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Select random elements from three consecutive columns in a big matrix?

Asked by AM-Laurentian on 4 Apr 2019
Latest activity Commented on by AM-Laurentian on 5 Apr 2019
Hello all,
I have a 12 by 12 matrix, and I want to select 10 random elements. The elements should be from only three consecutive columns.
For example, I want to use 10 random elements from columns (1, 2, 3) or they could be from columns (2, 3, 4) or columns(3, 4, 5) and so on.
Any help please.
Thanks

  3 Comments

10 from each of the columns? 10 total from those 3 columns, with no effort to balance how many are to be taken from each individual column?
Please do not ask the same question twice.
And it looks like you delete your question once answered, which is not going to go down well with us (and will be restored anyway).
I am really sorry, it was a mistake. I realized that the question was submitted twice, I tried to delete it as soon as it was submitted but I could not. I apologies for any inconvenience.

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2 Answers

Answer by Walter Roberson
on 5 Apr 2019

N = 10;
[r, c] = size(YourMatrix);
start_col = randi([1, c-2]);
chosen = YourMatrix(start_col * r - 1 + randi([1 3*r], 1, N));

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Answer by Jos (10584)
on 5 Apr 2019
Edited by Jos (10584)
on 5 Apr 2019

M = magic(5) % input data
N = 4 % number of selections
nC = 3 % number of consecutive columns
% engine
szM = size(M)
r = randi(szM(1), N, nC) % N x 3 random rows
c = randi(szM(2) - nC + 1, N, 1) % N random first columns
c = c + [0:nC-1] % add consecutive columns
R = M(sub2ind(szM, r, c)) % index into M

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