How to check this condition? (matlab programming)

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RS 2014-8-30

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评论： Image Analyst 2014-8-31

采纳的回答： Star Strider

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Hi,

I have an randomly generated 100 variables between 1 to 20

a=randi(20,1,100)

and another variable b by

b=randi(20,1,100)

Now I want to select 20 values from a and b such that a*b < 64..

how to select 20 such values from a and b so that the above condition is maintained?

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dpb 2014-8-30

With/without replacement? But, basically looks like an acceptance/rejection scheme w/ resampling would be the choice. Or, select from one then restrict selection from the other such condition is met; it is trivial in this case to compute the allowable range for the second given the first.

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Answer 1

Star Strider 2014-8-30

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At least as I understand the problem, this works:

a=randi(20,1,100);          % Initialise random integer arrays
b=randi(20,1,100);
k1 = 1;                     % Initialise counter and ‘v’
v = [];
while (k1 <= 20)            % Limit: 20 pairs of numbers
    c1 = a(randi(100));     % Random number from ‘a’
    c2 = b(randi(100));     % Random number frin ‘b’
    cv = [c1 c2];           % Vector of [a b]
    if prod(cv) < 64        % Check: a*b < 64
        v = [v; cv];        % If so, add [a b] to ‘v’
        k1 = k1 + 1;        % Increment counter and continue
    end
end

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Answer 2

Roger Stafford 2014-8-30

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The following code assumes there are at least 20 such pairs. If not, you will have to regenerate a and b and start over again.

   [p,q] = find((a.'*b)<64);
   r = randperm(size(p,1),20);
   aa = a(p(r));
   bb = b(q(r));

The two 20-element column vectors, aa and bb, will be such randomly selected pairs.

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Answer 3

Image Analyst 2014-8-30

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编辑：Image Analyst 2014-8-31

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Try this:

a=randi(20,1,100);
b=randi(20,1,100);
count = 0;
% Compute every product.
for ia = 1 : length(a)
  for ib = 1 : length(b)
    % Look for a product less than 64.
    if a(ia) * b(ib) < 64
      % Found one pair that works.
      count = count + 1;
      % Store it in "keepers" array.
      keepers(count, 1) = a(ia);
      keepers(count, 2) = b(ib);
    end
  end
end
% Print to command window:
keepers
% Keep only the first 20 of them or however many of them there are.
lastIndex = min(20, length(keepers));
keepers = keepers(1:lastIndex);

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RS 2014-8-31

Thank you all,

All 3 logic are very useful.

Image Analyst 2014-8-31

Note though that they are different. My code exhaustively checks every number in a multiplied by every number in b. So every pair is checked, and checked only once. Star's code takes random selections from a and b and checks them, so it might check (and select/keep) some products twice while others not at all .

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