Extracting rows form a matrix with indices.

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Paola on 21 Feb 2023
Commented: Paola on 23 Feb 2023
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

dpb on 21 Feb 2023
Edited: dpb on 21 Feb 2023
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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Paola on 23 Feb 2023
Thanks! It works

Adam Drake on 21 Feb 2023
% 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,:);
The desired rows are specified by B, all columns are specified by ':'.

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