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Java Forum / General / December 2005JAVA random number question
I have to write a JAVA code to generate 30 random number from 1 to 100 in a 5 x 6 board of cells.. but I don't know how to write two rules (1)The first rule is each number must be different in the 30 random number. (2)The second rule is in the 30 numbers, there should be at least 10 prime number in these 30 numbers. anyone can help me how to write these two rules? THANKS. Alex Chien >I have to write a JAVA code to generate 30 random number from 1 to 100 > in a 5 x 6 board of cells.. but I don't know how to write two rules [quoted text clipped - 6 lines] > > Alex Chien It violates Usenet etiquette to multipost questions like this. I just spent a fair while answering this one on comp.lang.java.help and I wouldn't have put so much time into my answer if I had realized you already had answers on another newsgroup. That just wastes my time, which is why it is rude for you to multipost. Rhino >>I have to write a JAVA code to generate 30 random number from 1 to 100 >> in a 5 x 6 board of cells.. but I don't know how to write two rules [quoted text clipped - 12 lines] > already had answers on another newsgroup. That just wastes my time, which > is why it is rude for you to multipost. Likely he assumes if he posts his assignment in enough places someone will do it for him -- LTP :) here is the article I've promised : Vitter, J.S. An efficient algorithm for sequential random sampling ACM Trans. Math. Softw., ACM Press, 1987, 13, 58-67 >I have to write a JAVA code to generate 30 random number from 1 to 100 > in a 5 x 6 board of cells.. but I don't know how to write two rules [quoted text clipped - 4 lines] > > anyone can help me how to write these two rules? THANKS. What do you want us to do? Do you not understand the assignment? We're not going to do it for you. Here is an idea. Make a list (array) of all the prime numbers 1 to 100, and then generate only prime numbers for the first 10 attempts. After ever item is added compare it against a list of every number already made to make sure there are no duplicates. This is the shitty way to do it. Better way is to make an array of every number 1 to 100 and then remove items from the array as they are used. Then you have no inefficient checking. Fill in the first 20 and keep track of the number of primes. If you don't have enough primes by the time you are out of spaces, fill the rest with prime numbers only -- LTP :) >Fill in the first 20 and keep track of the number of primes. If you don't >have enough primes by the time you are out of spaces, fill the rest with >prime numbers only warning Luc, this introduce a bias in the number of prime generated ... >This is the shitty way to do it. this is not that shitty if you look at the ratio he has 30 among 100, "most" of the time nextInt() will return an unused integer, (I ve tried it, it still working with 50 among 100 regards > >Fill in the first 20 and keep track of the number of primes. If you > >don't >>have enough primes by the time you are out of spaces, fill the rest with >>prime numbers only > > warning Luc, this introduce a bias in the number of prime generated ... Forcing primes into a "random" sequence produces biases. I VERY strongly feel that this is an assignment, which is designed to make the student think of a way to implement this; something that this student obviously does not want to do. -- LTP :) > I have to write a JAVA code to generate 30 random number from 1 to 100 > in a 5 x 6 board of cells.. but I don't know how to write two rules > (1)The first rule is each number must be different in the 30 random > number. > (2)The second rule is in the 30 numbers, there should be at least 10 > prime number in these 30 numbers. I doubt those two rules are sufficient. You must also specify what you mean by "random". Should all primes occour with equal probability? Should all non-primes occour with equal probability? How should the number of primes be distributed? I any number of primes between 10 and 25 equally likely, or are smaller numbers more likely than larger? How should the numbers be ordered? Without knowing these constraints, a simple solution would be: pick 10 primes at random pick 20 nonprimes at random (and "at random" might be not-very-random too) /L  SignatureLasse Reichstein Nielsen - lrn@hotpop.com DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html> 'Faith without judgement merely degrades the spirit divine.' >I doubt those two rules are sufficient. You must also specify what >you mean by "random". I interpreted the question to mean, integers are to be selected with a pseudorandom number generator with uniform distribution in the range 1 .. 100 without regard to primality. However, a solution with less than 10 primes is not acceptable as is a solution with any number repeated. Obviously that is not how you rapidly compute a solution, but I think that is one way of describing the solution. SignatureCanadian Mind Products, Roedy Green. http://mindprod.com Java custom programming, consulting and coaching. > >I doubt those two rules are sufficient. You must also specify what > >you mean by "random". [quoted text clipped - 6 lines] > Obviously that is not how you rapidly compute a solution, but I think > that is one way of describing the solution. Yes, barring any additional info, this seems to be the natural way to interpret the question. For the specific problem mentioned, this is likely fast enough. To speed things up, you could select 30 of the 100 integers (using the first 30 iterations of the standard shuffling algorithm), check if you have >= 10 primes. If not, repeat. Here's a somewhat more efficient method if the problem size increase. Break this problem into two pieces: (i) determine exactly how many primes to include, then (ii) randomnly choose the required number of primes and nonprimes as above. To do part (i), repeatedly generate Binomial(30, p) random variable until you get a number >= 10. Here p = fraction of primes in desired range. This has the desired distribution, and it should be reasonably efficient, even if the problem size increases. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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