On Fri, Jun 06, 2014 at 08:39:28PM +0200, meino.cra...@gmx.de wrote: > Canek Peláez Valdés <can...@gmail.com> [14-06-06 17:36]: > > On Thu, Jun 5, 2014 at 9:56 PM, <meino.cra...@gmx.de> wrote: > > > Hi, > > > > > > I am experimenting with the C code of the ISAAC pseudo random number > > > generator > > > (http://burtleburtle.net/bob/rand/isaacafa.html). > > > > > > Currently the implementation creates (on my embedded linux) 32 bit > > > hexadecimal output. > > > > So it's a 32 bit integer. > > > > > From this I want to create random numbers in the range of [a-Za-z0-9] > > > *without violating randomness* and (if possible) without throwing > > > away bits of the output. > > > > You mean *characters* int the range [A-Za-z0-9]? > > > > > How can I do this mathemtically (in concern of the quality of output) > > > correct? > > > > The easiest thing to do would be: > > > > ------------------------------------------------------------------------------- > > #include <time.h> > > #include <stdio.h> > > #include <stdlib.h> > > > > #define N (26+26+10) > > > > static char S[] = { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', > > 'K', 'L', 'M', > > 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', > > 'X', 'Y', 'Z', > > 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', > > 'k', 'l', 'm', > > 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', > > 'x', 'y', 'z', > > '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' }; > > > > int > > next_character() > > { > > // Use the correct call for ISAAC instead of rand() > > unsigned int idx = rand() % N; > > return S[idx]; > > } > > > > int > > main(int argc, char* argv[]) > > { > > // Use the correct call for initializing the ISAAC seed > > srand((unsigned int)time(NULL)); > > for (int i = 0; i < 20; i++) // --std=c99 > > printf("%c\n", next_character()); > > return 0; > > } > > ------------------------------------------------------------------------------- > > > > If the ISAAC RNG has a good distribution, then the next_character() > > function will give a good distribution among the set [A-Za-z0-9]. > > > > Unless I missunderstood what you meant with "create random numbers in > > the range of [a-Za-z0-9]". > > > > Regards. > > -- > > Canek Peláez Valdés > > Profesor de asignatura, Facultad de Ciencias > > Universidad Nacional Autónoma de México > > > > Hi, > > Thank you very much for the input! :) > > I have a question about the algorithm: > Suppose rand() has an equal distribution of numbers and furthermore > one has a count of 2^32 random numbers listed in numerical sort > order. > In this list each number would appear (nearly) with the same count: 1 > > To get an better imagination of that...suppose the rand() would only > return numbers in the range of 1...12 and the alphabet has only 8 > characters (as 2^32 is not devideable by 62) > > rand(): > 1 2 3 4 5 6 7 8 9 10 11 12 > > rand()%N : rand()%7 > 1 2 3 4 5 6 7 0 1 2 3 4 > > or in other words: An even distribution of numbers of rand() > would result in a unevenly distributed sequence of characters...or? > This would break the quality of ISAACs output. > > I am sure I did something wrong here...but where is the logic trap? >
This is the thing I explained in my message. -- -Matti
