Extracting rows form a matrix with indices.

Question

Paola 2023-2-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1915905-extracting-rows-form-a-matrix-with-indices

评论： Paola 2023-2-23

HI all,

i have a Matrix a=(24X30) and a vector b=(1,4,7,8,10,12) that represents the rows index. I would like to create a new Matrix C=(6X30). This matrix is composed of the 6 rows with indeces (1,4,7,8,10,12), the vector b.

How can I do that?

Thanks

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Answer 1

dpb 2023-2-21

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https://ww2.mathworks.cn/matlabcentral/answers/1915905-extracting-rows-form-a-matrix-with-indices#answer_1175915

编辑：dpb 2023-2-21

在 MATLAB Online 中打开

M=[[1:24].' randi(100,24,5)];       % a sample array
b=[1,4,7,8,10,12];                  % the indexing vector
N=M(b,:)                            % the new array
N = 6×6
     1    57     8    36     5    76
     4    84    18    83    22    27
     7     5    28    43    98    24
     8    46    27    91    59    88
    10     2    98    51    46    36
    12    91    21   100    33    33

illustrates vector indirect addressing does work.

NOTA BENE: the array M was created such that the first element is the row index value so can see which was selected by inspection to prove got what wanted...

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dpb 2023-2-21

在 MATLAB Online 中打开

M=[[1:24].' randi(100,24,5)];       % a sample array
b=[1,4,7,8,10,12];                  % the indexing vector
ix=randperm(numel(b))               % let's shuffle them around...
ix = 1×6
     5     4     2     3     1     6
N=M(b(ix),:)
N = 6×6
    10    24    28    71    27    13
     8    65    32    13    32    84
     4    97    77    44    19    36
     7    33    25    48     6    49
     1     7     1    28    73    63
    12    14    40    79    94    88

illustrates the addressing doesn't have to be in any specific order, either...

Paola 2023-2-23

Thanks! It works

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Answer 2

Adam Drake 2023-2-21

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1915905-extracting-rows-form-a-matrix-with-indices#answer_1175940

在 MATLAB Online 中打开

% Create Matrix A: 24 x 30 
A = randi(100,[24 30]);
% Create Vector B: desired row indices
B = [1,4,7,8,10,12];
% Create Matrix C: desired rows from Matrix A, specified by B
C = A(B,:);