# Randomly selecting a number from different arrays

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Sojung Youn on 13 Oct 2021
Edited: Matt J on 19 Oct 2021
Hello,
I am trying to randomly select numbers from 3 different arrays without having them side by side or repeated. For example, I have 3 arrays like below and pick 1 random number from them with the code below.
A = [11 11 11 12 12 12 13 13]
B = [21 21 21 22 22 22 23 23]
C = [31 31 31 32 32 32 33 33]
I would need 18 string of numbers and would there a way to pick 1 random number from A, and for the next number it would come from only B or C? (and after that, the number would only come from the other two list.)
Many thanks!

Matt J on 14 Oct 2021
A = [11 11 11 12 12 12 13 13];
B = [21 21 21 22 22 22 23 23];
C = [31 31 31 32 32 32 33 33];
M=[A;B;C];
I=mod(cumsum(randi(2,1,18)),3)+1;
J=randi(length(A),size(I));
selection=M( sub2ind(size(M),I,J) )
selection = 1×18
22 33 12 21 11 23 12 21 12 33 11 31 12 22 31 11 33 22
Matt J on 19 Oct 2021
You can't be guaranteed to get the same amount of numbers, because the rule you have specified in your post does not call for a pure permutation of the values in A,B,C. If you wanted a pure permutation you would simply do,
M=[A;B;C]
selection=M(randperm(numel(M)))
The rule you have specified, however, is that none of A,B,C can be selected twice in succession. This means, for example, that the sequence A,B,A,B,A,B,... with no occurence at all of C is hypothetically possible.