transform an empty matrix '0x0 double' into a matrix '0x1 uint8'

Question

Alberto Acri 2023-7-20

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1998683-transform-an-empty-matrix-0x0-double-into-a-matrix-0x1-uint8

评论： Walter Roberson 2023-7-20

采纳的回答： Walter Roberson

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Hello! How can I transform an empty matrix '0x0 double' into a matrix '0x1 uint8' as in the figure?

For example in my case, I have a cell 'test_p'. Rows 3 and 4 (0x0 double) should become like rows 1,2 and 5 (0x1 uint8).

test_p = importdata("test_p.mat");
M_0x1_uint8 = test_p{1, 1};
M_0x0_double = test_p{3, 1};

The uint8 command should do the trick. How can I apply it only to those '0x0 double' arrays ?

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Answer 1

Walter Roberson 2023-7-20

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https://ww2.mathworks.cn/matlabcentral/answers/1998683-transform-an-empty-matrix-0x0-double-into-a-matrix-0x1-uint8#answer_1276353

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mask = cellfun(@isempty, test_p);
test_p(mask) = {zeros(0,1,'uin8')};

Unless, that is, you might have entries that are (for example) '' (the empty character vector). In such a case

mask = cellfun(@(C) isdouble(C) && isempty(C), test_p);
test_p(mask) = {zeros(0,1,'uin8')};

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Alberto Acri 2023-7-20

Thank you for your reply @Walter Roberson. I am not clear on the difference of the two examples shown. Does the second example refer in the case where line 5 of test_p contains all 0x0 double elements?

Walter Roberson 2023-7-20

The first version of the code looks inside the cell array test_p and finds all of the entries that are "empty" and changes them all to 0 x 1 uint8.

The second version of the code looks inside the cell array test_p and finds all of the double precision entries that are "empty" and changes them all to 0 x 1 uint8.

The difference is that hypothetically your cell array might contain empty entries that are not double precision and that you might not want converted. For example it might contain empty character vectors or empty figure() handles that you want to leave alone.

There is another possibility that this code does not account for in the form posted: you might potentially have some double precision empty arrays that are not 0 x 0 that you do not want converted. For example you could theoretically have an 768 x 1024 x 3 x 0 array that you did not want converted.

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Answer 2

Matt J 2023-7-20

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1998683-transform-an-empty-matrix-0x0-double-into-a-matrix-0x1-uint8#answer_1276358

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x=zeros(0,0);
y=uint8( reshape(x,0,1) );
whos x y
  Name      Size            Bytes  Class     Attributes

  x         0x0                 0  double              
  y         0x1                 0  uint8               