Splitting a matrix based on certain values in the rows

Question

Mr. 206 2019-2-19

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https://ww2.mathworks.cn/matlabcentral/answers/445805-splitting-a-matrix-based-on-certain-values-in-the-rows

评论： Jos (10584) 2019-2-20

I have a matrix A like this:

A = [911 911; 
       0   2; 
       8   5;        
       7   3; 
     911 911;       
       5   3;      
       1   6;        
       6   7;      
     911 911;        
       3   5;         
       8   4];

I want to split the matrix A into three matrices (A1,A2,A3) based on the row values 911 like this:

A1 = [0 2; 8 5; 7 3]; 
A2 = [5 3; 1 6; 6 7];
A3 = [3 5; 8 4];

I need to do this thing inside a for loop which will give the spitted matrix one after another.

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Mr. 206 2019-2-19

This is just a dummy. It can be A, B,C or any other name.

Jan 2019-2-19

@Atta: A,B,C, ... suffers from exactly the same problems as A1, A2, A3, ... With using a cell array and and index, the code is fast and flexibel. You can e.g. simply run it for 12'781'986 rows without getting mad while typing the code with manually hidden names of variables.

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Answer 1

Stephen23 2019-2-19

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https://ww2.mathworks.cn/matlabcentral/answers/445805-splitting-a-matrix-based-on-certain-values-in-the-rows#answer_361729

编辑：Stephen23 2019-2-19

No loop required:

>> idx = cumsum(all(A==911,2));
>> row = 1:numel(idx);
>> fun = @(r){A(r(2:end),:)};
>> C = accumarray(idx,row(:),[],fun);
>> C{:}
ans =
   0   2
   8   5
   7   3
ans =
   5   3
   1   6
   6   7
ans =
   3   5
   8   4

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Mr. 206 2019-2-20

编辑：Mr. 206 2019-2-20

Thanks, I got it.

I just need another help, I have another 5 by 6 matrix (say D). I want to pad the matrices C{1}, C{2} and C{3} by some random number, so that the row numbers are equal.

For example, C{3} has 2 rows, so i need to put three e.g 911911 value at the bottom to make the row number equal to matrix D (5 by 6)

Jos (10584) 2019-2-20

在 MATLAB Online 中打开

% Pad each cell with rows of 911,
% so that it has the same number of rows as D
% using CELLFUN with an anonymous padding function using REPMAT
C2 = cellfun(@(m) [m ; repmat(911, size(D,1)-size(m,1),size(m,2))],  C)

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Answer 2

Jan 2019-2-19

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/445805-splitting-a-matrix-based-on-certain-values-in-the-rows#answer_361728

编辑：Jan 2019-2-19

在 MATLAB Online 中打开

A = [911 911; 
       0   2; 
       8   5;        
       7   3; 
     911 911;       
       5   3;      
       1   6;        
       6   7;      
     911 911;        
       3   5;         
       8   4];
   
index  = [find(A(:, 1) == 911); size(A, 1) + 1];
n      = numel(index) - 1;
Result = cell(1, n);
for k = 1:n
    Result{k} = A(index(k):index(k+1)-1, :);  % [EDITED]
end