Ric, As another relatively neophyte J-er, I must applaud your exposition in that it really cleared up for me how +/ worked over square matrices. I had always in my mind when working on problems thought of +/"1 as 'going across the rows', and +/"2 as 'going down the columns', which of course worked, and of course I in some sense knew (as a mantra almost) that '+/"1 inserts + between each rank 1 item of the noun it is applied to', but your explanation and example is clear enough to both make intuitive sense and be technically accurate. Thank you :)
On 29 April 2010 22:47, Sherlock, Ric <r.g.sherl...@massey.ac.nz> wrote: > > From: Robert O'Boyle > > > > I have come across a feature of J that I didn't expect. When I add > > rows of a matrix, I use +/"1 > > > > a > > 1 2 3 4 > > 1 2 3 4 > > 1 2 3 4 > > > > +/"1 a > > 10 10 10 > > > > And columns, > > > > +/"2 a > > 3 6 9 12 > > Although the result is the same as summing the columns, I suspect you are > misunderstanding rank in this case. Note to start with that not using rank: > +/ a > 3 6 9 12 > > Will give you the same result. The adverb insert ( / ) will place the verb > to its immediate left between each _item_ of the noun to its right. The noun > ( a ) is rank 2, and is therefore a list of rank 1 items. ( +/ a ) will > place + in between each item of ( a ), i.e. > 1 2 3 4 + 1 2 3 4 + 1 2 3 4 > 3 6 9 12 > > You can think of the rank conjunction as a way of feeding chunks of a noun > on its right, to the verb on the left. So ( +/"1 a ) will send rank 1 chunks > (vectors) of ( a ) to ( +/ ). i.e. similar to: > (+/ 1 2 3 4) , (+/ 1 2 3 4) ,: (+/ 1 2 3 4) > Insert will now place + between each item of the list: > (1+2+3+4) , (1+2+3+4) ,: (1+2+3+4) > > If you use ( +/"2 a ) then you are saying you want to feed ( a ) to ( +/ ) > in rank 2 chunks. Since ( a ) is only rank 2 (a matrix) this is the same as > sending all of ( a ) to ( +/ ) at once. i.e. the same as ( +/ a ). > > > By chance, I happened to 'misplace' insert and got an unexpected result > > Note also that the sentence ( a *"1/ b ) uses dyadic / and so is "table" > not "insert". > > I see Bob has replied along similar lines, I certainly endorse his > recommendation of Henry's book! > > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm > > ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
