How to divide a vector randomly in 3 groups?

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Hello everyone,
I have the following problem: I have a column vector A = [1:1000]' and I would like to divide it in 3 groups randomly. (For example in A1=[1:200], A2=[1:450], A3=[1:350]) It doesn't matter if the groups are contiguous. That is, if the first group is made up of sample 1 to 200, the second group should be made up of sample 201 to 650, etc.
The only requirement is that each group must contain at least 10% to 80% of the data. That is, there cannot be a group with 2 samples and the rest with 499 and 499.
Thanks in advance,
J.F.
  2 Comments
James Tursa
James Tursa on 19 Feb 2021
Each group contains 10% to 80% of the data, and you allow overlaps?
Javier Fuster
Javier Fuster on 20 Feb 2021
Yes. The goal is really to know how many samples each group has (number), so even if they overlap it would not be a problem.
Thanks for the reply.

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Accepted Answer

James Tursa
James Tursa on 24 Feb 2021
Edited: James Tursa on 24 Feb 2021
Based on my current understanding, maybe this rejection method might do what you want. Again, since there are only three groups the rejection percentage of about 23% might be tolerable to you.
A = your vector
n = numel(A);
n10 = floor(0.10*n);
while( true )
p = n10 + sort(randperm(n-2*n10+1,2)-1);
if( p(1) >= n10 && p(2)-p(1) >= n10 && n-p(2) >= n10 )
break;
end
end
G = {A(1:p(1)),A(p(1)+1:p(2)),A(p(2)+1:end)};

More Answers (1)

James Tursa
James Tursa on 20 Feb 2021
Maybe a simple loop:
n = numel(A);
n10 = floor(0.10*n)-1;
n80 = floor(0.80*n);
for k=1:3
k1 = randi(n-n10);
k2 = k1 + n10 + randi(min(n-n10-k1+1,n80-n10)) - 1;
G{k} = A(k1:k2);
end
  6 Comments
Javier Fuster
Javier Fuster on 20 Feb 2021
Yes, true, I meant the number of elements in each group in this first example, sorry for the confusion. If you know how to resolve the problem I would appreciate.
Thank you very much.
Javier Fuster
Javier Fuster on 22 Feb 2021
Hello again @James Tursa! Sorry for the inconvenence, do you know how I can resolve this problem? I have not managed to fix it ... Thank you again for your selfless help.
Regards,
J.F.

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