I'll be surprised if there's anything much better than the first program you offered. My only recommendation is to use Laminate rather than Stitch:
( ~.@[ ,: +//. )/ Doing this we hold to the row-wise orientation that is natural for Key, vs the column-wise orientation given in the inquiry. If there is some importance to the columnar orientation it can be managed outside the main verb, perhaps with Under Transpose: (~.@[ ,: +//.)/ &.|: arr The use of Nub to associate the keying values seems relatively awkward, but I'm not aware of an elegant alternative. -- Tracy ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
