What is wrong with using i = rand5() + rand5() instead of i = 5 * rand5 () + rand5() is that the former is not uniformly distributed between 1 and 10, as claimed. First, 1 never occurs. 2 occurs 1 out of 25 times, 3 occurs 2 out of 25, etc. (Think of rolling two ordinary dice.)
Dave On Sep 18, 6:19 am, eSKay <[email protected]> wrote: > one of the solutions given > athttp://stackoverflow.com/questions/137783/given-a-function-which-prod... > is: > > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > int i; > do > { > i = 5 * (rand5() - 1) + rand5(); // i is now uniformly random > between 1 and 25} while(i > 21); > > // i is now uniformly random between 1 and 21 > return i % 7 + 1; // result is now uniformly random between 1 and 7 > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > Why do we need to go for all this trouble?? > > Why not: > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > int i; > do > { > i = rand5() + rand5(); // i is now uniformly random between 1 and > 10} while(i > 7); > > // i is now uniformly random between 1 and 7 > return i; > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > Whats wrong with it?? > > On Sep 9, 12:34 am, Ramaswamy R <[email protected]> wrote: > > > > > Generate the random number 7 times. Sum them all and divide by 5. > > Theoritically it should be evenly distributed over 1-7. But owing to random > > number generators characteristics the sum of rand(5) called 7 times may not > > be perfectly evenly distributed over 1-7. > > A nice discussion on some neat options is available here > > -http://stackoverflow.com/questions/137783/given-a-function-which-prod... > > > Rejection technique is pretty standard for this and yet simple. > > > On Mon, Sep 7, 2009 at 8:56 AM, ankur aggarwal > > <[email protected]>wrote: > > > > Given a random number generator that generates numbers in the range 1 to > > > 5, how can u create a random number generator to generate numbers in the > > > range 1 to 7. (remember that the generated random numbers should follow a > > > uniform distribution in the corresponding range) > > > -- > > Yesterday is History. > > Tomorrow is a Mystery. > > Today is a Gift! That is why it is called the Present :). > > >http://sites.google.com/site/ramaswamyr- Hide quoted text - > > - Show quoted text - --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Algorithm Geeks" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/algogeeks -~----------~----~----~----~------~----~------~--~---
