transform an empty matrix '0x0 double' into a matrix '0x2 double'

Question

Alberto Acri 2023-7-20

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1998533-transform-an-empty-matrix-0x0-double-into-a-matrix-0x2-double

评论： Bruno Luong 2023-7-20

采纳的回答： Bruno Luong

test_p.mat

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Hello! How can I transform an empty matrix '0x0 double' into a matrix '0x2 double' as in the figure?

For example in my case, I have a cell 'test_p'. Rows 3,4,5 (0x0 double) should become like rows 1 and 2 (0x2 double).

test_p = importdata("test_p.mat");
data_0x2_double = test_p{1, 1};
data_0x0_double = test_p{3, 1};

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Answer 1

Bruno Luong 2023-7-20

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https://ww2.mathworks.cn/matlabcentral/answers/1998533-transform-an-empty-matrix-0x0-double-into-a-matrix-0x2-double#answer_1276173

编辑：Bruno Luong 2023-7-20

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test_p.mat

test_p = importdata("test_p.mat")
test_p = 5×1 cell array
    {0×2 double}
    {0×2 double}
    {0×0 double}
    {0×0 double}
    {0×0 double}
ir = cellfun('size',test_p,1)==0 & cellfun('size',test_p,2)==0
ir = 5×1 logical array
   0
   0
   1
   1
   1
test_p(ir) = {zeros(0,2)}
test_p = 5×1 cell array
    {0×2 double}
    {0×2 double}
    {0×2 double}
    {0×2 double}
    {0×2 double}

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Answer 2

Steven Lord 2023-7-20

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https://ww2.mathworks.cn/matlabcentral/answers/1998533-transform-an-empty-matrix-0x0-double-into-a-matrix-0x2-double#answer_1276243

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Since a 0-by-0 array and a 0-by-2 array have the same number of elements you can reshape one into the other.

A = zeros(0, 0)

A = []

B = reshape(A, [0 2])

B = 0×2 empty double matrix

C = reshape(B, 0, 0)

C = []

whos A B C

Name Size Bytes Class Attributes A 0x0 0 double B 0x2 0 double C 0x0 0 double

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Steven Lord 2023-7-20

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Your code below made an assumption about the user's data, one that's likely valid (and is valid WRT the original question) but in some cases may not be.

A = zeros(0, 0, 'single');
B = reshape(A, [0 2]);
C = zeros(0, 2);
whos A B C
  Name      Size            Bytes  Class     Attributes

  A         0x0                 0  single              
  B         0x2                 0  single              
  C         0x2                 0  double              

B and C are different classes. You could modify the call that creates C to always create it 'like' A or with the same type as A, but reshape will not change the type and is shorter than either of the calls that create C1 or C2 below.

C1 = zeros(0, 2, 'like', A)

C1 = 0×2 empty single matrix

C2 = zeros(0, 2, class(A))

C2 = 0×2 empty single matrix