all function B = all(A < 0.5,3)

Question

Marie Nancy B 2023-6-14

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1983194-all-function-b-all-a-0-5-3

编辑： Stephen23 2023-6-15

This is straight forward to me:

A = [0.53 0.67 0.01 0.38 0.07 0.42 0.69] 
A = 1×7
    0.5300    0.6700    0.0100    0.3800    0.0700    0.4200    0.6900
B = all(A < 0.5,1)
B = 1×7 logical array
   0   0   1   1   1   1   0

this also makes sense -

A = [0.53 0.67 0.01; 0.38 0.07 0.42 ] 
A = 2×3
    0.5300    0.6700    0.0100
    0.3800    0.0700    0.4200
B = all(A < 0.5,2)
B = 2×1 logical array
   0
   1

But why does this yield the same result?

A = [0.53 0.67 0.01 1.1; 0.38 0.07 0.42 0.01] 
A = 2×4
    0.5300    0.6700    0.0100    1.1000
    0.3800    0.0700    0.4200    0.0100
B = all(A < 0.5,3)
B = 2×4 logical array
   0   0   1   0
   1   1   1   1

OR

B = all(A < 0.5,4)
B = 2×4 logical array
   0   0   1   0
   1   1   1   1

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Stephen23 2023-6-15

编辑：Stephen23 2023-6-15

在 MATLAB Online 中打开

"But why does this yield the same result?"

Consider the sub-arrays (i.e. vectors) along each of the specified dimensions: what values do they have in them? Remember that all arrays implicitly have infnite trailing singleton dimensions: the fact that each of those vectors for dimensions 3 and 4 happen to have one element in them is irrelevent, ALL applies its algorithm on that one element. It might help to revise this: https://www.mathworks.com/help/matlab/math/multidimensional-arrays.html

Lets consider the top left corner of your data:

A = [0.53 0.67 0.01 0.38 0.07 0.42 0.69];
A(1,1,:) % top left, all elements along 3rd dimension
ans = 0.5300
A(1,1,1,:) % top left page1, all elements along 4th dimension
ans = 0.5300

You can repeat this yourself for every other element in your matrix.

Question: what do you expect the result to be? Why?

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Answer 1

the cyclist 2023-6-14

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1983194-all-function-b-all-a-0-5-3#answer_1255929

编辑：the cyclist 2023-6-14

在 MATLAB Online 中打开

The reason is that all MATLAB arrays have implied singleton dimension beyond the 2nd dimension, if they have not been actually specified.

A = [0.53 0.67 0.01 1.1;
     0.38 0.07 0.42 0.01];
size(A,47)
ans = 1

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Answer 2

Torsten 2023-6-14

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https://ww2.mathworks.cn/matlabcentral/answers/1983194-all-function-b-all-a-0-5-3#answer_1255934

移动：Torsten 2023-6-14

But why does this yield the same result?

Because A has only 2 dimensions, not 3 or 4. Thus "all" operates on all elements separately in both cases.

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VBBV 2023-6-15

在 MATLAB Online 中打开

Thus "all" operates on all elements separately in both cases., Yes, thats correct. But if one uses 'all' option it would do only once.

A = [0.53 0.67 0.01 1.1; 0.38 0.07 0.42 0.01] 
A = 2×4
    0.5300    0.6700    0.0100    1.1000
    0.3800    0.0700    0.4200    0.0100
B = all(A < 0.5,'all')
B = logical
   0
B = all(A < 0.5,3)
B = 2×4 logical array
   0   0   1   0
   1   1   1   1
B = all(A < 0.5,4)
B = 2×4 logical array
   0   0   1   0
   1   1   1   1