Choose pairs of integers without repeating this selection in a loop

Hi. I want select randomly two number by 'randsample' or 'randperm' function in a loop (for intermediate recombination in evolutionary strategy matlab code). But I want any pair of numbers can be selected only once. For example if selected [1 2], in next time [1 2] or [2 1] are not selected. please help me. Thanks

Answer 1

[Image Analyst]

Seems really, really easy. Did you try it? This is what I got:

theNumbers = randperm(50) % Whatever number you want.
for k = 1 : 2 : length(theNumbers)/2
  selection = theNumbers(k:k+1) % Report this selection to command window.
end

[Ehsan]

but i want when selected {1} for example, still it has a chance of being selected.

[Image Analyst]

I have no idea what that means. First of all, I have selection, not selected. Secondly it's a numerical array, not a cell array. And third, you said "any pair of numbers can be selected only once " which seems to be a direct contradiction of saying it still "has a chance of being selected."

[Ehsan]

for example:

A = {1,2,....,10};

Loop 1 : { 1 & 5 } selected

Loop 2 : { 5 & 7 } selected ( {5} has chance for selected )

Loop 3 : { 5 & 1 } selected >> #error >> next loop

Loop 4 : { 7 & 2 } selected

.

.

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Answer 2

[Geoff Hayes]

Hi Ehsan,

One option is to keep track of a list of all the possible choices for your recombination, randomly select a pair of indices into that list, remove those two from your list of choices and then repeat for the next random selection (the two that you removed previously will no longer be in the list to choose from):

N            = 26;                               % the max number of choices
recomChoices = 1:N;                              % the vector of choices
rndPairIndcs = randperm(length(recomChoices),2); % the randomly chosen indices for recombination
rndPair      = recomChoices(rndPairIndcs);       % the numbers/ids for recombination (i.e. 1,2)
recomChoices(rndPairIndcs) = [];                 % remove that pair from the list

Hope that this helps!

Geoff