cellfun with empty cell input

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Wesley Ooms 2015-1-7

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https://ww2.mathworks.cn/matlabcentral/answers/169166-cellfun-with-empty-cell-input

编辑： Wesley Ooms 2015-1-7

When I use cellfun with an empty cell as input, the output is an empty array. Is that a bug in matlab? How can that be avoided?

If i do x = arrayfun(@(i){y(i)},t),

and t is not an empty array, then x is a cell array and if t is empty, then x is an empty array which i can understand.

but if i do x = cellfun(@(i)y(i),num2cell(t)), i would expect x to be a cell always. no matter what t is.

Is it possible to always have x a cell array without checking if it is empty?

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Answer 1

Guillaume 2015-1-7

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https://ww2.mathworks.cn/matlabcentral/answers/169166-cellfun-with-empty-cell-input#answer_164210

在 MATLAB Online 中打开

x = cellfun(@fn, c)
x = arrayfun(@fn, a)

x will never be a cell array regardless of input cell array c or array a, unless function fn returns a cell array. Thus with,

x = cellfun(@(i) y(i), num2cell(t))

unless y(i) is a cell array, x will never be a cell array.

To force cellfun and arrayfun to return cell arrays, set 'UniformOutput' to false:

x = cellfun(@(i) y(i), num2cell(t), 'UniformOutput', false)

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Guillaume 2015-1-7

在 MATLAB Online 中打开

Well to be perfectly equivalent to arrayfun, your for loop should be something like:

%y = arrayfun(@fn, x);
y = zeros(size(x));
for i = 1:numel(x)
   tmp = fn(x(i));
   if i == 1 && iscell(tmp)
      y = cell(size(x));
   end
   y(i) = tmp;
end

Both behave the same way when passed an empty array.

The behaviour makes sense in any way. By default, arrayfun returns plain arrays and only returns cell arrays if explicitly asked or if the function called returned a cell array. When passed an empty array, arrayfun has no way to know that the function it never calls would return a cell array.

More practically, what are you actually trying to do? The examples that you've shown so far don't actually need any kind of arrayfun / cellfun. num2cell would be enough.

I also don't see why 'UniformOutput', false does not solve the problem.

Wesley Ooms 2015-1-7

编辑：Wesley Ooms 2015-1-7

Yes you are right that the code doesn't do much, but that is because i stripped all the functional code to isolate the problem. Now i also see that, as you mentioned, the arrayfun has no way to know that the function it never calls would return a cell array if it would have been called. So now instead of forcing t to be a cell, i was able to easily handle noncells if empty. That solved the problem. Thank you for your help.

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