I need to change n elements in an array chosen randomly

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Matthew Pickard 2021-11-10

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编辑： dpb 2021-11-11

How do I change n elements in an array but chosen randomly? The code below favors the first matrix elements evaluated which I don't want. I only want to set the matrix value to 1 only n - 1 times.

for i = 1:n
  %Starting from i only changes upper matrix triangle
  for j = i:n
    if i == j
      A(i,j) = 1;
    else
      r = randi(100);
      if r < 50
        if num_connected < n-1
          num_connected = num_connected + 1;
          A(i,j) = 1;
        end
      end
    end
  end
end

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Steven Lord 2021-11-10

How do I change n elements in an array but chosen randomly?

How specifically do you want to choose? Do you want to select exactly n different elements to change? Or do you want each element to have an n/numel chance of being chosen, which may not give you exactly n changed elements?

Matthew Pickard 2021-11-10

I want to select n -1 elements to change. The code above selects the first n -1 found randomly. But this is not distributed across all the ones looked at. Background: I have 10 vertices and I want to make 9 random connections between them. This is related to a minimu spanning tree analysis. Thanks for your attention.

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Answer 1

dpb 2021-11-10

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编辑：dpb 2021-11-11

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Not totally clear yet just what are valid locations to be changed -- the above code sets the diagonal elements to one anyways; if one wants N other random locations to be set to one, then something like

N=YourMagicNumber;
I=randperm(numel(M),N);         % select N random locations from the total indices available
[r,c]=ind2sub(size(A),I);       % check don't have diagonal element
if any(r==c)
  % regenerate any for which r,c are same here if that's important
end
A(I)=1;

Alternatively, to select only off diagonal elements a priori...

A=eye(M);                           % the starting array of size MxM
I=setdiff(1:numel(A),find(A));      % non-diagonal indices into A
N=YourMagicNumber;
I=I(randperm(numel(I),N));          % select N random locations from collection
A(I)=1;

No regeneration would be needed this way, the indices are exclusive of the diagonal elements.