Extracting rows form a matrix with indices.

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

Answer 1

[dpb]

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...

[dpb]

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]

Thanks! It works

Answer 2

[Adam Drake]

% 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 ':'.