Implicit expansion with empty arrays

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Matt J 2019-8-21

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https://ww2.mathworks.cn/matlabcentral/answers/477060-implicit-expansion-with-empty-arrays

评论： Bruno Luong 2019-8-22

采纳的回答： James Tursa

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I was just idly curious why scalar expansion of an empty array seems to work here (R2018a),

>> [1,2,3;4 5 6]-zeros(2,3,0)
ans =
  2×3×0 empty double array

but not here,

  >> [1,2,3;4 5 6]-zeros(2,0,0)
Error using  - 
Array dimensions must match for binary array op.

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James Tursa 2019-8-21

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编辑：James Tursa 2019-8-21

In the 1st case, you are expanding a dimension of 1 (the 3rd dimension of the first operand) to 0, so it is scalar expansion.

In the 2nd case, you are trying to expand a dimension of 3 (the 2nd dimension of the first operand) to 0, so it is not scalar expansion ... it is simply a dimension mismatch.

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Steven Lord 2019-8-22

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Rik wrote: That is just a case where the name doesn't match the operation.

Yes, but "scalar {or implicit} expansion except when the size of the other operand in a particular dimension is 0 in which case it is scalar {or implicit} contraction" is a bit of a mouthful.

Bruno wrote: So if you apply your method to

rand(3,10) + rand(2,10);

you would get 6 x 10 result.

But how would those inputs be replicated? repmat style or repelem style? Collated or uncollated?

>> x = reshape(1:6, [2 3]);
>> x1 = repmat(x, [3 1]);
>> x2 = repelem(x, 3, 1);
>> isequal(x1, x2) % false

If you're replicating in a singleton dimension, they're the same. Collating multiple copies of a 1-page document is the same as not collating them.

>> y = 1:10;
>> isequal(repmat(y, 3, 1), repelem(y, 3, 1)) % true

Rik wrote: Personally, I would have voted for extending bsxfun to support more functions, instead of enabling implicit expansion for all operations.

You can pass a function handle that accepts two inputs into bsxfun.

>> fun=@(a,b) a+b;
>> A=1:5;B=A';
>> C1 = bsxfun(fun, A, B);
>> C2 = A + B; % Implicit expansion
>> isequal(C1, C2) % true
>> bsxfun(@besselj, A, B) % works