Generate cell with random pairs without repetitions

Question

How to generate a sequence of random pairs without repeating pairs? The following code already generates the pairs, but does not avoid repetitions:

for k=1:8
    Comb=[randi([-15,15]) ; randi([-15,15])];
    T{1,k}=Comb;
end

When running I got:

T= [-3;10] [5;2] [1;-5] [10;9] [-4;-9] [-5;-9] [3;1] [-3;10]

The pair [-3,10] is repeated, which cannot happen.

PS : The entries can be positive or negative.

Is there any built in function for this? Any sugestion to solve this?

Possible duplicate of [Efficiently generating unique pairs of integers](https://stackoverflow.com/questions/15793172/efficiently-generating-unique-pairs-of-integers) — Leander Moesinger, Sep 08 '17 at 17:16
Thanks for the link (https://stackoverflow.com/questions/15793172/efficiently-generating-unique-pairs-of-integers) but the solutions presented only work for positive combinations. — hello123, Sep 08 '17 at 21:39

Luis Mendo · Accepted Answer · 2017-09-09T00:24:09.507

2

If you have the Statistics Toolbox, you can use randsample to sample 8 numbers from 1 to 31^2 (where 31 is the population size), without replacement, and then "unpack" each obtained number into the two components of a pair:

s = -15:15; % population
M = 8; % desired number of samples
N = numel(s); % population size
y = randsample(N^2, M); % sample without replacement
result = s([ceil(y/N) mod(y-1, N)+1]); % unpack pair and index into population

Example run:

edited Sep 09 '17 at 00:24

answered Sep 08 '17 at 17:15

Luis Mendo

110,752
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Thanks! It worked. But only for positive combinations. And I also wanted for negative combinations. – hello123 Sep 08 '17 at 21:33
1

@emanbap That's because you changed your question after I answered. Anyway, see edited answer – Luis Mendo Sep 09 '17 at 00:24

rahnema1 · Answer 2 · 2017-09-09T02:46:19.830

2

You can use ind2sub:

n = 15;
m = 8;
[x y]=ind2sub([n n],randperm(n*n,m));

edited Sep 09 '17 at 02:46

answered Sep 08 '17 at 17:28

rahnema1

15,264
3
15
27

1

Best approach so far, in my opinion. `ind2sub` can be easily generalized to more than `2` components – Luis Mendo Sep 08 '17 at 18:10
1

Do you have to switch the input order to `ind2sub`? – Sep 08 '17 at 19:48
@emanbap I updated the answer, as Jon noted the order of input to the ind2sub should be switched. – rahnema1 Sep 09 '17 at 02:48

score 1 · Answer 3 · answered Sep 08 '17 at 17:31

Two possibilities:

1.

M = nchoosek(1:15, 2);
T = datasample(M, 8, 'replace', false);

2.

T = zeros(8,2);
k = 1;
while (k <= 8)
  t = randi(15, [1,2]);
  b1 = (T(:,1) == t(1));
  b2 = (T(:,2) == t(2));
  if ~any(b1 & b2)
    T(k,:) = t;
    k = k + 1;
  end
end

The first method is probably faster but takes up more memory and may not be practicable for very large numbers (ex: if instead of 15, the max was 50000), in which case you have to go with 2.

Generate cell with random pairs without repetitions

3 Answers3