((!3)*!(7-3))%~!7 35
3!7 35 On Wed, Feb 26, 2014 at 8:25 AM, Jon Hough <[email protected]> wrote: > I have a question about doing combinations and permutations with J. > There is an easy to use factorial function (or is that verb) : ! > !5120 > Is there a combination verb? Or do I make my own? > ((!3)*!(7-3))%~!7 > Gives 7 choose 3. I think I butchered that. Is there a better way to do > this, without all the brackets? > Now I want to think about more complicated combinatorial objects: Steiner > Systemshttp://en.wikipedia.org/wiki/Steiner_system > Essentially a Steiner System is defined by three numbers a,b,c such that > we have a total of c points and we collect the points into subsets of size > b, such that for any "a" points (where a < b < c) there is exactly one > subset containing all three:example a,b,c=2,3,7 we have the S(2,3,7) > steiner system with points 0~6, the subsets are012 034 056 135 146 > 236 245Note, every pair of points is in exactly one subset. > For a given Steiner system, to calculate the number of subsets (in the > above case 7) there is a simple eqn: > num = (c choose a)/(b choose a) > I would like to write this as a verb (preferrably tacit).I have > immediately run into a problem:this verb would take three arguments. Not > sure how to do that. > I would like some help writing this verb. Thanks in advance. > Regards,Jon > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
