Delete part of a 4D array

Question

Suuz HMS 2024-3-28

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2100101-delete-part-of-a-4d-array

评论： Rik 2024-3-28，14:28

Hi! I have a 4D array with dimensions 7501 x 5 x 2 x 5 (data samples x dyads x rower x trials), and for some specific trials, I would like to remove some samples since the data is not gathered correctly.

For example, I would like to remove data samples 2501:7501 of dyad 1, both rower 1 and 2, trial 4. Is this possible?

I already tried

if (dyad == 1 && trial == 4)

data(2501:7501,:,:,:) = [];

end

But that does not seem to work, since then the data from all trials there after are also being removed (so that just 1:2500 remains).

If someone would have a tip, that would be very helpful, thanks!

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Answer 1

Rik 2024-3-28

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https://ww2.mathworks.cn/matlabcentral/answers/2100101-delete-part-of-a-4d-array#answer_1432871

在 MATLAB Online 中打开

Let's try something:

try
    data=rand(4,3,5,6);
    data(1:2,3,:,:)=[];
catch ME
    warning('error: %s',ME.message)
end
Warning: error: A null assignment can have only one non-colon index.

Why do you thing this happens? Perhaps it is easier to understand with fewer dimensions:

data = reshape(1:6,2,3)
data = 2×3
     1     3     5
     2     4     6
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>
data(2,3,:) = []
A null assignment can have only one non-colon index.

Suddenly it is clear why this doesn't work: you're trying to remove only part of the array. You need to remove full slices. You can remove multiple slices, but not part of slices.

The solution in this case would be to assign NaN, or use a cell, since that can contain empty values.

data(2501:7501,dyad==1,:,trial==4) = NaN;

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Suuz HMS 2024-3-28

Thank you for your respons!

When I use the option

data(2501:7501,1,:,4) = NaN;

it seems to work and the right samples get replaced by NaN, but when I try to plot the data, nothing comes up.

When I use a cell

data(2501:7501,1,:,4) = {};

I get the warning:

Conversion to double from cell is not possible.

Do you know how this can be solved?

Rik 2024-3-28，14:28

How are you plotting your data? Does any intermediary step compute some summary statistic in a way that doesn't properly deal with NaN values? I didn't hack your computer, so you will have to tell me or think of the underlying cause on your own.

As to your second point: You shouldn't assign an empty cell to elements of your double array. What I suggested is to use a cell array, and then wipe the contents of those cell elements. That has wider implications for the rest of your code (i.e. you will have to change a lot of your code).

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