alternatively

VxMax = max(cellfun(@(x) x.Vx, flow), [], 'all');

[Erick Oberstar]

1) VxMaxx = [Flow_PlaneA{:}]; VxMaxx = max([VxMaxx.Vx]); returns a 160400 element vector not a single number like I would expect

2) K>> VxMax = max(cellfun(@(x) x.Vx, Flow_PlaneA), [], 'all');

Error using cellfun

Non-scalar in Uniform output, at index 1, output 1.

Set 'UniformOutput' to false.

3) attempting to fix 2) K>> VxMax = max(cellfun(@(x) x.Vx, Flow_PlaneA,'UniformOutput',false),[], 'all');

Error using max

Invalid data type. First argument must be numeric or logical.

I'm a little lost

[Erick Oberstar]

Is the 'all' dimension flag not supported in Matlab 2018a?

below appears to be the correct cellfun formatting

K>> VxMax = cellfun(@(x) max(x.Vx,[],'omitnan','all'), Flow_PlaneA,'UniformOutput',false);

Error using max

Dimension argument must be a positive integer scalar within indexing range.

This formulation seems to match my double loop method answer:

VxMax = max(cell2mat(cellfun(@(x) max(max(x.Vx)), Flow_PlaneA,'UniformOutput',false)))

[Ameer Hamza]

Erik, the problem happened with the code in my answer because the arrangement of data is not obvious. Can you attach the variable Flow_PlaneA in a .mat file?

[Erick Oberstar]

A little big but here it is: https://uwmadison.box.com/s/8be2s8die5y3y5imzyf8xd49vvcxnxq1

[Ameer Hamza]

The solution could have been more efficient, but some of the cells are empty. The solution requires the creation of an intermediate variable. Try the following two alternatives

maxVx = max(arrayfun(@(x) max(max(x.Vx)), [Flow_PlaneA{:}]));

or

maxVx = max(max(cellfun(@helperFun, Flow_PlaneA)));

function y = helperFun(x)
    if ~isempty(x)
        y = max(max(x.Vx));
    else
        y = -inf; % smallest value
    end
end

'all' is not supported in R2018a, so two calls to max() are required.