Shuffle vector with constraints
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Hi,
I have an array of 50 1's, 50 2's, etc up to 50 8's. I would like to shuffle them with the constraint that two consecutive numbers cannot be the same. I tried several options but most are very cpu intensive or did not yield a good result. Are there computation efficient solutions to this problem?
1 Comment
Azzi Abdelmalek
on 14 Jan 2014
This is not enough as information. Because you can set them for example [1 2 3 1 2 3 1 2 3 . . .]
Accepted Answer
W. Owen Brimijoin
on 14 Jan 2014
A brute force method works fairly well in this scenario - 1) start with a shuffled version of the sequence, 2) find repeats, 3) swap any repeats to different locations... goto 2 (rinse and repeat). This sort of method would likely be terrible if you had fewer characters in your sequence, but for nine unique elements, it seems to halt very quickly.
Making your sequence:
seq = repmat([1:8],50,1); %make sequence
seq = [seq(:);repmat(9,30,1)]; %make sequence
Doing the reshuffling:
seq = seq(randperm(numel(seq))); %initial shuffle
old_idx= unique(find(diff(seq)==0));%find repeats
while ~isempty(old_idx), %continue until no repeats
new_idx = unique(setdiff([1:length(seq)],old_idx)); %find new spots
new_idx = new_idx((randi(length(new_idx),length(old_idx),1)))';
seq([new_idx;old_idx],:) = seq([old_idx;new_idx],:); %swap
old_idx= unique(find(diff(seq)==0));%find repeats
end
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More Answers (3)
Mischa Kim
on 14 Jan 2014
Edited: Mischa Kim
on 14 Jan 2014
Since I do not know what techniques you have tried, here's (another) one? Start with sorting the 1's, 2's, etc. in matrix form, i.e.,
1 1 1 1...
2 2 2 2...
3 3 3 3...
and perform permutations on the individual columns (e.g using randperm). Based on the entry of the last row in the first column randomly pick a column vector with a different first row entry and make it the new second column. Using this process work your way through all the columns (you might have to do some more permutations towards the final columns). Once done, transpose and concenate all of the column vectors.
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Pieter
on 14 Jan 2014
1 Comment
Mischa Kim
on 14 Jan 2014
Edited: Mischa Kim
on 14 Jan 2014
Hello Pieter,
- Understood. I used the permutation for shuffling with a certain level of randomness (while satisfying the constraint).
- You can still use the approach outlined above by randomly tossing in the 30 9's into the vector at the end.
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