How to check this condition? (matlab programming)

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RS
RS le 30 Août 2014
Commenté : Image Analyst le 31 Août 2014
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?
  1 commentaire
dpb
dpb le 30 Août 2014
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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Star Strider
Star Strider le 30 Août 2014
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

Plus de réponses (2)

Roger Stafford
Roger Stafford le 30 Août 2014
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.
Image Analyst
Image Analyst le 30 Août 2014
Modifié(e) : Image Analyst le 31 Août 2014
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);
  2 commentaires
RS
RS le 31 Août 2014
Thank you all,
All 3 logic are very useful.
Image Analyst
Image Analyst le 31 Août 2014
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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